Human Factors Guidelines for Road Systems 2021 Update, Volume 1: Updated and New Chapters (2022)

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HFG TUTORIALS VERSION 2.1 22-1 CHAPTER 22 TUTORIALS TUTORIALS FROM NCHRP REPORT 600: HUMAN FACTORS GUIDELINES FOR ROAD SYSTEMS Tutorial 1: Real-World Driver Behavior Versus Design Models...……………………….……22-2 Tutorial 2: Diagnosing Sight Distance Problems and Other Design Deficiencies.…………….22-9 Tutorial 3: Detailed Task Analysis of Curve Driving.………………………………………...22-35 Tutorial 4: Determining Appropriate Clearance Intervals…………………………………….22-38 Tutorial 5: Determining Appropriate Sign Placement and Letter Height Requirements...…...22-39 Tutorial 6: Calculating Appropriate CMS Message Length under Varying Conditions...……22-43 NEW TUTORIALS Tutorial 7: Joint Use of the Highway Safety Manual (HSM) and the Human Factors Guidelines for Road Systems (HFG) ................................................................................... 22-49 Tutorial 8: Using the HFG to Support a Road Safety Audit (RSA) ........................................ 22-71 Tutorial 9: Summary of HFG Topics ....................................................................................... 22-75

Tutorial 1: Real-World Driver Behavior Versus Design Models Much of the information on sight distance presented in Chapter 5 reflects the application of empirically derived models to determine sight distance requirements. Such models, while valu- able for estimating driver behavior across a broad range of drivers, conditions, and situations, have limitations. This tutorial discusses how driver behavior as represented in sight distance models may dif- fer from actual driver behavior. The design models presented in Chapter 5 use simplified con- cepts of how the driver thinks and acts. This simplification should not be viewed as a flaw or error in the sight distance equations. These models are a very effective way of bringing human factors data into design equations in a manner that makes them accessible and usable. After all, the intent of a sight distance equation is not to reflect the complexities of human behavior but to bring what we know about it into highway design in a concise, practical way. However, like any behavioral model, models for deriving sight distance requirements are not precise predictors of every case and there may be some limitations to their generality. Therefore, having an under- standing of certain basic principles of human behavior in driving situations is useful to better interpret these models and to understand how they may differ from the range of real-world driv- ing situations. Sight distance formulas for various maneuvers (presented in Chapter 5) differ from one another, but they share a common simple behavioral model as part of the process. The model assumes that some time is required for drivers to perceive and react to a situation or condition requiring a particular driving maneuver (i.e., PRT), which is followed by some time (i.e., MT) and/or distance required to execute the maneuver. Sight distance equations for some maneuvers may contain additional elements or assumptions; however, all have this basic two-stage model somewhere at their core. The two equations that follow show two versions of the general, two-component model. In both versions, the first term shows the distance traveled during the PRT component and the sec- ond term shows the distance traveled during the MT component. The difference is that the first equation shows a case where the distance traveled while executing the maneuver is based on the time required to make that maneuver (for example, the time to cross an intersection from a Stop), while the second equation shows a case where the distance traveled while executing the maneuver is based directly on the distance required to complete the maneuver (for example, braking distance for an emergency stop). For both forms of this general equation, vehicle speed (V) influences the second (MT) component. The general form of the sight distance equation is: Where: d = required sight distance V = velocity of the vehicle(s) tprt = PRT tman = MT dmanV = distance required to execute a maneuver at velocity V k = a constant to convert the solution to the desired units (feet, meters) d kVt kVt where maneuver time is iSD prt man= + , nput or d kVt d where maneuver tiSD prt manV= + , me is input HFG TUTORIALS Version 2.0 22-2

This model shows that the sight distance requirement is composed of (at least) two distances: there is a distance traveled while the driver perceives and evaluates a situation (determined by PRT and vehicle speed) and a distance traveled while executing the maneuver (determined by maneu- ver time/distance and vehicle speed). Figure 22-1 depicts the activities and sequence of activities associated with this simple model. As the figure shows, the PRT component is itself viewed as a series of steps. These individual steps are not explicit in the design equation but are included in the assumptions that underlie the PRT value. Design equations and their assumptions for specific maneuvers were discussed in Chapter 5. The sequential model of driver behavior shown in Figure 22-1 is a shared common conceptual underpinning of various sight distance equations. However, in some respects, we can consider this model to be a “convenient fiction,” in part because it depicts a simple, fixed, linear, and mechanistic process. While the model provides a useful basis for deriving approximate quantitative values for design requirements that work for many situations, real-world driving behavior is far more complex than the model suggests. While highway designers and traffic engineers are often required to work with less complex (i.e., imper- fect) models of human visual perception, attention, information processing, and motivation, it is important that they understand those factors that may affect the application of design sight distance models for specific situations. Such an understanding will help them to prevent, recog- nize, or deal with sight distance issues that may arise. For a particular situation, the standard sight distance design equation might either underestimate or overestimate the actual needs of a driver. Subsequent sections of this tutorial deal with specific factors that affect the driver response and provide guidance for working with them. Before these specific factors are considered, it will be useful to have an appreciation of how the simple driver models that underlie sight distance requirements contrast with the real complexities of driver behavior. There are a number of factors or conditions associated with driver responses to a hazardous event or object that are not reflected in the basic sight distance model, but nonetheless can have a profound effect on driver behavior and overall roadway safety: • Conditions or events that occur prior to a hazardous event/object becoming visible to the driver • How and when the driver processes relevant information • Driving as an “episodic” activity versus driving as a “smooth and continuous” activity • The nature of the hazardous object or event • The nature of the driver’s response • Individual differences across drivers • The quality and applicability of the empirical research used to develop the driver models Each of these is discussed in more detail below. 22-3 HFG TUTORIALS Version 2.0 Figure 22-1. Diagrammatic version of the basic sight distance model.

Conditions or Events that Occur Prior to a Hazardous Event/Object Becoming Visible to the Driver The model shown in Figure 22-1 is not sensitive to events that happen prior to the moment that the hazardous object or event becomes visible to the driver. In reality, the driver’s ability to react to a hazardous object or event may be strongly influenced by previously occurring condi- tions or events. For example, drivers traveling on a roadway with few access points and little traf- fic may be unprepared to stop for a slow-moving vehicle ahead. In contrast, if drivers had been encountering numerous commercial driveways and intersections, with entering truck traffic, they might more readily react. Roadway design and operational features in advance of a haz- ardous event/object becoming visible are potentially important influences on behavior that are not explicit in the basic sight distance model. Figure 22-2 shows an expansion of the basic model, with added “driver state” factors (e.g., anticipation, situational awareness, caution, and locus of attention) that increase or decrease the driver’s cognition preparation for a hazardous condition or event. In Figure 22-2, an addition component to the model is shown prior to the event becoming visible. One element of the additional component is cognitive preparation. This general term encompasses the various active mental activities that can influence response times and deci- sions, such as driver expectancies, situational awareness, a general sense of caution, and where attention is being directed by the driver. Part II: Bringing Road User Capabilities into High- way Design and Traffic Engineering Practice provides some further explanation of these factors. As the arrows in the figure show, the driver’s cognitive preparation as he or she encounters a hazardous object or event can influence the speed of detection, the speed and accuracy of recognizing the situation, and the speed and type of decision made about how to respond. The critical point is that the PRT associated with a particular hazardous object or event is influenced by the conditions or events preceding the driver’s perception of the haz- ardous object or event. The second element in the additional component in Figure 22-2 that occurs prior to the driver’s perception of the hazardous object or event is speed selection. As discussed earlier, speed can have perceptual effects, influencing how easily a target object is detected or how accurately gaps are judged. Speed may affect the driver’s sense of urgency, which can influence what maneu- ver options are considered and their relative appeal. Speed also may directly affect the difficulty, as well as the required time or distance, of the maneuver. Therefore, the driver’s speed choice prior to the event may influence the driver’s decision process; it may also influence the time avail- able for the driver’s response. HFG TUTORIALS Version 2.0 22-4 Figure 22-2. Added elements to basic sight distance behavioral model.

The basic sight distance behavioral model (Figure 22-1) makes assumptions about driver cog- nitive state and speed choice as the hazardous event is encountered. In reality, the driver does not arrive at the situation as a “blank slate.” The locus of a sight distance problem, or its solu- tion, therefore may turn out to be in advance of the problem site itself. How and When the Driver Processes Relevant Information The basic sight distance model shows a chain of mental and physical events taking place in the following sequential fashion: 1. A hazardous object or event becomes visible. 2. The presence of this object or event is detected by the driver. 3. The object or event is recognized and understood by the driver. 4. The driver makes a decision about what maneuver is needed to avoid or respond to the object or event. 5. The maneuver is initiated. 6. Once initiated, the maneuver is fully executed. Each event in this chain takes some amount of time to occur, and—according to the basic model—one step does not begin until the previous step is complete. This assumed “serial pro- cessing” model is indeed one way a driver might respond, but it may not be typical. For exam- ple, if a driver sees some vague object ahead of the vehicle that might or might not be in the roadway, he or she may begin to brake even before the object is fully recognized. Also, once the object is fully recognized, the maneuver may be reconsidered (e.g., stopped, slowed, accelerated, or otherwise revised). Contrary to the serial processing assumed by the basic model, the mental processes shown by the various boxes in Figure 22-1 may actually occur in parallel, in a differ- ent sequence, or with modifications (feedback loops) as the process progresses. The assumed lin- ear response sequence is therefore really a simplified case used for design purposes. It should not be viewed as a universal or invariant representation of the more complex perceptual and cogni- tive activity in complex driving situations. Importantly, consistency in geometric design is required in order to meet driver expectations and to avoid surprising the driver. Driving as an “Episodic” Activity versus Driving as a “Smooth and Continuous” Activity Related to the previous point, the basic sight distance model reflects an “episodic” perspective of real-world driving. That is, some object or event becomes visible, and some driver maneuver(s) in response to the object or event are initiated and executed. Then, another object or event becomes visible, and another maneuver takes place. Real-world driving however, is normally smooth and continuous; it is not a jerky sequence of separate, individual episodes. Yet for ease of analysis, we often break driver behavior into individual events each requiring their own separate response, or we treat the roadway as a succession of discrete segments or zones. To the driver, though, the road- way and the driving task are generally smooth and continuous. Real drivers do not just react to events that randomly occur; they plan and predict and manage and adapt to events as they go along. Adopting an “episodic” perspective is useful for developing models of driver behavior that are both simple and reasonably predictive. A “smooth and continuous” perspective of real-world driving is much more difficult to model and quantify, especially in a manner that will easily generate a sim- ple design parameter. From a human factors perspective, sight distance models are based on a lit- tle bit of driver performance data that describe how a driver might react, but may not reflect how drivers always or even typically behave. The use and application of the simpler sight distance model 22-5 HFG TUTORIALS Version 2.0

is generally reasonable from a design perspective, however, because it is somewhat conservative. Specifically, those drivers who encounter a situation without planning or anticipation are those most likely to be in need of the full sight distance requirement. The Nature of the Hazardous Object or Event For each sight distance design application, the analysis is based around some object, event, or roadway feature to which the driver must respond with a driving maneuver. That object, event, or roadway feature might be debris in the roadway, braking by a vehicle ahead, an approaching vehicle on a conflicting path, a freeway lane drop, a change in signal phase, a pedestrian entering the road, a railroad gate, an animal, a vehicle entering from a driveway, or many other things. The PRT process begins with the potentially hazardous object or event (the “visual target”) becoming visible to the driver followed by some time to visually detect and recognize that target. Design equations have to include some estimate of when a target becomes visible and how long driver reaction will take. The many examples of potential hazards suggest just how different these may be as visual targets; therefore, making a single assumption is an obvious simplification. A target object may be large or small, bright or dull, familiar or unfamiliar, moving or stationary, or have other attributes that affect the driver’s ability to accurately and quickly detect and recognize it. Explicitly or implicitly, design equations have to make some assumption about the characteris- tics of the visual target. Furthermore, visibility conditions may vary with weather, glare, light condition, roadway lighting, and intervening traffic (especially truck traffic). Again, design equa- tions must be based on some assumption about visibility conditions. A PRT model requires the user to be able to specify the point in time or space that the hazard becomes visible to the driver. However, this too may be an oversimplification. For example, there is usually no sharp threshold where an object in the road suddenly goes from being invisible to visible. Most hazards do not occur all at once, but evolve over some time, such as a vehicle mov- ing into a lane in front of a driver. Some events might have a preview, such as a vehicle positioned in a driveway prior to its pulling out or children playing near the road prior to entering the road. Some events might have multiple cues; for example, a freeway lane drop has an initial taper, lane markings, and the point where the lane finally disappears. Sometimes the important visual target is not the hazard object or event itself but a cue about the hazard; for example, brake lights on a vehicle ahead may be a warning cue about a sudden severe deceleration, but they may also reflect a minor tap on the brake. Drivers cannot respond to the brake light in the same way they respond to recognition of the actual deceleration. Overall then, the driver’s response to a hazardous event or object will reflect specific physical characteristics, visibility conditions, and the evolving nature of the hazard itself. The Nature of the Driver’s Response The behavioral components of sight distance models are based around some very specific maneuver in response to the object/event, with fixed assumptions about response parameters. For example, when responding to an unexpected need to stop, AASHTO (2004) assumes a braking maneuver with a deceleration of 3.4 m/s2 (11.2 ft/s2). Braking may be a reasonable response to assume, and 3.4 m/s2 may be a reasonable deceleration to assume, but this certainly does not mean that braking at this level is always the driver’s response to an unexpected hazard. The maneuver time and maneuver distance components of sight distance models are in many cases based on good empirical research and human factors considerations and work well for most applications. Still, the use of a single standard value is a convenient simplification. Actual maneuvers can be influenced by various factors. The perceived urgency of the situation (based on available time/ HFG TUTORIALS Version 2.0 22-6

distance, driver/vehicle capabilities) determines options and shapes the way drivers respond, and often multiple options are available to the driver. For example, for an unanticipated stop, a driver may brake severely, or brake gradually and steer around, or swerve sharply. The surrounding physical, traffic, and social environment will affect these options: is there a lane or shoulder to steer around, are there adjacent or following vehicles, is the obstacle a piece of debris or a child, is there a passenger in the vehicle? Drivers also make trade-offs between speed versus control when executing maneuvers. The AASHTO deceleration value of 3.4 m/s2 represents an estimate of a “comfortable deceleration” with which almost all drivers can maintain good vehicle control. In this sense it is appropriate for general design, but does not necessarily describe what drivers can do or actually do under all conditions or circumstances. Furthermore, once a driver initially selects and begins to execute a particular maneuver, the maneuver is not simply executed in a fixed man- ner. As Figure 22-2 illustrates, the situation is monitored and the maneuver is re-evaluated as it is being executed. The response may be refined or modified as it progresses. Drivers may not respond to a situation with a maximum response (e.g., maximum braking or steering), but may initiate a more controlled action and monitor the situation before committing to a more extreme action. For instance, they may begin gradual braking and check their mirrors for following traf- fic before decelerating more sharply or swerving. Individual Differences Across Drivers The diverse driving population ranges widely in capabilities and behaviors. Drivers vary in experience, visual acuity, contrast sensitivity, useful field of view, eye height, information process- ing rate, tolerance for deceleration, physical strength, and other factors related to PRT and MT. A design equation will typically be based around a design driver with some assumed set of attri- butes. To be conservative, the assumptions do not usually represent a typical driver, but rather reflect less capable drivers (e.g., 15th percentile in terms of some attribute). Assumptions are made about the state of the driver as well. For example, data are generally based on drivers who are sober and alert. Yet impaired or fatigued drivers may represent a large part of the crash risk. Alcohol, drugs, medication, and fatigue can have dramatic effects on the psychological processes that underlie PRT and MT. Driver distraction by activity within the vehicle is also a common occur- rence that is not reflected in the design model. In-vehicle technologies, such as cell phones, nav- igation systems, and infotainment systems, are increasingly common. The multitasking driver is an increasing concern, but PRT models do not reflect this possibility. The Quality and Applicability of the Empirical Research Used to Develop the Driver Models The values used in design equations may or may not be derived from good empirical sources. In some cases (e.g., brake reaction time), there are numerous empirical studies and reasonably good agreement among them. In other cases, empirical data are very limited, are of lesser qual- ity, or are only weakly applicable to the design issue in question. The quality and applicability of the numbers that come from empirical studies are sometimes questionable on a number of grounds: the sample of drivers may be small or unrepresentative; the situations evaluated may be limited and may not generalize well; the research may be out of date (given changes in road- ways, traffic, vehicles, traffic control devices, and driver norms); the research setting (test track, simulator, laboratory) may lack validity; and results may conflict with results from other stud- ies. It would be wrong to assume that sight distance design equations are necessarily based on a strong, high-quality empirical foundation that readily generalizes to all cases. Another concern related to data quality and applicability is the inability of general design equations based on simple behavioral models to incorporate site-specific considerations. Empir- 22-7 HFG TUTORIALS Version 2.0

ical observations made at the site may be at variance with the predicted behaviors. Even when design equations are based on “good” data, the generality of the models suggests that credence should be given to any empirical data that can be collected at the site itself. In summary, sight distance requirements are based on a highly simplified and mechanistic model of driver behavior and capabilities. This approach is reasonable and generally success- ful. The general assumptions often work well enough to approximate the needs of most driv- ers; however, it is important to recognize that this simple model has a number of limitations as a description of actual driver performance. When difficult sight distance problems are being diagnosed or addressed, it may be useful for the highway designer or traffic engineer to recog- nize how design models simplify driver actions and to acknowledge the realities of more com- plex driver perception and behavior. HFG TUTORIALS Version 2.0 22-8

Tutorial 2: Diagnosing Sight Distance Problems and Other Design Deficiencies Introduction The previous sections of this document—especially Chapter 5—have provided design guide- lines for human factors aspects of various sight distance concepts. However, for users to imple- ment these guidelines in a practical sense, it is desirable to provide a procedure for their operational application. Therefore, this section comprises a hands-on tool whereby practition- ers can apply human factors techniques to analyze sight distance problems and other design defi- ciencies at a selected highway location. A starting point for development of the current procedure was a review of previously docu- mented procedures for conducting on-site driving task analyses (Alexander & Lunenfeld, 2001) that applied techniques such as commentary drive-thru procedures to generate checklist subjective- scaled ratings of hazard severity and information load. The current in-situ sight distance diagnos- tic procedure includes application of previously available engineering tools, e.g., AASHTO (2004) analyses of geometric requirements and MUTCD (FHWA, 2003) traffic control device require- ments, and augments these techniques with those sight distance concepts presented in Chapter 5 of this HFG. This sight distance diagnostic procedure consists of a systematic on-site investigation tech- nique to evaluate the highway environment to support the concepts of interest, i.e., SSD, PSD, ISD, and DSD. The highway location is surveyed, diagrammed, and divided into component sections based on specific driving demands (e.g., requirement to perform a maneuver). Then each section is analyzed in terms of its suitability to support the required task (e.g., informa- tion provided to driver and allotted time to the complete required task). This procedure enables the practitioner to compare the available sight distance with the required sight distance to safely perform the driving task. The Six-Step Procedure The procedure consists of the following six steps: 1. Collect field data to describe roadway characteristics and other environmental factors affect- ing sight distance requirements and driver perception of a potential hazard. 2. Conduct engineering analyses applying traditional techniques, e.g., AASHTO design criteria and MUTCD compliance, to initially assess site characteristics or deficiencies. 3. Examine crash data and prepare collision diagram to seek possible association between safety and a sight distance problem. 4. Establish component roadway sections in which drivers respond to specific visual cues in order to initiate a maneuver to avoid a hazard. 5. Analyze driving task requirements (PRT and MT) and determine the adequacy of each com- ponent roadway section to support these requirements. 6. Develop engineering strategies for improvement of sight distance deficiencies. A flow diagram overview of the process is shown in Figure 22-3. Following the description of the six-step procedure, an example application is provided. 22-9 HFG TUTORIALS Version 2.0

H FG T U TO RIA LS Version 2.0 22-10 Figure 22-3. Flow diagram of six-step diagnostic process.

22-11 HFG TUTORIALS Version 2.0 Step 1A: Identify Hazard and Prepare Site Diagram The specific hazard location under investiga- tion is identified and the approach roadway is diagrammed. Example of hazards requir- ing sight distance consideration and the associated sight distance concepts are as follows. • A hidden intersection (SSD) • An exit from a shopping mall in a heavily lit (e.g., visually cluttered) setting (DSD) • A vehicle approaching an intersection (ISD) • An oncoming vehicle in a passing zone (PSD) Note distances from hazard to the following features: (1) traffic control devices, (2) intersecting driveway or roadways, and (3) sight distance obstructions. References: Lunenfeld, H., and Alexander, G. J. (1990). A User’s Guide to Positive Guidance (FHWA- SA-90-017). Washington, DC: FHWA. Procedure Product/Application Step 1B: Collect Operating Speed on Approach Spot speeds for randomly selected vehicles are to be observed at a sufficient advance distance upstream from the hazard beyond which slowing in response to the hazard is expected. Candidate speed collection tech- niques are radar/laser detection, automated speed recorders, and manual timing. Refer- ences noted in the column to the right describe appropriate procedures to ensure random vehicle selection and suitable sam- ple sizes. In the event that the approach roadway section is characterized by horizontal or vertical curvature, speed collection points should be selected so as to represent opera- tional speeds at these locations. The product of this step will be a statistical distribution of speeds from which means and/or percentile values will be applied to estimate vehicle speed for the approach roadway under study. References: Hanscom, F. R. (1987). Validation of a non-automated speed data collection methodology. Transportation Research Record, 1111, 54–61. Robertson, H. D. (Ed.). (2000) Manual of Transportation Engineering Studies, Wash- ington, DC: Institute of Transportation Engineers. Procedure Product/Application Step 1: Collect Field Data This step involves making specific field measurements and observations. Data are to be gath- ered both at the location of the designated hazard as well as the approach roadway section imme- diately in advance of the hazard. Approach distances over which field measurements should be gathered are determined from Table 22-1 at the end of this step. Approach distances were derived from approximated perception-reaction and sign reading times applied to the desig- nated operating speeds.

HFG TUTORIALS Version 2.0 22-12 Step 1C: Observe Erratic Vehicle Maneuvers on Approach Observations of vehicle movements should be considered in situations of sufficiently high traffic volumes to justify this type of study, e.g., 100 vehicles per hour (vph) and above. Typical target vehicle behaviors indicative of a sight distance problem are sudden slowing (e.g., observable break light activation) and abrupt lane changes when these maneuvers are not induced by other vehicles in the traffic stream. A considerable literature base is available regarding the conduct and interpretation of “traffic conflicts” studies; however, the reader is cautioned that traffic conflicts studies are limited to interactions between vehicles. A sight distance–induced erratic maneuver, on the other hand, can involve a single vehicle. Methodological literature addressing conflicts study is helpful with respect to observational techniques. The outcome of this step should be insight- ful with respect to possible sight distance– induced vehicle behaviors. References: Parker, M. R., and Zegeer, C. V. (1989). Traffic Conflict Techniques for Safety and Operations (FHWA-IP-88-026 (Engineer’s Guide) and FHWA-IP-88-027 (Observer’s Guide)). Washington, DC: FHWA. Taylor, J. I., and Thompson, H. T. (1977). Identification of Hazardous Locations: A Users Manual (FHWA-RD-77-82). Washington, DC: FHWA. Procedure Product/Application Step 1D: Inventory Existing Traffic Control Devices Document existing signs, signals, and pave- ment markings along with their respective distances from the hazard under study. Document the age of these signs, signals, and markings, as well. The letter heights and mounting heights of signs need to be re- corded. Document any visual obstructions. The resulting device inventory will be sub- sequently applied in this diagnostic analy- sis to evaluate the suitability of provided information, as well as visual distractions and information processing demands on motorists as they approach the hazard under study. Procedure Product/Application Step 1E: Measure Existing Geometric Sight Distances Existing geometric sight distance limitations along the approach to the hazard must be measured in accordance with AASHTO crite- ria. Specifically, sight distance observations should be made from an elevation above the pavement that equals the design driver eye height (i.e., 3.5 ft) to a point ahead that is 2.0 ft above the pavement. This step will yield the length of specific roadway subsections along the approach in which drivers must observe and process available information (e.g., roadway features and other vehicles). References: AASHTO (2011). A Policy on Geometric Design of Highways and Streets. Washington, DC. Procedure Product/Application

22-13 HFG TUTORIALS Version 2.0 Step 1F: Note Factors Affecting Flow Speeds Certain roadway environmental features are known to affect drivers’ selection of speed. Examples are pavement defects, narrow shoulder widths and protruding bridge piers, abutments, guardrails, median barriers, etc. Documentation and general awareness of these factors are important because subse- quent minor highway improvement proj- ects may result in higher highway speeds, thus producing increased sight distance requirements. Procedure Product/Application Step 1G: Note Visual Distractions at Hazard Location Certain environmental conditions are known to produce “visual clutter” (i.e., distractions that make hazards more difficult for drivers to perceive). Examples include (1) off-roadway lighting, (2) commercial signing in driver field of view, (3) complex urban intersection designs, (4) high volumes of vehicular/ pedestrian movement (including bicycles), and (5) proliferation of intersec- tion traffic control devices. Observations should document drivers’ field of view at SSD from hazard (e.g., AASHTO (2011)). This inventory of visual distractions will be subsequently applied in a human factors analysis to determine the applicable sight distance criterion (e.g., DSD, to address driver perception and information- processing time requirements at the hazard location). References: AASHTO (2011). A Policy on Geometric Design of Highways and Streets. Washington, DC. Procedure Product/Application Step 1H: Note Visual Distractions Along Approach Roadway As in Step 1G, visual environmental condi- tions along the approach to the hazard also may produce driver distractions. These need to be included in the field data collec- tion process. Observations should document drivers’ field of view at DSD from hazard (e.g., AASHTO (2011)). This inventory of visual distractions will be subsequently applied in a human factors analysis to determine the applicable sight distance criterion to address driver infor- mation-processing time requirements on the approach to the hazard location. References: AASHTO (2011). A Policy on Geometric Design of Highways and Streets. Washington, DC. Procedure Product/Application

HFG TUTORIALS Version 2.0 22-14 Step 1I: Label the Diagram with Specified Symbols SDHAZ—Sight distance to a potential hazard. The point at which a location or object is first detectable to an approaching motorist. A—Point of required action. The location where an intended maneuver (e.g., hazard avoidance) is to be completed. SDTCD—Sight distance to a traffic control device. The point at which the device is first detectable to an approaching motorist. TCD—Traffic control device. The location of the device that warns of the hazard, measured as a distance from the location or object about which information is provided. The inclusion of uniform symbols on the site diagram will facilitate the subsequent sight distance analysis (see Figure 22-4). Procedure Product/Application Approach Distance to Hazard (ft) Estimated Operational Speed (mi/h) Visually Cluttered Environment Visually Non-Cluttered Environment Additional, when TCDs Present 25 360 180 95 30 440 220 110 40 580 290 150 50 730 370 185 60 880 440 220 70 1030 520 260 Table 22-1. Recommended approach distance to hazard for collection of field data. ASDHAZSDTCD TCD (SDTCD) (TCD) (SDHAZ)) Figure 22-4. Example symbol diagram: A two-lane 55-mi/h roadway approaches a 35-mi/h curve.

22-15 HFG TUTORIALS Version 2.0 Step 2: Conduct Preliminary Engineering Analyses This step involves the application of traditional traffic engineering techniques (e.g., AASHTO Design Policy geometric design criteria and DSD warrant) as a preliminary determinant of site deficiencies. In addition, the placement of traffic control devices needs to be examined in terms of MUTCD requirements. Step 2A: Examine Hazard Location with Respect to AASHTO Design Criteria To ensure a valid engineering diagnosis of sight distance to a hazard, it is necessary to first assess whether the hazard location itself has any inherent design shortcomings. One geometric deficiency potentially associated with a hazard location might be roadside that fails to meet requirements of the AASHTO Roadside Design Guide. Other examples are (1) a high-crash intersection may be deficient with respect to existing corner sight distance (AASHTO, 2011) and (2) in the case of a high incidence of run- off-road crashes, observed operational speeds (from Step 1A above) may differ significantly from the design speed upon which the curve radius and super elevation of the curve under consideration were based (AASHTO, 2011). The resulting analytical steps ensure that the hazard location itself is free of any inherent design shortcomings that have the potential for confounding the intended sight distance diagnosis. References: AASHTO (2002). Roadside Design Guide. Washington, DC. AASHTO (2011). A Policy on Geometric Design of Highways and Streets. Washington, DC. Procedure Product/Application Step 2B: Examine Approach with Respect to AASHTO Design Criteria As with the procedure noted in Step 2A, to ensure the integrity of the overall sight dis- tance diagnosis, it is necessary to assess whether the approach to the hazard loca- tion has any inherent design shortcomings. (For example, a substandard lateral clear- ance to a roadside object along the approach may create a visual obstruction, thus producing an unintended sight distance limitation.) Likewise, crest vertical sight dis- tances along the approach should be consis- tent with observed operational speeds gathered during Step 1B. The resulting analytical steps ensure that the approach to the hazard is free of any inher- ent design shortcomings that have the potential for confounding the intended sight distance diagnosis. Procedure Product/Application

HFG TUTORIALS Version 2.0 22-16 Step 2D: Examine Approach with respect to DSD Warrants The approach to the hazard location also must be examined for conditions of visual clutter meeting requirements for DSD application. In particular, these conditions could take the form of roadside distrac- tions and/or complex TCDs at intersections along the approach. Visual clutter along an approach to a hazard detracts from drivers’ perception of the haz- ard. When DSD-warranting conditions are found to exist along an approach to a hazard, the distraction is sufficient such that available sight distance to the hazard must be restricted to that distance beyond the distraction. Procedure Product/Application Step 2E: Examine Traffic Control Devices with Respect to MUTCD Criteria The MUTCD (FHWA, 2009) prescribes device placement criteria for signs, signals, and markings. Devices at both the hazard location and along the approach need to be examined for MUTCD compliance. Note that the MUTCD establishes manda- tory, recommended, and optional require- ments for the application of TCDs. The examination conducted in this step (as well as Steps 4 and 5) should reflect these MUTCD criteria. The output of this step will reveal whether in- adequate traffic control device application (e.g., insufficient warning distance or inap- propriate warning message) constitutes possi- ble sources of driver confusion. Inappropriate or inadequate TCD information can result in longer information processing times, thereby creating an artificial sight distance problem. References: FHWA (2009). Manual on Uniform Traffic Control Devices (MUTCD). Washington, DC. Procedure Product/Application Step 2C: Examine Hazard Location with Respect to Possible DSD Warrants AASHTO (2011) (e.g., section on DSD) notes a distinction between typical stopping sight distances and those in which drivers are required to make complex decisions (i.e., in which drivers require PRT beyond the design value [which is typically 2.5 s]). The DSD criterion applies to a difficult-to- perceive information source in a roadway environment that may be visually cluttered. Therefore, the hazard location needs to be examined for conditions of “visual noise” from competing sources of information (e.g., roadway elements, traffic, TCDs, and advertising signs). Specific sources of visual clutter were also noted in Step 1E. When DSD-warranting conditions are found to exist, apply the sight distance requirements noted in AASHTO (2011), rather than con- ventional stopping distances based on a 2.5-s PRT. References: AASHTO (2011). A Policy on Geometric Design of Highways and Streets. Washington, DC. Procedure Product/Application

22-17 HFG TUTORIALS Version 2.0 Step 3: Apply Crash Data This step involves the integration of traffic crash data into the analysis. The objective is to locate specific crash-prone locations within the roadway segment, which may be indicative of sight distance problems. The practitioner is cautioned that the absence of crashes does not rule out the existence of a sight distance problem, as crashes are probabilistic events and reporting requirements are variable. Step 3A: Establish Typologies and Frequency by Spot Locations A review of crash data will reveal the occur- rence of various types in close vicinity to the hazard under study. The associated pre- collision paths and their proximity to high- way features may suggest the existence of a sight distance problem. Certain crash types are typically associated with specific sight distance problems: • Run-off-road, fixed-object crashes (SSD) • Side-swipe, rear-end crashes (PSD) • Right-angle, rear-end crashes (ISD) A collision diagram is used to summarize crash types by location. For examples, see Robertson et al. (2000) and Hostetter and Lunenfeld (1982). References: Robertson, H. D., Hummer, J. E., and Nel- son, D. C. (Eds.) (2000). Manual of Trans- portation Engineering Studies. Washington, DC: ITE. Hostetter, R. S., and Lunenfeld, H. (1982). Planning and Field Data Collection (FHWA- TO-80-2). Washington, DC: FHWA. Procedure Product/Application Step 3B: Assess Suitability of Crash Sample While well-documented procedures exist to statistically establish crash causation (see Council et al. (1980)), this level of sophistica- tion is not necessary for the diagnosis of a sight distance problem. It is desirable (to the extent possible based on available crash data) to establish causation inferences based on crash patterns and to rule out non-sight-distance causal effects. A reasonable level of confidence (albeit logic- based rather than statistically rigorous) regarding crash causation is possible based on the following: • Inferences based on crash patterns rather than a single event • Occurrences whereby non-sight-distance factors can be logically ruled out. References: Council, F. M., Reinfurt, D. W., Campbell, B. J., Roediger, F. L., Carroll, C. L., Dutt, A. K., and Dunham, J. R. (1980). Accident Research Manual (FHWA-RD-80-016). Washington, DC: FHWA. Procedure Product/Application

HFG TUTORIALS Version 2.0 22-18 Step 3C: Examine Potential Sight Distance Causation Effect Certain patterns of crash behaviors (i.e., pre-collision maneuvers) are sugges- tive of sight distance problems: for exam- ple, single-vehicle or run-off-road crashes with a fixed object that may appear visible under some conditions but may not be easily detectable to drivers during condi- tions of more limited visibility (e.g., dark- ness). These patterns need to be examined to determine whether sight distance is a potential causal factor (i.e., adequate night- time sight distance conveyed by TCDs). A collision diagram can be descriptive of the location and nature of a sight distance haz- ard, thus supporting a hypothesis regarding the effect of a sight distance problem. Procedure Product/Application

22-19 HFG TUTORIALS Version 2.0 Step 4A: Establish and Plot Action Points Along Approach Segment Specific locations within the study road- way section requiring a driver action (e.g., maneuver) will be identified and plotted. For example, the hazard under study is the key point where action (e.g., driving at the posted speed) is likely required. Where a maneuver (e.g., deceler- ating) is necessary prior to reaching the hazard, the “compliance point” is the point where the maneuver is initiated (e.g., start of the deceleration distance). In the event that the approach roadway section requires some intermediate action (e.g., merging from a dropped traffic lane), this action also needs to be identified and plotted. Action points on the site diagram prepared in Step 1 should be indicated on the dia- gram by the symbol A. A series of sequen- tial action points may be designated as A1, A2, etc. The developed site diagram will indicate specific points where vehicle actions are required. Examples are as follows: • Approach maneuver (such as slowing) as required by the hazard under study • Any intermediate actions (e.g., required lane change) on the approach to the haz- ard under study Procedure Product/Application Step 4: Establish Roadway Segments The practitioner specifies component roadway approach segments in a manner to support the detailed human factors analysis in Step 5. Separate approach roadway segments are theoretically required for driver PRT and hazard avoidance maneuver functions. The product of this section is a series of driver task diagrams that depict the point where driver actions are required to avoid a potential hazard, information sources that warn of the hazard, and drivers’ available sight dis- tances to perform the necessary information-processing and maneuver tasks.

HFG TUTORIALS Version 2.0 22-20 Step 4B: Establish and Plot Information Sources and Associated Sight Distances Along Approach Segment Any driver action (e.g., hazard avoidance) must be based on information available to the driver. In this step, drivers’ information sources that inform an intended action must be located and documented. Informa- tion to the driver should be available from (1) detection of the hazard and/or (2) traffic control devices pertaining to the hazard. The following information/detection sources were noted on the site diagram in Step 1I: • Initial point of sight distance to the hazard identified by the symbol SDHAZ • Location of TCD providing information regarding the hazard identified by the symbol TCD • Initial point of sight distance to the appli- cable TCD identified by the symbol SDTCD In this step, separate plots of component information-processing segments may be helpful. The developed site diagram will indicate specific points where information pertain- ing to the hazard is available to the driver. Examples are as follows: • Point of initial detection opportunity on an approaching of both the hazard and any traffic control device warning of the hazard. • Specific locations of any TCDs advising of the hazard. NOTE: In the event that the hazard under study is not detectable (i.e., defined in the visual field), the symbol SDHAZ would not appear on the diagram. In such instances the required sight distance to action point (A) will be determined in Step 5. Procedure Product/Application

22-21 HFG TUTORIALS Version 2.0 Step 4C: Define Component Driver Response Sections Within Approach Segment Distinctly different driver information- processing tasks are associated with each detection and maneuver activity. In this step, roadway sections will be designated and plotted to illustrate the required travel distances over which the driver would per- form these varied information-processing and maneuver tasks. Depending upon physical characteristics of the roadway section under study, four dis- tinct driver response cases are possible: Case 1: Direct line of sight to hazard SDHAZ ➞ A Case 2: Intervening traffic control device (i.e., warning of hazard) SDTCD ➞ LDTCD ➞ TCD ➞ A Case 3: Intervening (e.g., distracting) hazard (A2) within sight line of first hazard (A1) SDHAZ1 ➞ SDHAZ2 ➞ A2 ➞ A1 Case 4: Intervening traffic control device and dis- tracting hazard SDTCD ➞ LDTCD ➞ TCD ➞ SDHAZ2 ➞ A2 ➞ A1 The product of this step is a diagrammed set of roadway component sections, each corre- sponding to specific information-processing and maneuver driver tasks. The distance over which a driver can react to a detectable hazard is the roadway sec- tion SDHAZ ➞ A. In this roadway section, the driver would detect the hazard and per- form any required preparatory maneuver (e.g., decelerating). Likewise, the distance over which a driver reacts to an advance traffic control device is the roadway section SDTCD ➞ TCD. In this roadway section, the driver has the opportunity to detect the sign and compre- hend the sign’s message. The message becomes readable at the point LDTCD (i.e., the legibility distance from the sign), which will be computed and located during Step 5. In the final approach section to the hazard, TCD ➞ A, the driver would complete the decision-making and maneuver tasks. Procedure Product/Application

HFG TUTORIALS Version 2.0 22-22 Step 5A: Determine the Relevant Geometric Design Sight Distance Application The analysis of driving task requirements involves application of the appropriate sight distance value for the given task. Sight distance requirements (to accommo- date both the information-processing and maneuver tasks) approaching action points (A) will fall into one of the following cate- gories (depending upon roadway environ- ment condition), which were identified in Section 5.2: • Stopping sight distance (SSD) • Intersection sight distance (ISD) • Decision sight distance (DSD) • Passing sight distance (PSD) The result of this task is the specification of the applicable procedure (e.g., engineering design formula) for the computation of SDHAZ corresponding to each identified haz- ard or action point. The required sight dis- tance based on application of the appropriate design formula is applied to determine the required length of the roadway segment under study. Procedure Product/Application Step 5: Analyze Component Driving Task Requirements In this step, the practitioner applies human factors principles (comprising information- processing and decision-making criteria) to ensure the adequacy (or to quantify the shortcoming) of the approach roadway to allow for time/distance hazard avoidance requirements.

Step 5B: Determine Driving Task Requirements Within Each Component Roadway Segment Case 1: Direct line of sight to hazard; no traffic control SDHAZ ➞ A In this case, PRT and MT are determined from Section 5.2. Case 2: Intervening traffic control device (i.e., warning of hazard) SDTCD ➞ LDTCD ➞ TCD ➞ A 1. Driver must detect traffic control device: SDTCD ➞ LDTCD 2. Driver must read or otherwise compre- hend message and may begin decision process: LDTCD ➞ TCD (Legibility distance will be determined in Step 5C.) 3. Decision and maneuver must be completed: TCD ➞ A Case 3: Intervening, distracting hazard at A2 within sight line of first hazard at A1 SDHAZ1 ➞ SDHAZ2 ➞ A2 ➞ A1 1. Driver requires longer PRT due to com- plex visual scene ahead: SDHAZ1 ➞ SDHAZ2 ➞ A Consider DSD application. 2. Driver may require longer MT due to complexity of maneuver and visual scene: SDHAZ2 ➞ A2 ➞ A Case 4: Intervening traffic control device and dis- tracting hazard at A2 within sight line of first hazard A1 SDTCD ➞ LDTCD ➞ TCD ➞ SDHAZ2 ➞ A2 ➞ A1 1. Driver must detect traffic control device: SDTCD ➞ LDTCD 2. Driver must read or otherwise compre- hend message and may begin decision process: LDTCD ➞ TCD 3. Driver may require longer MT due to complexity of maneuver and visual scene: SDHAZ2 ➞ A2 ➞ A Procedure Driver information-processing demands vary as a function of environmental factors, according to the four cases indicated below. Identify separate PRT and MT components of the driving task for each of the four cases. Specific values of PRT and MT will be deter- mined subsequently. 22-23 HFG TUTORIALS Version 2.0

HFG TUTORIALS Version 2.0 22-24 Step 5C: Quantify the Applicable PRT and MT Requirements for Each Driving Task Component No TCDs present: SDHAZ ➞ A Apply applicable PRT and MT requirement corresponding to predetermined condition (i.e., SSD, ISD, DSD, or PSD as determined in Step 5A). TCDs present: SDTCD ➞ LDTCD Drivers should be able to detect a TCD prior to time required to comprehend its message; 2.5 s is desirable, although less time may be adequate (e.g., second, third, etc. in a sequence). LDTCD ➞ TCD LDTCD is the “legibility distance” or approach distance at which a traffic control device mes- sage is comprehended. A detailed discussion in the following paragraphs addresses the LDTCD for signs. In the case of pavement markings, LDTCD is the advance distance at which the marking is visually recognized. The LDTCD for a sign is the distance at which its legend is read or its symbol message is comprehended. PRT requirements for signs consist of reading times for the message leg- end and symbol as follows (Smiley, 2000): Reading Time = 1*(number of symbols) + 0.5*(number of words and numbers) [s] The minimum reading time is 1 s. For mes- sages exceeding four words, the sign requires multiple glances; the driver must look back to the road and at the sign again. Therefore, for every additional four words and numbers, or every two symbols, an additional 0.75 s should be added to the reading time. TCDs present (continued): This segment must be sufficient in length to accommodate the reading time noted above. However, its length is constrained by letter height (i.e., limited to 40 ft for every inch of letter height). For example, a 4-in. letter-height sign must be read within a distance of 4 × 40 = 160 ft. On a 40 mi/h (58.8 ft/s) roadway, the driver is limited to a maximum of 160/58.8 or 2.7 s to read the sign. Moreover, the traffic engineer must consider that the driver can not be expected to fixate on the sign. Considering the driver’s alerted state after reading the sign, decision time (i.e., time to make a choice and initiate a maneuver if required) can range from 1 s for common- place maneuvers (e.g., stop, reduce speed) to 2.5 s or more when confronted with a com- plex highway geometric situation. LDTCD ➞ TCD ➞ A While the required MT may be initiated prior to passing the TCD, it must be com- pleted in the above-noted segment. MT val- ues associated with designed sight distance considerations are treated in Chapter 5. Additional literature sources of extensive maneuver time data are available (Lerner, Steinberg, Huey, and Hanscom, 1999). References: Lerner, N. D., Steinberg, G. V., Huey, R. W., and Hanscom, F. R. (1999). Under- standing Driver Maneuver Errors, Final Report (Contract DTFH61-96-C-00015). Washington, DC: FHWA. Smiley, A. (2000). Sign design principles. Ontario Traffic Manual. Ottawa, Canada: Ontario Ministry of Transportation. Procedure The general model to be applied for quantifying driver task requirements (i.e., required PRT and MT) is SDTCD ➞ LDTCD ➞ TCD ➞ A. Driver task requirements are determined for each task as follows.

22-25 HFG TUTORIALS Version 2.0 Step 5D: Assess the Adequacy of the Available Sight Distance Components Case 1: Direct line of sight to hazard; no traffic con- trol SDHAZ ➞ A Does the subsection length SDHAZ ➞ A allow sufficient time for the driver to perform any required hazard avoid- ance maneuver? Case 2: Intervening traffic control device (i.e., warning of hazard) SDTCD ➞ LDTCD ➞ TCD ➞ A Does the subsection length, SDTCD ➞ LDTCD allow sufficient time (minimum 1.5 s) for the driver to detect the traffic control device? Does the subsection length, SDTCD ➞ TCD allow sufficient time for the driver to detect and read the traffic control device? Does the subsection length, TCD ➞ A allow sufficient time for the driver to per- form any required hazard avoidance maneuver? Case 3: Intervening, distracting hazard at A2 within sight line of first hazard at A1. SDHAZ1 ➞ SDHAZ2 ➞ A2 ➞ A1 Does then subsection length SDHAZ1 ➞ A1 allow sufficient time for the driver to process and respond to the intervening distraction (i.e., apply DSD criteria) and perform any required hazard avoidance maneuver? Case 4: Intervening traffic control device and dis- tracting hazard A2 within sight line of first hazard A1. SDTCD ➞ LDTCD ➞ TCD ➞ SDHAZ2 ➞ A2 ➞ A1 Does the subsection length, SDTCD ➞ LDTCD allow sufficient time (2.5 s desir- able; minimum 1.0 to 1.5 s) for the driver to detect the traffic control device? Does the subsection length, SDTCD ➞ TCD allow sufficient time for the driver to detect and read the traffic control device? Does the subsection length, TCD ➞ A1 allow sufficient time for the driver to process and respond to the intervening distraction (i.e., apply DSD criteria) and perform any required hazard avoidance maneuver? Procedure

Step 6: Develop Engineering Strategies for Improvement of Sight Distance Deficiencies In this final step, the practitioner recommends improvement (e.g., traffic control device appli- cations or minor design modifications) to correct deficiencies. HFG TUTORIALS Version 2.0 22-26 Step 6A: Apply Traffic Engineering and Highway Design Principles to Component Sight Distance Deficiencies Case 1: Direct line of sight to hazard; no traffic control SDHAZ ➞ A Available sight distance to hazard, SDHAZ, is less than required based on Step 5B results. Case 2: Intervening traffic control device (i.e., warning of hazard) SDTCD ➞ LDTCD ➞ TCD ➞ A Total available sight distance less than the required sight distance from Step 5C. Case 3: SDHAZ1 ➞ SDHAZ2 ➞ A2 ➞ A1 Available sight distance to hazard, SDHAZ, is less than required based on Step 5B results. Case 4: SDTCD ➞ LDTCD ➞ TCD ➞ SDHAZ2 ➞ A2 ➞ A1 Total available sight distance less than the required sight distance from Step 5C. Add warning traffic control device, increas- ing warning distance as shown in Case 2 below. If LDTCD ➞ TCD is inadequate (i.e., infor- mation overload): • Apply “information spreading” by adding more devices, each with less information • Increase legibility distance (e.g., by increasing letter size) If LDTCD ➞ TCD ➞ A is inadequate: • Increase warning distance, SDTCD ➞ LDTCD, via improving the TCD’s legibility distance • Apply larger device, increase letter size • In DSD condition, add conspicuity device (e.g., flashing beacon) or consider ITS application. If SDTCD ➞ LDTCD ➞ TCD is inadequate: • Reduce information load on existing TCDs • Apply additional TCDs (e.g., delineation devices, advance supplemental devices) to convey essential information. Add warning traffic control device, achiev- ing increased warning distance. Apply combination of Case 2 solutions noted above. Procedure Product/Application

22-27 HFG TUTORIALS Version 2.0 Example Application: Sight Distance Diagnostic Procedure The example driving situation consists of a 55-mi/h, two-lane rural roadway that approaches a 35-mi/h curve followed by a stop-controlled intersection. The intersection approach is to a main highway, which requires application of destination guide signing. Driver requirements in this situation are as follows: 1. Reduce speed from 55 to 35 mi/h to negotiate curve 2. Process traffic control device information related to intersection (e.g., destination name sign) 3. Stop for intersection Step 1: Collect Field Data and Prepare Site Diagram The labeled site diagram is shown in Figure 22-5. Step 2: Conduct Preliminary Engineering Analyses This example requires a sight distance analysis to two separate potential hazards. The first is a 35-mi/h curve that requires slowing from 55 mi/h; and the second is an intersection that is heav- ily signed with a stop sign and two guide signs, containing multiple route shields, symbols, and destination names. The approach roadways to each hazard point are separately treated as fol- lows: (1) curve approach and (2) signed intersection approach. Curve Approach Segment Steps 2A through 2D: Examine Site with Respect to AASHTO Design and DSD Criteria. For the purpose of this example, it is assumed that geometrics conform to AASHTO and that DSD criteria (e.g., visually cluttered environmental conditions) do not apply. Step 2E: Examine Traffic Control Devices for Compliance with the MUTCD. The MUTCD specifies requirements for warning signs. The curve warning sign in the example is a “W1-2, Hor- izontal Alignment Sign” with a 35-mi/h advisory speed plate. Section 2C-05 of the MUTCD spec- ifies an advance placement guideline for warning signs. Given the requirement to slow from 55 to 35 mi/h, the minimum recommended distance in Table 2C-4 (located on page 2C-5) is 138 ft (FHWA, 2003). Figure 22-5. Example site diagram.

Signed Intersection Approach Segment Steps 2A through 2D: Examine Site with Respect to AASHTO Design and DSD Criteria. For the purpose of this example, it is assumed that geometrics conform to AASHTO and that DSD criteria (e.g., visually cluttered environmental conditions) do not apply. Step 2E: Examine Traffic Control Devices for Compliance with the MUTCD. This segment is a stop-controlled intersection approach containing signs to multiple routes and destinations. The MUTCD provides requirements for guide signs on conventional roads. Signs in the example consist of a “directional assembly” with destination name signs and route shields. Required advance distances and spacing of these signs is given in Figure 2D-2 (FHWA, 2009). Typically, when a series of guide signs is placed sequentially along the approach to an inter- section there is a 100- to 200-ft separation between the first two signs. The minimum spacing between signs is 100 ft, which is intended to enable drivers to read the entire message on both signs. Section 2D.06 requires 6-in. letter heights for a 35-mi/h roadway (FHWA, 2009). Specifications for stop sign size and placement are contained in Chapter 2A of the MUTCD. As shown in Figure 2A-2, the stop sign should be set back a minimum of 12 ft from the intersection. The recommended letter height is 8 in. (FHWA, 2009). Step 3: Apply Crash Data Not conducted as part of this example. Step 4: Establish Roadway Segments This example requires a sight distance analysis to two separate potential hazards. The first is slowing from 55 mi/h to 35 mi/h, the posted curve advisory speed; and the second is a stop- controlled approach to an intersection containing signs to multiple routes and destinations. As above, the approach roadways are discussed separately. Curve Approach Segment. The roadway segment requiring the driver to slow from 55 mi/h to 35-mi/h is labeled in accordance with Steps 4A and 4B and is shown below. The two sight dis- tance driver response scenarios follow: • Case 1, direct line of sight to hazard (i.e., 55-mi/h speed zone to 35-mi/h curve): SDHAZ ➞ A • Case 2, intervening traffic control device (i.e., 35-mi/h advisory speed sign warning of hazard): SDTCD ➞ LDTCD ➞ TCD ➞ A This roadway is diagrammed in Figure 22-6. Signed Intersection Approach Segment. On this roadway section, motorists traveling at 35-mi/h are confronted with a stop-controlled intersection and two guide signs containing destination names and route shields. Because sight distance to the intersection is limited by a curve on the approach, a sight distance analysis is critical. The component section diagram is labeled in accordance with Steps 4A and 4B and shown below. The sight distance driver response scenarios follow: HFG TUTORIALS Version 2.0 22-28 Figure 22-6. Curve approach segment diagram.

• Case 1, direct line of sight to hazard (i.e., 35-mi/h speed zone to intersection): SDHAZ ➞ A • Case 2: Three intervening traffic control devices – A route shield assembly: SDTCD1 ➞ LDTCD1 ➞ TCD1 ➞ A – A destination name sign: SDTCD2 ➞ LDTCD2 ➞ TCD2 ➞ A – A stop sign: SDTCD3 ➞ LDTCD3 ➞ TCD3 ➞ A This roadway segment is diagrammed in Figure 22-7. Step 5: Analyze Component Driving Task Requirements Curve Approach Segment. The roadway section, requiring the driver to slow from a 55-mi/h speed zone to a 35-mi/h curve, considers sight distance to the curve and legibility distance requirements posed by the advisory speed sign. Step 5A: Determine the Relevant Design Sight Distance Application. The applicable design sight distance is slowing sight distance—the required distance for a driver to observe the curve ahead and adjust speed accordingly. In the event that certain visual noise conditions or other fac- tors are present that would render the curve difficult to perceive, then the practitioner must con- sider applicable DSD criteria (discussed in Chapter 5). Where a traffic control device is present, driver information-processing time is required to observe and comprehend the sign as well as slow to a safe curve negotiation speed. In the current example (i.e., a rural uncluttered environ- ment), DSD criteria are not applied. Step 5B: Determine the Driving Task Requirements. Considering the two possibilities (i.e., Case 1 in which the driver observes the curve ahead without seeing the sign, and in Case 2 whereby the driver observes and comprehends the sign), the requirements for each are as follows: • Case 1, direct line of sight to hazard (i.e., 55-mi/h speed zone to 35-mi/h curve): SDHAZ ➞ A The sight distance requirement in this case is simply that the driver observes the curve ahead and slows to a safe speed. • Case 2, intervening traffic control device (i.e., 35-mi/h advisory speed sign warning of hazard): SDTCD ➞ LDTCD ➞ TCD ➞ A The sight distance requirement in this case is that the driver observes the sign, comprehends the sign message, and slows to a safe speed. 22-29 HFG TUTORIALS Version 2.0 Figure 22-7. Intersection approach segment diagram.

Step 5C: Quantify the Applicable PRT and MT Requirements for Each Driving Task • Case 1, direct line of sight to hazard (i.e., 55-mi/h speed zone to 35-mi/h curve): SDHAZ ➞ A Because DSD does not apply (determined previously), the design PRT value of 2.5 s is applied; thus the PRT component of sight distance is 202 ft (i.e., 2.5 s times 80.85 ft/s). The MT require- ment (4.0 s) is derived from the need to slow from 55 mi/h to 35 mi/h at a comfortable decel- eration level (i.e., .23g), which requires 261 ft. Thus the total PRT and MT sight distance requirement is 463 ft. The comfortable deceleration level is derived from Table 2-25 of AASHTO (2011). (For safety purposes, wet weather deceleration is considered.) However, AASHTO (2011) acknowledges that its deceleration data may be outdated and that more rapid (albeit uncomfortable) deceler- ations are common. A typical such deceleration is .35g (Knipling et al., 1993), resulting in an MT of 2.6 s. It is also known that most reasonably alert drivers are able to initiate braking within a PRT of 1.6 s (Chapter 5). Applying these performance parameters to slowing from 55 to 35 mi/h, the total required PRT distance is 129 ft plus 172 ft MT distance, or 301 ft. It is unlikely that the need to slow to 35 mi/h would be visually evident from an advance dis- tance of either 301 or 463 ft. Therefore, the critical sight distance consideration is based on the application of the speed advisory sign. • Case 2, intervening traffic control device (i.e., 35-mi/h advisory speed sign warning of hazard): SDTCD ➞ LDTCD ➞ TCD ➞ A In this case the driver needs to detect the sign, read the sign, and decelerate to the safe curve speed. A critical requirement for sight in advance of a sign (i.e., allowing time to comprehend the sign’s message) is known as legibility distance. There is a considerable body of knowledge regarding sign legibility distance requirements (Smiley, 2000). For simple warning signs, the MUTCD specifies an advance placement guideline, which includes “an appropriate legibility distance” of 175 ft for word legend signs or 100 ft for symbol signs. The MUTCD sign placement requirement to allow for slowing from 55 to 35 mi/h is 350 ft. Driver requirements imposed by the MUTCD rule in this case are as follows: Given that 2.0 s are needed to detect and comprehend (e.g., minimum 1.0 s for detection plus 1.0 s for symbol comprehension) the simple warning sign message prior to the initiation of slowing, the deceleration requirement would be .32 g or approximately the equivalent slowing rate of skidding on wet pavement. In this example the required PRT and MT distances would be 161 and 189 ft respectively, for a total of 350 ft. For signs with complex messages (i.e., sets of destination names or symbols in combination with symbols), message comprehension may require significantly more legibility distance. The next example illustrates such a situation. Signed Intersection Approach Segment. On this roadway section, motorists traveling at 35 mi/h are confronted with a stop-controlled intersection and two guide signs containing des- tination names and route shields. Because sight distance to the intersection is limited by a curve on the approach, a sight distance analysis is critical. Step 5A: Determine the Relevant Design Sight Distance Application. As the driver approaches a stop-controlled intersection, there must be sufficient available stopping sight dis- tance (Chapter 5) to enable stopping at the stop line. (While negotiation of the intersection involves the application of intersection sight distance, the current example is limited to approaching the intersection.) HFG TUTORIALS Version 2.0 22-30

Step 5B: Determine the Driving Task Requirements. Considering the two possibilities (i.e., Case 1 in which the driver proceeds to the intersection ahead while ignoring the signs, and Case 2 whereby the driver observes and comprehends the intermediate signs), the requirements are as follows: • Case 1, direct line of sight to hazard (i.e., 35-mi/h speed zone to intersection): SDHAZ ➞ A The sight distance requirement (to accommodate travel time) in this case is simply that the driver observes the intersection ahead and safely slows to a stop. • Case 2, three intervening traffic control devices, i.e.: – A route shield assembly: SDTCD1 ➞ LDTCD1 ➞ TCD1 ➞ A – A destination name sign: SDTCD2 ➞ LDTCD2 ➞ TCD2 ➞ A – A stop sign: SDTCD3 ➞ LDTCD3 ➞ TCD3 ➞ A TCD1 is a route shield assembly bearing two route designations; TCD2 is a destination guide sign with two destination names and directional arrows; and TCD3 is a stop sign. The sight distance requirement in this case is that the driver detects and comprehends the signs and slows to a safe stop at the stop line. Step 5C: Quantify the Applicable PRT and MT Requirements for Each Driving Task • Case 1, direct line of sight to hazard (i.e., speed reduction from 35 mi/h to stop at the stop line): SDHAZ ➞ A The design stopping sight distance does not accommodate information-processing requirements of the intervening guide signs. The AASHTO design SSD value (AASHTO, 2004) for a 35-mi/h approach is the range of 225 to 250 ft, which accounts for both the PRT and MT tasks. However, this 225- to 250-ft sight distance would barely accommodate the physical placement of the two guide sign assemblies that are shown in the Figure 22-7. Moreover, the information- processing load imposed by the signs requires significant attention in terms of sight distance requirements. Therefore the Case 2 condition is treated below. • Case 2, intervening traffic control device (i.e., guide signs): SDTCD ➞ LDTCD ➞ TCD ➞ A The general model (above) entails the following considerations. First, there must be sufficient sight distance so that the sign is detected prior to the time required to comprehend the sign’s message, thus application of the SDTCD term. This advance distance is not specified in the MUTCD. Nevertheless, 2.5 s is desirable for this sign detection task, although less time may be adequate as motorists who are looking for signs are generally aware of the expected posi- tion in their field of view. The more essential approach sight distance to a traffic control device is that required to comprehend its message. LDTCD refers to legibility distance—the approach distance at which a TCD legend is read or its symbol message is comprehended. The legibility distance of a legend sign is determined by mul- tiplying a legibility index (i.e., the distance at which a given unit of letter height is readable) by the letter height. The applicable legibility index values are shown in Table 22-2. For example, the legibility distance typically associated with 6-in. letter height is 240 ft (40 times 6). 22-31 HFG TUTORIALS Version 2.0

The legibility distance of symbol signs has been researched in a laboratory study (Dewar, Kline, Schieber & Swanson, 1994) and found to significantly exceed that of legend signs (despite the high degree of variability in the study data). For example, the mean legibility distance for the right curve arrow symbol was determined to be 283 m (with a standard deviation of 68 m). Considering that a 55-mi/h approach allowing a 2.5-s advance sight distance and 1.0-s reading time would consume only 86 m, pure symbol signs are not expected to result in an information- processing problem. The required PRT for this example roadway segment consists of three components: detecting the signs, comprehending the sign messages, and detecting the intersection. Each is separately discussed. Sign Detection. Upon a driver’s detection of the first sign, the second and third signs would require minimal detection time. The recommended detection time for the first sign is 2.5 s; however, the second two signs are likely to be detected much more rapidly. “Alerted” PRT responses are known to occur in as little as 1.0 to 1.5 s. Moreover, signs can be quickly detected as drivers know where to look for signs and typically scan toward expected sign locations. Therefore, a conservative sign detection PRT for the example roadway segment is (2.5 + 1.5 + 1.5) or 5.5 s. Sign Comprehension. Sign comprehension consists of reading the sign plus making the result- ant decision (e.g., right or left turn in response to the sign’s information). The PRT require- ment (Smiley, 2000) is based on sign-response reading and decision time, for which general rules are noted in Table 22-3. HFG TUTORIALS Version 2.0 22-32 Metric US Customary 4.8 meters/ centimeter of letter height 40 feet/ inch of letter height Table 22-2. Legibility index. Comprehension Task PRT Requirements Reading Time requirements for reading the sign are 0.5 s for each word or number, or 1 s per symbol, with 1 s as a minimum for total reading time. In the event of the sign’s containing redundant information, the reading time computation should be limited to critical words. The suggested formula for estimating sign reading time is: Reading time = 1(number of symbols) + 0.5(number of words and numbers). For messages exceeding four words, the sign requires multiple glances, which means the driver must look back to the road and at the sign again. Therefore, for every additional four words and numbers, or every two symbols, an additional 0.75 s should be added to the reading time. When the driver is sufficiently close to see a sign at an angle, the sign is not visible for the last 0.5 s. Therefore, 0.5 s should be added to the required reading time. An exception applies to signs requiring a maneuver before the sign is reached, as no further reading is required. Deciding Considering the driver’s alerted state having read the sign, decision time can range from 1 s for commonplace maneuvers (e.g., stop or reduce speed) to 2.5 s or more when confronted with a complex highway geometric situation. Table 22-3. General rules for sign comprehension PRT requirements.

The first guide sign assembly contains two numbers and two symbols, requiring 3.0 s of read- ing time; the second contains two designation names and two symbols, also requiring 3.0 s; and the third is a simple and familiar one-word regulatory sign, requiring 1 s. Thus the total sign reading time is 7.0 s. This estimate is highly conservative, as drivers would likely scan the guide signs seeking only a particular name or route number; however, it is necessary to provide suf- ficient information-processing sight as some drivers may need the entire set of information. An additional 3.0 s is considered for decision time responses to the three signs. Thus the total com- prehension time for the three signs is 10 s. Intersection Detection Distance. As noted above under the Case 1 (SDHAZ ➞ A) discussion, the stopping sight distance requirement considers a 2.5-s PRT. A summary of the above-noted PRT requirements, if separately considered, is shown in Table 22-4. The sum of PRT requirements would apply to a serial task process. However, a realistic assessment of PRT requirements considers that many of the tasks in Table 22-4 are concurrent. For example, stop sign comprehension would not logically entail a separate process of perceiving the intersection, thus conceivably reducing the total PRT by 2.5 s. In addition, following a driver’s 2.5-s detection of the initial sign, the subsequent two signs would likely be detected with a minimum detection time (e.g., 1.0 s rather than 1.5 s), thus conceivably reducing the total PRT by another 1.0 s. Therefore, subtracting 3.5 s from the serial total of 19.5 s, the estimated PRT requirement becomes 16.0 s. The MT requirement (i.e., to slow from 35 mi/h to a stop at the specified AASHTO g-force) calculates to 4.7 s over a distance of 120 ft. The extent to which the deceleration process would occur concurrently with the various sign-response tasks is uncertain. However, it is logical (and best serves liability concerns) to allow time for comprehension of all signs prior to the initiation of the slowing response. Therefore, the overall sight distance requirement is approximately 16.0 s of sign information processing at 35 mi/h (51.45 ft/s) or 823 ft, plus the 120-ft deceleration distance, for a total of 943 ft. (Actual requirements will reflect real-world conditions. If possible, data should be col- lected at the relevant sites.) A final consideration is the necessity that drivers have sufficient time to comprehend a sign’s message during the interval when the message is discernable. Therefore, an essential sight dis- tance diagnostic step is to compare the available sign legibility distance (i.e., available reading distance) with distance traveled during reading PRT (i.e., required reading distance and deci- sion time). Table 22-5 contrasts the distance traveled during PRT with the legibility distance. While the guide signs in this example accommodate both reading time and associated deci- sion time, the decision component of PRT can obviously be accomplished after the driver passes the sign. 22-33 HFG TUTORIALS Version 2.0 Driving Task PRT Requirement (s) Perceive initial guide sign 2.5 Perceive next three signs @ 1.5 s/sign 4.5 Comprehend initial guide sign 4.0 Comprehend second guide sign 4.0 Comprehend stop sign 2.0 Perceive intersection 2.5 Total 19.5 Table 22-4. Summary of PRT requirements.

Step 6: Develop Engineering Strategies for Improvement of Sight Distance Deficiencies Not conducted as part of this example. HFG TUTORIALS Version 2.0 22-34 Sign Legibility Distance (ft) PRT Distance (ft) 1. 6-in. letters: 2 Numbers + 2 Symbols 240 231 2. 6-in. letters: 2 Numbers + 2 Symbols 240 231 3. 8-in. letters: 1 Word 320 51 Table 22-5. Contrast of distance traveled during PRT with legibility distance.

22-35 HFG TUTORIALS Version 2.0 Tutorial 3: Detailed Task Analysis of Curve Driving A task analysis of the different activities that drivers must conduct while approaching and driving through a single curve (with no other traffic present) was conducted to provide qualita- tive information about the various perceptual, cognitive, and psychomotor elements of curve driving. Consistent with established procedures for conducting task analyses (Campbell and Spiker, 1992; Richard, Campbell, and Brown, 2006; McCormick, 1979; Schraagen, Chipman, and Shalin, 2000), the task analysis was developed using a top-down approach that successively decomposed driving activities into segments, tasks and subtasks. The approach used in this tutorial was specifically based on the one described in Richard, Campbell, and Brown (2006); readers interested in additional details about the methodology should consult that reference (available at http://www.tfhrc.gov/safety/pubs/06033/). The curve driving task was broken down into four primary segments, with each segment gen- erally representing a related set of driving actions (see Figure 22-8). The demarcation into seg- ments was primarily for convenience of analysis and presentation and does not imply that the curve driving task can be neatly carved up into discrete stages. Within each segment, the indi- vidual tasks that drivers should or must perform to safely navigate the curve were identified. Moreover, these driving tasks were further divided based on the information-processing ele- ments (perceptual, cognitive, and psychomotor requirements) necessary to adequately perform each task. The perceptual requirements typically refer to the visual information about the curve and the surrounding roadway that drivers need to judge the curvature, determine lane position and heading, etc. The cognitive requirements typically refer to the evaluations, decisions, and judgments that drivers have to make about the curve or the driving situation. The psychomo- tor requirements refer to the control actions (e.g., steering wheel movements, foot movements to press brake, etc.) that drivers must make to maintain vehicle control or to facilitate other information acquisition activities. The task analysis presented in Table 22-6 shows the driving tasks and corresponding information-processing subtasks associated with driving a typical horizontal curve, approach- ing from a long tangent. Drivers must also engage in other ongoing safety-related activities, such as scanning the environment for hazards; they may also engage in in-vehicle tasks such as adjust- ing the radio, using windshield wipers, or consulting a navigation system (just to name a few). However, these more generic tasks are not included in the task analysis in order to emphasize those tasks and subtasks that are directly related to curve driving. 2. Curve Discovery 3. Entry and Negotiation 4. Exit1. Approach Tangent Point Point of Curvature 75 -100 m (˜4 sec) Figure 22-8. The four primary segments of the curve driving task.

HFG TUTORIALS Version 2.0 22-36 Table 22-6. Driving tasks and information-processing subtasks associated with a typical curve. Driving Task Perceptual Requirements Cognitive Requirements Ps yc ho mo to r Re qu ir em en ts 1. Approach 1. 1 Lo ca te be nd In sp ec t fo rw ar d ro ad wa y sc ene fo r ev id en ce of be nd Re co gn iz e vi su al cu es in di ca ti ng depa rt ur e fr om st ra ig ht pa th Ey e mo ve me nt s n eeded fo r sca nni ng 1. 2 Ge t av a ila bl e sp eed in fo rm at io n fr om si gn ag e Vi su a lly sc an en vi ro nm ent fo r si gn ag e Re ad an d in te rp re t si gn in fo rm at io n He ad an d ey e mo ve me nt s n eeded fo r sca nni ng 1. 3 Ma ke in it ia l sp eed ad ju st me nt s L ook at sp eedom eter Re ad sp eedom eter in fo rm at io n an d co mp ar e to po st ed sp eed Ex ec ute ne ce ssa ry f oot mo ve me nt s to ac hi ev e de si re d sp eed ch an ge 2. Curve Discovery 2. 1 De te rm in e cu rv at ur e L ook at ro ad wa y an d en vi ro nm ent fe at ur es at cu rv e lo ca ti on Es ti ma te cu rv e an gl e ba se d on vi su al im ag e an d ex pe ri en ce He ad an d ey e mo ve me nt s n eeded fo r sca nni ng 2. 2 Asse ss ro ad wa y co nd it io ns (e .g ., lo w fr ic ti on , p oor vi si b ilit y) L ook at ro ad wa y in fr ont of ve hi cl e De te rm in e co nd it io ns re qu ir in g (a ddi ti on al ) sp eed re du ct io ns Ex ec ute ne ce ssa ry f oot mo ve me nt s to ac hi ev e de si re d sp eed ch an ge 2. 3 Ma ke a ddi ti on al sp eed ad ju st me nt s L ook at sp eedom eter an d/ or vi ew sp eed cu es fr om en vi ro nm ent Re ad sp eedom eter an d/ or ju dge sa fe sp eed ba se d on cu es an d ex pe ri en ce Ex ec ute ne ce ssa ry f oot mo ve me nt s to ac hi ev e de si re d sp eed ch an ge 2. 4 Ad ju st ve hi cl e pa th fo r cu rv e entr y L ook at ro ad wa y/ la ne ma rk in g in fo rm at io n in the i mmedi at e fo rw ar d vi ew De te rm in e the am ount of st eer in g wh eel di sp la ce me nt re qu ir ed to ac hi ev ed de si re d la ne po si ti on He ad an d ey e mo ve me nt s n eeded fo r vi ew in g, an d pr ec is e ar m mo ve me nt s fo r st eer in g co nt ro l 3. En tr y an d Ne go ti at io n 3. 1 Ad ju st sp eed ba se d on cu rv at ur e/ la te ra l a cce le ra ti on Pe rc ei ve la te ra l a cce le ra ti on an d l ook at ro ad wa y mo ti on cu es Ju dge sa fe sp eed ba se d on vi su al cu es an d ex pe ri en ce or re ad sp eedom eter Ex ec ute ne ce ssa ry f oot mo ve me nt s to ac hi ev e de si re d sp eed ch an ge 3. 2 Ma in ta in pr op er tr aj ec to ry L ook at ta ngent po in t or in tended di re ct io n De te rm in e am ount of st eer in g wh eel di sp la ce me nt re qu ir ed to ac hi ev ed de si re d he ad in g He ad an d ey e mo ve me nt s n eeded fo r sca nni ng , an d pr ec is e ar m mo ve me nt s fo r st eer in g co nt ro l 3. 3 Ma in ta in sa fe la ne po si ti on L ook at ro ad wa y/ la ne ma rk in g in fo rm at io n in the i mmedi at e fo rw ar d vi ew De te rm in e am ount of st eer in g wh eel di sp la ce me nt re qu ir ed to ac hi ev ed de si re d la ne po si ti on He ad an d ey e mo ve me nt s n eeded fo r vi ew in g, an d pr ec is e ar m mo ve me nt s fo r st eer in g co nt ro l 4. Ex it 4. 1 Acce le ra te to a ppr op ri at e sp eed L ook at sp eedom eter an d/ or vi ew sp eed cu es fr om en vi ro nm ent Re ad sp eedom eter an d/ or ju dge sa fe sp eed ba se d on cu es an d ex pe ri en ce Ex ec ute ne ce ssa ry f oot mo ve me nt s to ac hi ev e de si re d sp eed ch an ge 4. 2 Ad ju st la ne po si ti on L ook se ve ra l se co nd s ah ea d do wn the ro ad wa y De te rm in e am ount of st eer in g wh eel di sp la ce me nt re qu ir ed to ac hi ev ed de si re d he ad in g He ad an d ey e mo ve me nt s n eeded fo r sca nni ng , an d pr ec is e ar m mo ve me nt s fo r st eer in g co nt ro l The primary source of information for segment tasks was the comprehensive driving task analysis conducted by McKnight and Adams (1970); however, other research more specifically related to curve driving were also used: • Donges, E. (1978). Two-level model of driver steering behavior. Human Factors, 20(6), 691–707. • Fitzpatrick, K., Wooldridge, M. D., Tsimhoni, O., Collins, J. M., Green, P., Bauer, K. M., Parma, K. D., Koppa, R., Harwood, D. W., Anderson, I., Krammes, R. A., and Poggioli, B. (2000). Alternative Design Consistency Rating Methods for Two-Lane Rural Highways. Final Report. (FHWA-RD-99-172). McLean, VA: FHWA.

• Groeger, J. A. (2000). Understanding Driving: Applying Cognitive Psychology to a Complex Everyday Task. Hove, U.K.: Psychology Press. • Krammes, R. A., Brackett, R. Q., Shafer, M. A., Ottesen, J. L., Anderson, I. B., Fink, K. L., Collins, K. M., Pendleton, O. J., and Messer, C. J. (1995). Horizontal Alignment Design Con- sistency for Rural Two-Lane Highways. Final Report. (FHWA-RD-94-034). McLean, VA: FHWA. • McKnight, A. J., and Adams, B. B. (1970). Driver Education Task Analysis. Volume I. Task Description. (DOT HS 800 367). Washington, DC: National Highway Traffic Safety Administration. • Pendleton, O. J., and Messer, C. J. (1995). Horizontal Alignment Design Consistency for Rural Two-Lane Highways. Final Report. (FHWA-RD-94-034). McLean, VA: FHWA. • Richard, C. M., Campbell, J. L., and Brown, J. L. (2006). Task Analysis of Intersection Driving Scenarios: Information Processing Bottlenecks (FHWA-HRT-06-033). Washington, DC: FHWA. Available at http://www.tfhrc.gov/safety/pubs/06033/ • Salvendy, G. (Ed.). (1997) Handbook of Human Factors and Ergonomics (2nd ed.). New York: Wiley. • Serafin, C. (1994). Driver Eye Fixations on Rural Roads: Insight into Safe Driving Behavior. (UMTRI-94-21). Ann Arbor: University of Michigan Transportation Research Institute. • Underwood, G. (1998). Eye Guidance in Reading and Scene Perception. Oxford: Elsevier. • Vaniotou, M. (1991). The perception of bend configuration. Recherche Transports Securite, (7), 39–48. For the most part, these references and the other research provided information about which tasks were involved in a given segment, but not complete information about the spe- cific information-processing subtasks. To determine this information, the details about the information-processing subtasks and any other necessary information were identified by the authors based on expert judgment and other more general sources of driving behavior and human factors research (e.g., Groegor, 2000; Salvendy, 1997; Underwood, 1998). 22-37 HFG TUTORIALS Version 2.0

HFG TUTORIALS Version 2.0 22-38 Tutorial 4: Determining Appropriate Clearance Intervals Methods for determining appropriate clearance interval length vary from agency to agency, and there is no consensus on which is the best method. The Institute for Transportation Engi- neers recommends several procedures for determining clearance interval duration in a 1994 informational report (see ITE, 1994) on signal change interval lengths. These methods include: 1. A rule of thumb based on approach speed, such as this one presented in the ITE Traffic Engineering Handbook (Pline, 1999): – Yellow change time in seconds = operating speed in mi/h/10 – Red clearance interval = 1 or 2 s 2. Formulas for calculating interval lengths based on site, vehicle, and human factors charac- teristics, such as this equation (from Pline, 1999): Where: CP = non-dilemma change period (change + clearance intervals) t = perception-reaction time (nominally 1 s) V = approach speed, m/s [ft/s] g = percent grade (positive for upgrade, negative for downgrade) a = deceleration rate, m/s2 (typical 3.1 m/s2) [ft/s2 (typical 10 ft/s2)] W = width of intersection, curb to curb, m [ft] L = length of vehicle, m (typical 6 m) [ft (typical 20 ft)] 3. A uniform clearance interval length—Various studies report that uniform value of 4 or 4.5 s for the yellow change interval length throughout a jurisdiction is sufficient to accom- modate most approach speeds and deceleration rates. Refer to Determining Vehicle Signal Change and Clearance Intervals (ITE, 1994) for more discussion on this. The Manual on Uniform Traffic Control Devices (FHWA, 2007) states that a yellow change interval should be approximately 3 to 6 s, and the Traffic Engineering Handbook (Pline, 1999) states that a maximum of 5 s is typical for the yellow change interval. The red clearance interval, if used, should not exceed 6 s (FHWA, 2007), but 2 s or less is typical (Pline, 1999). The traffic laws in each state may vary from these suggested practices. ITE recommends that the yellow interval not exceed 5 s, so as not to encourage driver disrespect for signals. CP t V a g W L V = + ± + + 2 64 4.

22-39 HFG TUTORIALS Version 2.0 Tutorial 5: Determining Appropriate Sign Placement and Letter Height Requirements When determining the appropriate sign placement, it is important to consider a number of driver-related factors. The Traffic Control Devices Handbook (Pline, 2001) describes a process that utilizes these factors and is the basis for the steps described below. This method is mostly focused on guide and informational sign applications. Step 1. Calculate the Reading Distance The reading distance is the portion of the travelling distance allotted for the driver to read the message, based upon the time required to read it (reading time). The Traffic Control Devices Handbook outlines two methods for calculating the reading time. The first method, used by the Ontario Ministry of Transportation, is described in the following three steps: 1. Allocate 0.5 s per word or number and 1 s per symbol, with a 1-s minimum for the total reading time. This time should only include critical words. Drivers do not need to read every word of each destination listed on a sign to find the one they are looking for. For example, assume they are reading a sign with two destinations: Mercer St. and Union St., each with a direction arrow. Drivers only need to read the word Mercer to realize that is not the street they are looking for and the word Union to know that is their destination. They then only need to look at the arrow for Union St. 2. “If there are more than four words on a sign, a driver must glance at it more than once, and look back to the road and at the sign again. For every additional four words and numbers, or every two symbols, an additional 0.75 s should be added to the reading time.” (Ontario Ministry of Transportation Traffic Office, 2001) 3. If the maneuver does not begin before the driver reaches the sign, add 0.5 s to the reading time. This extra time is to account for the extreme viewing angle immediately before the driver passes the sign, which prohibits reading. If the maneuver has already begun, the driver does not need to continue to read the sign, and thus does not need more time. These three steps are summarized in Table 22-7. Step 1 Step 2 Step 3 Base Reading Time (BRT) Are there more than 4 words? Does the maneuver initiate before passing the sign? Yes: Add time based on the BRT 2 < BRT 4 Add 0.75 s 4 < BRT 6 Add 1.50 s 6 < BRT 8 Add 2.25 s …etc Yes: Add 0 s BRT (s) = 0.5x + 1y where: x = the number of critical words/ numbers in the message y = the number of critical symbols in the message No: Add 0 s No: Add 0.5 s Table 22-7. Three-step method for calculating base reading time.

HFG TUTORIALS Version 2.0 22-40 Another method for calculating reading time, cited in previous studies, applies to complex signs in high-speed conditions. The formula provided is: After finding the reading time, convert it into a reading distance by multiplying by the travel speed. Step 2. Calculate the Decision Distance The decision distance is the distance required to make a decision and initiate any maneuver, if one is necessary. After reading the sign, the driver needs this time to decide his/her course of action based upon the sign’s message. Decision times range as follows: • 1 s for simple maneuvers (e.g., stop, reduce speed, choose or reject a single destination from a D1-1 sign) • 2.5 s or more for complex maneuvers (e.g., two choice points at a complex intersection) After finding the decision time, convert it into the decision distance by multiplying by the travel speed. Step 3. Calculate the Maneuver Distance The maneuver distance is the distance required to complete the chosen maneuver. The maneu- ver distance depends on the course of action decided upon by the driver and the travel speed. The sign placement should consider all of the maneuvers that could be chosen based upon the message. An example of required maneuver distances is provided in Table 22-8 for lane changes in preparation for a turn. These distances do not apply to situations in which drivers must stop. For high-volume roadways, more time may be needed to find a gap, while for low-volume roadways, some of the deceleration distance may overlap with the lane change distance. Reading Time (s) (Number of Familiar W= 0 31. ords)+1 94. Table 22-8. Maneuver distances required for preparatory lane changes. Operating Speed (mi/h) Gap-Search Distance (ft) Lane Change Distance (ft) Deceleration Distance (ft) Non-Freeway Maneuver Distance Requirements 25 66 139 77 35 92 195 154 45 119 251 257 55 145 306 385 Freeway Maneuver Distance Requirements 55 218 306 308 65 257 362 462 70 277 390 549 Source: Pline (2001)

22-41 HFG TUTORIALS Version 2.0 Step 4. Calculate the Information Presentation Distance The information presentation distance is the total distance from the choice point (e.g., inter- section) at which the driver needs information. This distance is calculated using the following formula: Step 5. Calculate the Legibility Distance The legibility distance is the distance at which the sign must be legible. This distance is based upon the operating speed and the advance placement of the sign from the choice point. The leg- ibility distance is calculated using the formula below: Step 6. Calculate the Minimum Letter Height The minimum letter height is the height required for the letters on the sign based upon the legibility distance calculated above. It is also based upon the legibility index provided in the MUTCD (30 ft/in.). Another consideration is the minimum symbol size. The minimum symbol size is based upon the legibility distance of the specific symbol that is being used. Table 22-9 contains daytime leg- ibility distances for five types of symbols based upon research (Dewar et al., 1994). From these legibility distances, we can obtain two general trends: (1) legibility distances vary by sign type and (2) legibility distances are greatly reduced for older drivers. Legibility distances for symbols are generally greater than for word messages. Example Application As an example, a driver approaches an intersection on a 35-mi/h (51 ft/s) roadway. The driver needs to read a simple designation sign (D1-1) that contains one destination word and Minimum Letter Height (in.) Legibility Dist= ance (ft) Legibility Index (ft/in.) Legibility Distance Information Presentatio= n Distance Advance Placement− Information Presentation Distance Reading D= istance Decision Distance Maneuver Distanc+ + e Table 22-9. Daytime legibility distances of five symbol types by age group. Daytime Legibility Distances (ft) Symbol Type Number of Signs Young Middle-Aged Old Mean Warning 37 736.4 714.7 581.5 677.6 School 2 573.3 634.7 501.2 569.7 Guide 21 472.3 461.5 366.0 433.3 Regulatory 12 464.4 437.9 367.4 423.1 Recreational 13 321.1 292.6 228.9 280.8

HFG TUTORIALS Version 2.0 22-42 one symbolic arrow. The sign is placed 200 ft in advance of the intersection. The legibility index is assumed to be 30 ft/in. (FHWA, 2009). See Figure 22-9. 1. Reading Distance (ft) = [(1 s/word)(1 word) + (0.5 s/symbol)(1 symbol)](51 ft/s) = 77 ft 2. Decision Distance (ft) = (1 s/simple decision)(1 simple decision)(51 ft/s) = 51 ft 3. Maneuver Distance (ft) = Gap Search + Lane Change + Deceleration = 92 ft + 195 ft + 154 ft = 441 ft 4. Information Presentation Distance (ft) = Reading Distance + Decision Distance + Maneuver Distance = 569 ft 5. Legibility Distance = Information Presentation Distance – Advance Placement = 569 ft – 200 ft = 369 ft 6. Letter Height = (369 ft)/(30 ft/in.) = 12 in. (when rounded to the nearest inch) Figure 22-9. Graphic illustrating the example application of a driver approaching an intersection. Information Presentation Distance (569 ft) Maneuver Distance (441 ft) Reading Distance (77 ft) Decision Distance (51 ft) Gap Search (92 ft) Lane Change (195 ft) Deceleration (154 ft) Legibility Distance (369 ft) Advance Placement (200 ft)

22-43 HFG TUTORIALS Version 2.0 Tutorial 6: Calculating Appropriate CMS Message Length under Varying Conditions The amount of information that can be displayed on a CMS is limited by the amount of time that the driver has to read the message. This amount of time in turn is determined by the legi- bility distance of the sign and the traveling speed of the passing vehicle. The legibility distance is the maximum distance at which a driver can first read a CMS message. According to Dudek (2004), this distance depends upon a number of factors including: • Lighting conditions • Sun position • Vertical curvature of the roadway • Horizontal curvature of the roadway • Spot obstructions • Rain or fog • Trucks in the traffic stream These obstructions and visibility limitations reduce the amount of time that the sign is within view or legible, ultimately requiring a reduction in the amount of information that is displayed on the CMS. The information that can be displayed is measured in information units. An infor- mation unit is a measure of the amount of information presented in terms of facts used to make a decision. For example, the location of the problem, the audience that is affected by the problem, and the recommended action to take are each 1 information unit. To determine the appropriate number of information units for display on a CMS, the following steps should be considered. Step 1. Determine the Legibility Distance for the CMS The maximum legibility distance for a CMS depends on the design characteristics of the sign (Dudek, 2004). These characteristics include the display type, character height, character width, character stroke width, and the font displayed. The base legibility distances found in Table 22-10 are presented in Dudek (2004) and are based on the results of several studies. The distances are based on all uppercase letters, 18 in. character heights, approximately 13 in. character widths, and approximately 2.5 in. stroke widths. Note that all of the information for light-emitting diode signs provided in this tutorial applies only to the newer aluminum indium gallium phosphide (or equivalent) LEDs. Table 22-10. CMS legibility distances for varying lighting conditions. Legibility Distances (ft) Lighting Light-Emitting Diode Fiberoptic Incandescent Bulb Reflective Disk Mid-Day 800 800 700 600 Washout 800 800 700 400 Backlight 600 500 400 250 Nighttime 600 600 600 250 Source: Dudek (2004)

HFG TUTORIALS Version 2.0 22-44 Step 2. Use the Driver Speed to Find the Base Maximum Number of Information Units Allowed in a Message The maximum number of information units is derived from the legibility distance of the CMS (which depends on the technology used) and the speed of the passing vehicles. The faster that the passing drivers are going, the less time they have to read the CMS message. Also, because the legibility distance of the sign depends upon the technology used, the number of information units also varies with the technology that is used. Finally, the diverse technologies perform differ- ently under changing conditions. Table 22-11 presents the base maximum number of informa- tion units that can be presented for assorted CMS technologies, under several ambient lighting conditions. Step 3. Adjust for Adverse Roadway and Environmental Conditions There are many roadway and environmental conditions that reduce the visibility of CMSs and thus require a reduction in information units. Dudek (2004) provides further guidance on the exact number of information units that should be used under different conditions. The follow- ing sections describe how various conditions and factors lead to trade-offs in the number of information units that may be displayed. Vertical Curves The reduction in information units required for vertical curves depends on the design speed of the curve as well as the CMS offset from the road and mounting height. The following general relationships apply to CMSs on vertical curves: • As the design speed of the curve decreases, the number of information units that may be used decreases. • As the horizontal offset from the road increases, the number of information units that may be used decreases. • As the mounting height of the CMS decreases, the number of information units that may be used decreases. Table 22-11. Maximum number of information units per message for various technologies at different speeds. Maximum Information Units per Message Light-Emitting Diode Fiberoptic Incandescent Bulb Reflective Disk Lighting 0-35 mi/h 36-55 mi/h 56-70 mi/h 0-35 mi/h 36-55 mi/h 56-70 mi/h 0-35 mi/h 36-55 mi/h 56-70 mi/h 0-35 mi/h 36-55 mi/h 56-70 mi/h Mid-Day 5 4 4 5 4 4 5 4 3 5 4 3 Washout 5 4 4 5 4 4 5 4 3 4 3 2 Backlight 4 4 3 4 3 2 4 3 2 2 1 1 Nighttime 4 4 3 4 4 3 4 3 3 3 2 1 Source: Dudek (2004)

22-45 HFG TUTORIALS Version 2.0 In general, permanent CMSs that are mounted over the roadway are not affected by crest ver- tical curves (Dudek, 2004). Horizontal Curves The main concern with CMSs located on horizontal curves is the obstruction of the sign by roadside objects. Permanent CMSs that are mounted above or adjacent to the travel lanes will likely be high enough to be seen over any roadside obstructions. However, portable CMSs are usually closer to the ground and more likely to be obscured by obstructions. In general, the num- ber of information units that may be used decreases when: • The obstruction gets closer to the roadway • The curve radius decreases (i.e., for tighter curves) Rain Rain does not generally affect CMSs (Dudek, 2004). However, when the intensity of the rainfall increases to 2 in./h or more, the visibility of the sign can be impacted. When the operating speed of the roadway is over 55 mi/h, Dudek (2004) recommends that the number of information units displayed on portable LED CMSs should be reduced by 1 information unit. Portable LED CMSs often use fewer pixels per character, and thus have lower luminance levels per character than permanent CMSs, which are relatively unaffected even in heavy rain- fall. Therefore, signs utilizing other technologies should use fewer information units in heavy rainfall. Fog Fog can affect visibility even more than heavy rain. Generally, Dudek (2004) does not recom- mend a reduction in information units for permanent LED CMSs because of fog. A reduction is not necessary because of the high character luminance and contrast of permanent LED CMSs. However, portable LED CMSs require a reduction. The number of information units that may be used decreases when: • The visibility range decreases • The offset from the road increases Trucks on the Roadway Large trucks pose sight obstructions for other vehicles on the roadway. When a driver’s view of a CMS is obscured by a truck, the driver has the option to change his/her traveling speed or position to see around the truck. However, as the number of trucks on the roadway increases, the amount of space that is available for drivers to do this repositioning decreases. Thus, the more trucks that are on the roadway, the more likely they are to impair the view of a CMS for other drivers. Step 4. Adjust for Blanking Time Greenhouse (2007) found that inserting a 300-ms blank screen between phase 1 and phase 2 of a portable message sign improves comprehension. The study is further discussed in the guideline

HFG TUTORIALS Version 2.0 22-46 for Displaying Messages with Dynamic Characteristics. Although the blanking time was only tested between phases 1 and 2 (not between 2 and 1), it is reasonably conceivable that drivers who see a blank between phases 1 and 2, but not between phases 2 and 1, would reverse the order of the phases and possibly have trouble understanding the message. Dudek (1992) recommends that blank time and/or asterisks should be displayed between cycles of a message that contains three or more phases (on one-word or one-line signs). Because one-word and one-line signs are more lim- ited in the amount of information that they can display at one time, the phases may not make sense independently and drivers who read later phases before phase 1 may not understand the message. Thus, giving an indication of where the message is in the cycle gives drivers an idea of their location in the cycle. Overall, drivers may use the blanking time to determine where they are in the message cycle, even before the message is legible to them. There are additional benefits in terms of message com- prehension as shown by Greenhouse (2007). However, the insertion of blanking time reduces the total available time for the driver to read the message, potentially requiring a reduction in information units. Thus, there is a trade-off between the benefits of providing blanking time and the number of information units that may be contained in the message. Step 5. Display the Resulting Number of Information Units After the calculations and adjustments from Steps 1 through 4 are performed, the result will be the number of information units that may be displayed in the message. If there are still more information units in the message than should be displayed, they should be reduced using the fol- lowing steps, until the appropriate number of information units is reached (steps and examples adapted from Dudek (2004)). Step 5A: Omit and Combine Information Units First, attempt to reduce the number of information units without losing content by following the steps below. • Omit unimportant words and phrases Example: Original Message: Shortened Message: ROAD CLOSED AHEAD ROAD CLOSED DUE TO CONSTRUCTION 1 MILE FOLLOW DETOUR ROUTE FOLLOW DETOUR The word “Ahead” is unnecessary as drivers will assume the closure is ahead. The reason is less important than the location of the closure.

22-47 HFG TUTORIALS Version 2.0 • Omit redundant information Example: Original Message: Shortened Message: MAJOR ACCIDENT MAJOR ACCIDENT ON I-276 NORTH PAST I-80 PAST I-80 2 LEFT LANES CLOSED 2 LEFT LANES CLOSED KEEP RIGHT If the CMS is on I-276, the same freeway as the accident, the information is evident to the drivers and may be omitted. The information units “2 Left Lanes Closed” and “Keep Right” are redundant because drivers can assume that if the two left lanes are closed, they will need to move to the right. • Combine base CMS elements Example: Original Message: Shortened Message: TRUCK ACCIDENT FREEWAY CLOSED PAST I-80 EXIT AT I-80 ALL LANES CLOSED FOLLOW DETOUR AT I-80 I-287 NORTH TRAFFIC EXIT AT I-80 FOLLOW DETOUR In the example above, the incident descriptor, incident location, and lanes closed mes- sage elements are combined into the information unit “Freeway Closed”. The location of the closure can be eliminated because the action element “Exit at I-80” describes the location. Step 5B. Reduce the Number of Audiences in the Message Example: Original Message: Shortened Message: I-76 CLOSED I-76 CLOSED BEST ROUTE TO BEST ROUTE TO PHILADELPHIA/I-95 PHILADELPHIA USE RTE-73 NORTH USE RTE-73 NORTH When using this reduction technique, message designers must use their judgment to decide which audience is more important to address in the message. In the previous example, the audience “Philadelphia/I-95” was reduced from 2 information units to 1 information unit, “Philadelphia”.

HFG TUTORIALS Version 2.0 22-48 Step 5C. Use Priority Reduction Principles If the message still contains more information units than should be displayed, the information units should be reduced in order of priority. The priority order is derived from the information drivers need the most in order to make driving decisions. In Table 22-12, the information units are listed in priority order, with number 1 being the highest priority information. If the closure is due to roadwork, the effect on travel and good reason for following the action should be eliminated. Even though the incident/roadwork descriptor is useful to driv- ers, it may be replaced with the lanes closed element if necessary. When choosing information units to eliminate, the designer should start deleting units from the bottom of these priority lists first (i.e., element numbers 8 or 6). More examples of the application of these steps can be found in Dudek (2004). Table 22-12. Order of priority for information units. Lane Closures Freeway/Expressway Closures 1. Incident Descriptor 2. Incident Location 3. Lanes Closed 4. Speed Reduction Action (if needed) 5. Diversion Action (if needed) 6. Audience for Action (if needed) 7. Effect on Travel (if needed) 8. Good Reason for Following Action (if needed) 1. Closure Descriptor 2. Location of Closure 3. Speed Reduction Action (if needed) 4. Diversion Action 5. Audience for Action (if needed) 6. Effect on Travel (if needed)

HFG TUTORIALS VERSION 2.1 22-49 TUTORIAL 7: JOINT USE OF THE HIGHWAY SAFETY MANUAL (HSM) AND THE HUMAN FACTORS GUIDELINES FOR ROAD SYSTEMS (HFG)2 Background and Objectives Using the Highway Safety Manual (HSM; AASHTO, 2010) and the Human Factors Guidelines for Road Systems (HFG; Campbell et al., 2012) together provides a valuable resource for designing safer roadways. The HSM is used to quantify the effects of design and operational decision-making on crash frequency and severity outcomes thus estimating the crash potential of roadway infrastructure alternatives. The HFG is used to facilitate operational decisions that reduce crash potential by providing the currently accepted factual information and insight on road users’ needs. Together these two documents enable highway planners, designers, operations engineers, and traffic engineers to incorporate features that promote the explicit consideration of crash potential for new or upgraded roadways. Using the HSM and the HFG together means highway planners, designers, operations engineers, and traffic engineers can better diagnose and address the contributing factors to crashes and thereby increase the effectiveness of project treatment and selection. Joint use of the HSM and the HFG will improve end users’ ability to select roadway design and operational elements based on the best- available data and promote an improved level of highway safety. The objective of this tutorial is to summarize a process that facilitates the combined use of the HSM and the HFG to support improved countermeasure selection for road safety. This tutorial is intended to be a short, instructive, and readily useful document that explains the joint use of these resources and provides state and local agencies a tool to enhance data-driven decision-making. Overview of the HSM The HSM was developed to provide tools and resources to help transportation practitioners make data driven decisions to reduce crashes on all levels of roadways. Prior to the development of the HSM, there was no single resource to include crash potential in the project development and planning process. The use of the HSM has allowed for practitioners to not only include crash potential in decision-making, but to utilize data as the cornerstone of those decisions. While the HSM is geared towards transportation professionals, including but not limited to planners and engineers, the guidance is provided with three key assumptions. In particular, the material is geared towards individuals who:  Have been exposed to the basic fundamentals of transportation safety,  Are familiar to basic statistics and data interpretation, and 2 This tutorial is adapted from Campbell, Hull, and Maistros (2018).

HFG TUTORIALS VERSION 2.1 22-50  Can exercise sound traffic safety and operational judgement. The HSM provides guidance for traditional descriptive methods of analysis for traffic safety, but also drives the practice forward with predictive methods for crash analysis. When planning a new road or modifying an existing roadway, highway designers and traffic engineers need to review and predict the expected safety of the new design. The methodologies presented in the HSM provides analysis tools for many types of roadway segments and intersections. The relative crash potential of design alternatives and traffic control can be reviewed and compared. The HSM focuses on quantifying safety performance within the domains of planning, design, operations, and maintenance, and – as seen in Figure 1 below – is divided into four parts. Figure 1. The 4 Parts of the Highway Safety Manual (HSM) In Vol. 1 (Part A) the user is provided with an introduction to the HSM, knowledge about human factors and the general fundamentals of highway safety performance. Vol. 1 (Part B) covers the roadway safety management process. This part of the manual provides methods for network screening, crash diagnosis to understand potential contributing factors and identifying treatments, and methods for selecting and prioritizing projects. These methods can be used in near term planning analysis and individual site evaluation and countermeasures development. In Vol. 2 (Part C) the predictive method is introduced for different facility types. The facility types are two-lane rural highways, multilane rural highways, and urban and suburban arterials. A Supplement, published in 2014, presents the predictive method for freeways, ramps, and ramp terminals. This project developed the predictive method for freeways and interchanges. The predictive method can be used to estimate the change in crash frequency or severity at a site as a function of traffic volume or roadway characteristics. These methods can be used in corridor level alternatives analysis or more detailed site design. Vol. 3 (Part D) provides Crash Modification Factors (CMFs) for use with Part B and existing facilities. Part D can only be used with Part B. Part C has its own CMFs for each facility type. CMFs are used to estimate the change in crash frequency that can be associated with a particular treatment. CMFs can be used in the design exception process or site-specific evaluation process. Part A: Introduction, Human Factors, and Fundamentals Part B: Roadway Safety Management Part C: Predictive Method Part D: Crash Modification Factors

HFG TUTORIALS VERSION 2.1 22-51 Together, the HSM and the HFG can supplement traditional sources of roadway design information and help improve decisions that reduce crash potential. They are both guidance documents intended to provide practitioners with practical information about quantitative crash reductions and human factors. There is valuable information in these documents that, when used in coordination, can reduce crash frequency and severity. Specifically, the HFG and HSM provide complementary information for finding countermeasures to design and operational factors leading to crashes. A Process for Joint Use of the HSM and the HFG In order to make the most effective and efficient use of the HSM and the HFG, a process comprised of the following five steps is recommended: 1. Collect Site Data 2. Review Site and Existing Conditions 3. Identify Potential Countermeasures 4. Develop and Prioritize Countermeasures 5. Conduct Safety Effectiveness Evaluation Each of the steps (the key five steps are in blue) shown in the following diagram are discussed separately below. Figure 2. A Process for the Joint Use of the HSM and the HFG Step 1- Collect Site Data Objective of Step 1: Compile data necessary to assess the traffic safety performance at the intended site quantitatively and qualitatively.

HFG TUTORIALS VERSION 2.1 22-52 Data is the basis for defining how to improve traffic safety performance and understanding how to reduce the number and potential for crashes at a particular site. The three critical types of data needed for this process include crashes, traffic volumes, and recent projects that might impact crash potential (e.g., lane widening projects, horizontal curvature, or the installation of new sidewalks). All three of these data types are subject to the following criteria:  Scale: Data may be presented at finite individual levels or at more summary aggregate levels within a data set. It is critical to understand the nature and scale of the data being used in the analysis process to ensure the appropriate measures are captured and addressed. As needed, describe and define scales and the units of measure associated with the data.  Timeliness: It is important to understand the timeframe of each data set used in this process. Because there are at least three sets of data being used, understanding when each data source was collected, what time periods it covers ensures data comparisons are valid, and that current issues are being addressed. Outdated information may lead to less accurate or effective results. One of the cornerstones of the HSM is the inclusion of crash rates, either in terms of crashes per year or crashes per traffic volume. These rates require multiple sources of information. While it is not useful to develop crash rates with crash data from 2015 and traffic data from 1997, the newest available data may vary by data type. For instance, crash data may be available for the most recent calendar year, while traffic data may lag a year behind. Some variances in timeliness are anticipated but due diligence should be exercised to acquire the most recent available data.  Quality/limitations: All data sets have known limitations or potentially even quality issues. Understanding the where data is incomplete or known to have incorrect information is critical to successful analysis throughout this process. The following data are typically obtained from a Department of Transportation (DOT), city, or Metropolitan Planning Organization (MPO). Understanding which jurisdiction(s) is (are) responsible for the section of road being analyzed is important in determining the most valid source of crash/conflict data. There may be multiple sources of crash data for a particular facility if transportation departments and law enforcement maintain separate databases. 1a. Compile Crash Data  Scale: Crash data is commonly collected at three levels; crash, vehicle, person. Crash level data is the most summative and generally includes the information that describes the whole crash or facts applying to all parties involved in the crash (i.e., time, location, highest injury severity in a crash). Vehicle level data includes a record for each vehicle involved in the crash, and person level data included a record for each individual involved in the data.  Timeliness: A minimum of five years of crash data is recommended for analysis. While electronic crash reporting has greatly reduced the processing time for crashes, it is still common to have delays of a year or more. As such, the most recent crash data may not cover the most recent calendar year.  Quality/limitations: Each crash database has its own intricacies and limitations. Take time to either read the metadata associated with the crash data, or to talk with the crash data managers to understand the limitations or known quality issues.

HFG TUTORIALS VERSION 2.1 22-53 In some situations, crash data availability may be limited. The HSM and HFG are both driven by data and by addressing safety issues identified through data analysis. Effort should be made to capture available safety data; however, there are options for conducting analyses without data. The HSM, specifically, can estimate the number of crashes at a site with safety performance functions (SPFs). These estimations can then be adjusted with potential improvements/ changes to estimate crash potential. While these processes have the flexibility to estimate safety benefits without crash data, the results are much more reliable and reflect the specific project location characteristics. 1b. Compile Volume Data (including recent project site revisions)  Scale- Volume data is needed for each transportation mode in the analysis scope. Certain modal volumes such as bicycles and pedestrians are often difficult to obtain. Guidance for obtaining these volumes may be found in the NCHRP Report 770 Estimating Bicycling and Walking for Planning and Project Development: A guidebook (TRB, 2014); an additional source is Washington DOT’s Methods for Estimating Bicycling and Walking in Washington State (Nordback and Sellinger, 2015). It is also critical to use volume data with enough information to distinguish important traffic patterns. For example, if an intersection with two- way stop control is being evaluated, it is important to know not just the total volume of the intersection, but the major versus minor road traffic and any turning traffic as well.  Timeliness- The most recent volume data available should be used. If there was a major change in traffic during the crash analysis period, it may be prudent to use volume data reflecting this change for the appropriate years.  Quality/limitations- Similar to scale, it is important to understand which modes and traffic patterns the volume data pertains to. Any quality issues or known limitations of the crash/conflict data should be described. Patterns of crashes may also appear in the volume data; Example 50 percent of the crashes occur during the are peak hours 7-9 a.m. turning left NB. Field Investigation if possible should be conducted during that timeframe. Also, roadway facilities are subject to a wide variety of revision projects, from lane widening, changes in horizontal curvature, and the installation of sidewalks to new mailboxes and signs. The scale, timeliness, and known limitations in the type, amount, or quality of the data associated with these projects should be considered. Projects occurring within the data analysis period need to be considered for their impact on traffic patterns and safety performance. If major projects occurred just before the crash data period being analyzed, it may be prudent to expand the crash data period, if possible. Data and project plans are not necessarily data available for all projects at a site. For example, smaller projects such as signs, post boxes, or other installations may not require project plans but may affect sight lines or clear zones. Anticipated Outcome of Step 1: A time-consistent collection of crash, traffic, and project data describing the safety, behavioral, and roadway data relevant to the site.

HFG TUTORIALS VERSION 2.1 22-54 Step 2- Review Site and Existing Conditions 2a. Conduct a Site Visit Objective of Step 2a: Conduct a qualitative analysis of existing roadway, behavioral, and human factors conditions at the site. Visit the site3 under daytime and nighttime conditions, during the peak and off-peak hours. By visiting the site at a variety of times, it will help to understand the conditions facing road users4 under a variety of circumstances. The purposes of these visits are to determine the conditions facing road users and potentially affecting traffic safety. Conditions may include infrastructure, behavioral, and operational factors.  Infrastructure: Focus on the built environment at the site. What is the state of the infrastructure road users are dealing with? Items to consider include pavement and sidewalk condition, sign and marking visibility, drainage issues, shoulder condition, vegetation maintenance, properly functioning signals (including pedestrian signals), roadway/ intersection lighting, differences, if any, in upstream geometric or traffic control conditions that may impact user decisions, etc.  Behavioral and Human Factors: How are road users responding to the built environment and traffic within and upstream of the project? Items to consider include drivers’ use of passing or turning lanes, line of sight issues (to both signal and other traffic control devices), bicyclists or pedestrians traveling along shoulders, pedestrian mid-block crossings, etc.  Operational: How can drivers be better directed and informed to move more efficiently and with less potential for a crash? Items to consider include large queues from turning traffic, traffic frequently blocking intersections, signal timing, non-conforming signing and marking, cues that might lead to road user indecision, etc. These sites visits are similar to Road Safety Audits (RSAs) — and may be conducted as informal RSAs. An RSA is a formal safety performance examination of an existing or future road or intersection by an independent, multidisciplinary team. These assessments usually offer a qualitative approach to understanding safety and potential alternatives, but do not always provide the quantitative approach of this HSM/HFG process. While a formal RSA may not be completed, there are several guiding prompt sheets developed for RSAs that may be useful during a site visit. Figure 3 below provides an example prompt sheet for existing roadways. Several prompt sheets for specific audits are available from the FHWA. 5 3 If a site visit is not possible, a videolog or Google maps Street View could perhaps be used. 4 Or virtual drivers/road users; new plans or designs being evaluated will only have virtual drivers, not real drivers. 5 More information about RSAs can be found at https://safety.fhwa.dot.gov/rsa/

HFG TUTORIALS VERSION 2.1 22-55 Figure 3. Existing Road Audit Prompt Sheet (Ward, 2006) 2b. Identify Crash Patterns and Contributing Factors Objective of Step 2b: To quantitatively describe the safety performance at the site by mode, year, crash type, and contributing factor. Crash examples are illustrated but if such data not available, conflict statistics, such as police reports/citations, can be substituted for crashes. Understanding crash trends for the site is crucial for quantitatively assessing the site’s potential for crash reduction. There are several ways to effectively summarize crash trends, and there is no one correct set of tables or charts to develop. While the quantity, format, and style of the trends are up to the developer, it is important to clearly and completely describe the crash statistics by mode, year, and severity. As noted in Step 1, it is also important to use the number of crashes and the count of injuries for the crash trend analysis. The following three figures illustrate the data breakdowns that should be included in the crash summary; additional tables or graphs may be included as needed. Existing Road Audit Road Functions, Classification, Environment Road Alignment and Cross Section Auxiliary Lanes Intersections Interchanges Signs and Lighting Marking and Delineation Barriers and Clear Zones Traffic Signals Pedestrians and Bicyclists 1 Visibility, sight distance 1 Tapers 1 Location 1 Visibility, sight distance 1 Lighting 1 General issues 1 Clear zones 1 Operations 1 General issues 2 Design speed 2 Shoulders 2 Visibility, sight distance 2 Lanes, shoulders 2 General signs issues 2 Centerlines, edge-lines, lane li 2 Barriers 2 Visibility 2 Pedestrians 3 Speed limit/speed zoning 3 Signs and markings 3 Signing and marking 3 Signing, marking, delineation 3 Sign legibility 3 Guideposts and reflectors 3 End treatments/ Crash cushions 3 Placement of signal heads 3 Bicyclists 4 Passing 4 Turning traffic 4 Layout and "read- ability' (perception) by drivers 4 Pedestrians, bicyclists 4 Sign supports 4 Curve warning and delineation 4 Pedestrian railing 4 Public transport 5 "Readability" (perception) of the alignment by drivers 5 Pedestrians, bicyclists 5 Lighting 5 Visibility of barriers and fences 6 Human factors 6 Lighting 7 Widths 8 Shoulders 9 Cross slopes 10 Slide slopes 11 Drains 12 Combinations of features PROMPT LIST 6 (1 OF 2) Anticipated Outcome of Step 2a: A qualitative analysis of traffic, behavioral, and roadway issues at the site.

HFG TUTORIALS VERSION 2.1 22-56 Figure 4. Example of crashes by year and severity in both tabular and graphic layouts Figure 5. Example of crash counts by crash type and crash severity in tabular and graphic layouts Figure 6. Example of crash counts by contributing factors and crash severity in tabular and graphic layouts Crash  Year  Fatal (K)  Serious  Injury (A)  Evident  Injury (B)  Possible  Injury (C)  PDO  Total  2009  0  1  2  0  5  8  2010  0   0  0  1  9  10  2011  1   0  4  0  10  15  2012  0  1  0  1  9  11  2013  0  1  3  1  6  11  Total  1  3  9  3  39  55  0 5 10 15 20 2009 2010 2011 2012 2013 Fatal Serious Injury Evident Injury Possible Injury PDO Collision  Type  Fatal  (K)  Serious  Injury (A)  Evident  Injury (B)  Possible  Injury (C)  PDO  Total  Rear‐End  0  0  0  0  23  23  Vehicle  Hit  Pedestrian  1  2  7  0  0  10  Sideswipe  0  0  0  1  8  9  Vehicle  Hits Utility  Pole  0  0  1  1  7  9  Left‐Turn  (Minor to  Major)  0  1  1  1  1  4  Total  1  3  9  3  39  55  0 5 10 15 20 25 Rear-End Vehicle Hit Pedestrian Sideswipe Vehicle Hits Utility Pole Left-Turn (Minor to Major Fatal Serious Injury Evident Injury Possible Injury PDO Contributing  Factors  Fatal  (K)  Serious  Injury (A)  Evident  Injury (B)  Possible  Injury (C)  PDO  Total  Speed greater  than conditions  0  1  6  1  14  22  Inattention  0  0  0  0  10  10  Under influence  of alcohol  1  1  1  0  4  7  Following too  closely  0  0  1  1  4  6  Failure to yield  ROW  0  0  1  1  3  5  Under influence  of drugs  0  0  0  0  3  3  Exceeding speed  limit  0  1  0  0  0  1  Driver operating  device  0  0  0  0  1  1  Total  1  3  9  3  39  55  0 5 10 15 20 25 Speed greater than conditions Inattention Under Influence of Alcohol Following too closeley Failure to Yield ROW Under Influence of Drugs Exceeding Speed Limit Driver Operating Device Fatal Serious Injury Evident Injury Possible Injury PDO

HFG TUTORIALS VERSION 2.1 22-57 Develop a statement of significant trends and findings. Based on the totality of the crash/conflict record, a short summary of significant trends or findings should be developed. The state of significant trends and key findings should focus on the more severe crashes and look for crash types and contributing factors where the severe crashes are over-represented. For example, in Figure 5 above, the crash types with the greatest number of crashes was rear-end (10), however, all of the crashes were non-injury, PDO crashes. Conversely, all ten of the ‘Vehicle hit pedestrian’ crashes were either fatal, serious, or evident-injury crashes. The number of pedestrian crashes would be a significant finding that would warrant identification and perhaps some brief discussion. Note in the preceding three figures how the manner in which the total number of crashes is presented in each of the tables is different, but that all of the figures are equally important and are needed to illustrate the breadth of the potential contributing factors and crash types. 2c. Conduct Human Factors Evaluation Objective of Step 2c: Review crash or conflict data summaries and the site visit results to identify possible human factors issues for consideration in developing robust, science-based safety countermeasures. Below, we provide a brief overview of the human factors approach to road safety, followed by a set of specific steps needed to conduct a human factors evaluation of existing conditions at an existing site. Overview of the Human Factors Approach to Road Safety. A human factors evaluation of a specific roadway site starts with an understanding of the human factors approach to driving and road safety performance. At its core, this approach considers and accounts for road user needs, capabilities, and limitations in: (1) the design and operation of roads, vehicles, and pedestrian/bicycle/transit facilities (2) the identification of causal factors underlying conflicts and crashes. Figure 7 below illustrates how the road user, vehicles and the roadway environment intersect and interact together during the driving task to support safety. Anticipated Outcome of Step 2b: A quantitative description of the safety challenges at the site with an understanding of the critical crash types, severities, and contributing factors

HFG TUTORIALS VERSION 2.1 22-58 Figure 7. The Human Factors approach to road safety involves three factors We expand upon these factors in more detail below but, briefly, the ‘road user’ factor includes capabilities and limitations such as age, training, road familiarity, experience, and possible impairment; the ‘environment’ factor includes elements like road geometry, traffic control devices, and the luminance levels of signs and markings; and the ‘vehicle’ factor includes automobile and truck components like tires, brakes, or special safety systems. Safety performance reflects how well these components interact and work together to support full and accurate extraction of information, understanding of environmental cues and emerging situations, that lead to effective decision-making by road users. Specifically:  The road user… − Samples information from the environment Controls vehicles, bicycles, or walks through a facility − Maneuvers through the environment  The environment… − Provides information to the road user − Helps develop and support user expectations − Responds to the road user (e.g., a driver) through actions of other road users  The vehicle… − Triggers some responses in the environment − Serves as a stimulus to other road users (e.g., pedestrians and bicyclists pay close attention to nearby vehicles) − Provides feedback to all road users (driver, pedestrians, bicyclists) A useful way to think about how road users – particularly drivers – interact with the road environment is to examine typical visual scanning behaviors and patterns. Figure 8 below illustrates such visual scanning behaviors; a key element is that road users are continually seeking to extract the Most Meaningful Information (MMI) from the roadway scene, by scanning segments of the roadway in iterative steps. Sampling frequency will vary, and can be influenced by the road user, type of operation, roadway type, and environment. In this context, the specific MMI for a specific user at a Road User VehicleEnvironment

HFG TUTORIALS VERSION 2.1 22-59 given roadway location will also vary, and could range from roadway edges or markings, to signs or traffic control devices, to objects moving within the scene, such as pedestrians or bikes. Figure 8. Road users constantly seek out the Most Meaningful Information (MMI) from the roadway Critically, it is often the interactions between road users, vehicles, and the environment that lead to errors, conflicts, crashes, and fatalities. Specifically, errors do not generally reflect the breakdown or occurrence of a single factor—as in models of crashes that refer to a broken link in a causal chain—but, rather, reflect a confluence of factors that occur more or less simultaneously. For example, a crash doesn’t generally happen just because a driver is older; rather it might happen to an older driver, driving at night, under bad weather conditions, when faced with a sign that may be wordy, complicated, damaged, or worn out. As seen below in Figure 9, this view of crashes is well- supported by crash data (see Treat et al., 1979). The figure shows that while drivers contributed to 93 percent of crashes, they were the sole cause of only 57 percent of crashes; the crash percentages in the shaded regions of the figure highlight the role of driver/roadway/vehicle interactions as causal factors in crashes. Figure 9. Relative roles of driver, environmental, and vehicle factors in crashes (from Treat et al., 1979; see Figure 2-2)

HFG TUTORIALS VERSION 2.1 22-60 Consider some other examples with multiple factors: 1. On a wet road, worn brakes or tires leading to increased stopping distances and an increased probability of a rear-end crash. 2. No lighting and dark clothing with or without a pedestrian crosswalk increasing the probability of a pedestrian crash in nighttime conditions. 3. Drivers impaired by distraction, drugs, or alcohol; who are more easily confused by complex geometrics, signing; and who have an increased probability of a crash. The critical take-away here is to consider the full range of contributing factors that interact with a specific context and eventually lead to errors and crashes. Develop the Human Factors Interaction Matrix (HFIM). The Haddon Matrix was developed by William Haddon (see Haddon 1964, 1972, and 1980) for improving emergency responses for people injured in crashes and provides a technique and tool for looking at factors related to personal attributes, vehicle attributes, and environmental attributes before, during and after an injury or death (see also Table 6-1 from the HSM). The goal for the HFG/HSM user is similar; i.e., to think about and to list the individual road user, vehicle, and environment factors (and any possible interactions) that could contribute to driver confusion, misperceptions, high workload, distraction, or other problems and errors. As we conduct site-specific human factors evaluations, we use a modified Haddon Matrix to help identifying road user and other factors that could be contributing to a reduction in road safety across a range of various scenarios and driving situations. We call this the Human Factors Interaction Matrix (HFIM); Table 1 below shows a blank HFIM. From the left, the first four columns of this table are to be filled out as part of this step in the ‘joint use’ process - Step 2. The last column – Relevant Sections in the HFG – will be filled out later as part of Step 3. The HFIM is typically a one- way table organized by columns. Each of the illustrated examples provided in later sections of this tutorial will also include completed HFIM tables. It should be noted that the information in the HFIM is organized and should be interpreted by columns not rows, and that adjacent information (information to the left or right) is not necessarily related. Table 1. Blank HFIM to be completed during Steps 2 and 3 Road User Vehicle Environment Interactions Relevant Sections in the HFG A version of the HFIM is shown in Table 2 and it is “filled-in” with a set of factors - as examples- that could contribute to reductions in roadway safety. Note that this is not a comprehensive set of factors, but only a list of possible factors that could apply to a range of crash scenarios. Filling out this table requires thinking about and listing the individual road user, vehicle, and environment factors (and any possible interactions) that could contribute to reductions in road safety performance.

HFG TUTORIALS VERSION 2.1 22-61 When completing the HFIM, users should recognize that the individual elements and their interactions need to reflect the specific type of vehicles and types of road users using the infrastructure. For example, you can have large semi-trucks or some roads but not on others; e.g., some bridges and parkways. Also, you cannot have pedestrians on interstates (except in emergency). Certain kind of situations – such as work zones – require special consideration of the locations of maintenance staff and temporary signage. While the focus of the table should be on potential contributors to errors and crashes, the table could also include positive elements; i.e., compensatory features of the site that could mitigate the factors that are potentially contributing to crashes. Overall, the factors included in the HFIM should reflect careful consideration of the situation at hand, the crash record, and all possible impacts on the safety performance of the facility. Finally, keep in mind that solutions (in the form of countermeasures or treatments) are not being sought at this stage. The focus here is on identifying and understanding those roadway components that could be contributing to driver confusion, misperceptions, high workload, distraction, or other potential driving errors at a particular site. Table 2. Partial set of candidate road user, vehicle, and environment factors for the HFIM Road User Vehicle Environment • Age • Vision • Experience • Cognitive ability • Language/Culture • Road familiarity • Impairment (drugs, alcohol, fatigue) • Physical abilities • Training • Attitudes • Types of road users • Behaviors (speeding, distraction) • Vehicle type • Steering capabilities • Braking capabilities • Engine characteristics • Safety features • Vehicle height • Headlamps • Distractions • Length • Speed • Traffic volume • One-way flow • Two-way flow • Control type • Functional class • Lane width • Intersections • Shoulder width • Sight distance • Pavement type and condition • Bicyclists • Distractions • Enforcement • Parking lane • Bike lane • Shared lane Roadside • Grades • Curvature • Signs and markings • Weather • Land use • Pedestrians • Urban • Rural • Time of day • Light condition • Road Segments • Scenic/interest attractions • Upstream elements In filling out Table 1, the objective is to consider and document the possible road user, vehicle, and environment issues that could be contributing to confusion, errors, and crashes at the site or traffic situation that you are evaluating. Key inputs to the process of completing Table 1 include: the basic crash or conflict data compiled in Step 1, the site visit results from Step 2a (including relevant site data such as the types of vehicles, cross section dimensions, traffic volumes, speed limits, kinds of traffic control), considerations of infrastructure elements upstream of individual crash sites, and the crash data summaries from Step 2b.

HFG TUTORIALS VERSION 2.1 22-62 At this early stage in the process, it is best to generate a HFIM table that is inclusive of all possible factors impacting safety performance, rather than exclusive. Specifically, the HFIM should include any factors and combinations of factors (interactions) that could reasonably contribute to the known or suspected opportunities for reducing crash potentials at the site under investigation. Broad considerations should include crash or conflict: type, frequency, severity; and contributing factors, as well as on-the-scene observations of the facility and representative traffic movements (including pedestrians, bicyclists, and transit vehicles). In this regard, some interactions may be quantitative and very specific, while others may be qualitative and reflect possible impacts. As noted above, most crashes (or conflicts and near-misses) are going to be the results of interactions among two or more of the individual factors listed in the HFIM. Therefore, identifying known or possible crash- or conflict-relevant interactions will be critical to identifying possible treatments in Step 3. Carefully think as a virtual user through ways in which the individual factors could – in combination - create confusion, distraction, uncertainties, or misperceptions on the part of road users. Document those factors that are likely to negatively affect driving scenarios and road user behaviors, especially considering the site-specific crash and safety data. Some specific questions to ask include the following:  What is the nature of the crashes/conflicts observed; are there any discernible patterns between or among PDO crashes/conflicts and crashes/conflicts involving serious injuries or fatal crashes?  What can the most common crash types (e.g., rear-end crashes or ‘vehicle hit pedestrian,’ tell us about the relative contribution of road user, vehicle, and environment issues to the crashes?  What are the most common contributing factors cited in the crash records?  From the perspective of a road user (both the typical road user, as well as a road user who may be older or impaired in some way), what might be some sources of confusion when trying to extract the most meaningful information (MMI) from the road geometry and traffic control information? In general, is there consistency and meaning to the geometrics, signs, markings, and traffic control devices that road users rely on?  How might unique issues associated with vehicle type be contributing to crashes or conflicts? For example, does vehicle size play a role in how well the driver or road user can see signs, markings, and other road users, or how well the driver can navigate through roadway with narrow lanes and no shoulders?  Are there any unique environmental and road conditions that substantially increase road user stress, comprehension time to roadway information, or response maneuvers to hazards at conflict points? Examples may be short acceleration/deceleration lanes, unexpected roadside hardware placement, unexpected quickly narrowed lane widths, significant visibility restrictions, steep grades, etc.  In general, are there any unclear or misleading cues between the roadway and user? Such cues may be the lack of a timely (or conflicting) presentation of geometric, signing or pavement marking information. Examples are: using non-standard or non-maintained lanes or shoulders, less that standard sight distance for conditions, less than standard distance for acceleration or deceleration of vehicle, or where the speed differential is more than 15 mph.

HFG TUTORIALS VERSION 2.1 22-63  Consider not just the factors that were present at the exact time of a crash, but also factors or events that could have occurred prior to the crash. Step 3 - Identify Potential Countermeasures6 Step 3a. Identify Potential Countermeasures from the Highway Safety Manual As previously stated, the HSM includes four distinct sections (Parts A, B, C, and D) with varying intended uses and methods. It may be valuable to review Chapter 2 (Human Factors, Chapter 5 (Diagnostics) and Chapter 6 (Selecting Countermeasures) of the HSM as an orientation to this general topic. Step 3 of the ‘joint use’ process involves using either the predictive method (Part C) or CMFs (Part D) to estimate the effects of various potential site treatments. Using the Predictive Method (Part C of the HSM) Objective: Use safety performance functions (SPFs) to determine the predicted crash frequencies under existing and proposed conditions. The predictive method employs SPFs to estimate the predicted crash frequencies for a network, segment, or particular site for specific base conditions. SPFs are regression models, which use traffic volume and other site criteria to predict crash frequency. When using the predictive method, SPFs may be developed for the specific area being analyzed, or use general SPFs provided in the HSM. Developing reliable and accurate SPFs can be a considerable effort and may not always be feasible for evaluating alternative countermeasures for a single site. In addition to ongoing research in the area, the FHWA has extensive materials available for developing SPFs (see Srinivasan and Bauer, 2013). The SPFs currently available in the HSM were developed with input data from several states and cover:  Rural two-lane, two-way roads,  Rural multilane highways,  Freeways, 6 While this tutorial focuses on the HSM and the HFG as key sources for countermeasures, there are many additional sources that can and should be consulted when looking for safety countermeasures. These include sources published by the USDOT, such as behavioral countermeasures found in NHTSA’s Countermeasures that Work publications; see also: https://www.ghsa.org/resources/countermeasures Anticipated Outcome of Step 2c: A HFIM that is populated with possible road user, vehicle, and environment issues – and their interactions – that could be impacting the safety performance of the facility under investigation.

HFG TUTORIALS VERSION 2.1 22-64  Suburban arterials, and  Urban arterials. For analysis of a site not included in the preceding list of SPFs, the predictive method may not be the most efficient option for evaluation potential site improvements. The HSM breaks the predictive method into an 18-step process as summarized in a flowchart in HSM Figure C-2. The following steps highlight the key methods in comparing treatments through the predictive method. A. Predictive method Step 9- For the selected site, determine and apply the appropriate Safety Performance Function (SPF) for the site’s facility type and traffic control features. B. Predictive method Step 10- Apply appropriate Part C Crash Modification Factors (CMFs) to the results of the SPF to adjust the predicted average crash frequency to site-specific geometric design and traffic control features. C. Predictive method Step 11- Apply calibration factors to the results of Step 10 as appropriate for the jurisdiction and time period of the analysis. D. Predictive method Step 17- Determine if there is an alternative design, treatment, or forecast to be evaluated, and conduct alternative analysis as needed. E. Predictive method Step 18- Evaluate and compare results. To aid in the predictive method process, spreadsheet-based tools are available for use with rural two- lane, two-way roads, rural multilane highway, suburban arterials, and urban arterials.7 These tools guide users through the application of SPFs, CMFs, and calibration factors for individual sites. The Interchange Safety Analysis Tool – Enhanced is another spreadsheet tool that allows analysis of freeways, ramps and ramp terminals. Likewise, the Crash Prediction Module (CPM) of the Interactive Highway Safety Design Model (IHSDM) [www.ihsdm.org] also facilitates safety analysis of all facility types covered by the HSM. Using Crash Modification Factors (Part D of the HSM) Objective: Use crash modification factors to determine the predicted crash frequencies and number of crashes under existing and proposed conditions. CMFs are multiplicative factors relating to specific roadway treatments and their potential impact on crashes. CMFs less than one represent crash reductions while those greater than one represent potential increases in crashes. The Part D of the HSM is a catalog of treatments and associated crash modification factors when appropriate. The CMF Clearinghouse also provides a large catalog of CMFs (www.cmfclearinghouse.org/). CMFs are applied by multiplying the selected value to the existing conditions crashes or crash frequencies for a single potential treatment. The use of Part D CMFs permits the calculation of expected crashes for a broader set of proposed projects than the SPF base conditions of Part C. With the vast number of CMFs available, particularly from the CMF Clearinghouse, there are several things to consider before selecting a CMF value. A significant part 7 These tools as well as additional guidance for the HSM may be found at www.highwaysafetymanual.org/Pages/tools_sub.aspx#4.

HFG TUTORIALS VERSION 2.1 22-65 of the HSM is to realize when working with CMF’s that they are statistics without causation and often have multiple CMFs from one research study. Human Factors is the bridge to better decision- making. Often in CMF research a target crash is identified with a CMF value, but then the research bundles in “ALL Crashes” CMF, KABC Crashes bundled CMF, Wet Weather CMF, Darkness CMF, etc. What CMF should be used? Often the user picks the one that gives the best B/C number even if severe crashes are unrelated to the causation. Human Factors RSA process should be if you can’t link patterns and observed behavior to the CMF, its effectiveness should be ranked low or dismissed. Earlier RSA’s were to have multiple CMF recommendations High Cost, Medium Cost and Low Cost for an observed Safety Risk in the field. Giving the client multiple options, the easiest one with the biggest B/C number and the most roadway infrastructure improvement often gets picked. Not the most effective hard choice CMF. IDOT has incorporated linking causation to CMF in the RSA Flowchart Process. Human Factor RSA process has reduced the number of CMFs recommend but increased the confidence level of effectiveness. Example, uncoordinated signals conditioned drivers to speed to beat the next signal resulting in angle and rear-end crashes with driver at fault being from one direction. Change the behavior change the crash pattern. Signals were coordinated and timed that a consistent phasing would change the light to red well ahead of the last vehicle to beat the previous signal. The reward of speeding was removed, behavior changed and crashes from that direction were reduced. Quality- CMFs may be developed from any level of research and submitted by the researcher. The CMF Clearinghouse include a star rating quality to allow users to identify CMFs with low bias potential. Higher star ratings indicate more reliable CMFs. Human Factors reduces the trap of high quality CMF with the wrong application.  Crash type- CMFs may only be developed for a particular crash type. It is important to use a CMF that applies to the crashes being addressed at the site. It would not be appropriate to address left-turn crash with a CMF developed for rear-end crashes.  Crash severity- Similar to crash type, a particular CMF may have only been developed for a specific crash severity. It is important to use a CMF that applies to the crash severity being addressed at the site.  Area type- CMFs may be developed specifically for rural, urban, or suburban locations. It is important that the selected CMF is applicable to the site area.  Traffic volume- Not all CMFs include a traffic volume range, however, if one has been indicated by the study, it is important that the traffic volume range applies to the site being addressed. There are several additional sources of CMFs being developed and publicized. Recently, several states have developed their own lists of CMFs. These lists may have been developed for specific implementations, such as project funding prioritization, and should be thoroughly researched before using the CMF values they contain. In many situations, a practitioner may decide to implement multiple treatments at a given site. It is important to remember that CMF values represent the anticipated safety impacts of a single treatment alone. Using published CMF values is encouraged when comparing potential treatments, however, when multiple CMFs are to be used the change in crash potential is not additive. To estimate the change associated with multiple treatments at a site, the generally accepted method is to

HFG TUTORIALS VERSION 2.1 22-66 determine the combined CMF of the treatments by multiplying them together. For example, if three treatments (Treatment 1, Treatment 2, and Treatment 3) having their own representative CMFs (CMF1, CMF2, and CMF3) are being installed at Site A, the combined CMF123 would be calculated as: CMF123= CMF1 x CMF2 x CMF3 The HSM does not recommend analyzing more than three treatments at a time in this fashion. Step 3b. Identify Potential Countermeasures from the HFG Objective of Step 3b: Create a list of HFG contents (i.e., solutions, countermeasures, and design options) corresponding to design characteristics or crash reduction opportunities within the road system. The crash or conflict data compiled in Step 1, the site visit results from Step 2a, the crash or conflict data summaries from Step 2b, and the HFIM from Step 2c should have produced a comprehensive list of the:  Number and nature of the crashes or conflicts observed, including contributing factors,  Roadway and traffic engineering elements that could be associated with crashes or observed factors that might lead to crashes within the facility,  Possible road user, vehicle, and environment issues – and their interactions, and  Potentially challenging driving scenarios and road user behaviors. At this point, the HFIM results developed in Step 2 are expanded through direct use of the HFG. The HFG should be used to provide insight into the underlying crash or potential crash reduction opportunities from a road user perspective and to develop candidate solutions, countermeasures, and design options. This will include several activities. First, using the Step 2 results – and especially the HFIM – as a starting point, generate a list of keywords that can be used to characterize the nature and causes of the crashes, conflicts, and related safety outcomes. Review the Table of Contents and the Index in the HFG to identify Chapters, Guidelines, and Tutorials that seem broadly relevant to the HFIM interactions and underlying safety issues. In this regard, the .pdf version of the HFG can be manually searched (using the search tool) for each keyword to identify potentially useful HFG contents. Next, examine the individual Chapters/Guidelines/Tutorials more closely to determine whether the materials in the initial list will indeed be useful for the specific site/problems under consideration. The resulting Chapters, Guidelines, and Tutorials can be listed in a new column that can be added to the rightmost column of the HFIM (Column 5 of the HFIM; see Table 1 above). Anticipated Outcome of Step 3a: Quantitative estimation of potential impacts on crashes for each potential solution.

HFG TUTORIALS VERSION 2.1 22-67 Finally, go back to the individual Chapters/Guidelines/Tutorials cited in the HFIM and – for each candidate guidelines being considered for application – more closely examine the Design Guidelines, Discussion, and Design Issues subsections from the HFG in more detail. For each safety issue or risk listed in the HFIM, identify or list – as appropriate:8  Relevant road user needs, capabilities, or limitations,  Relevant road user perception or performance issues,  Specific HFG recommendations, countermeasures, or design options, and  Relevant data sources or research studies that could support specific design changes or enhancements. Specifically, the contributions or insights provided by individual HFG guidelines can be summarized in a simple list or table. Also, review and consider: (1) the cross references within the HFG and whether related guidelines beyond the initial list might contain useful information, (2) relevant tutorials to identify useful information, (3) trade-offs related to design and road user performance, and (4) whether the differences between the “as-built” roadway and the HFG recommendations are likely to result in safety improvements. Step 3c. Optional Site Visit After a list of potential countermeasures has been developed and quantitatively analyzed, it may be useful to return to the field or project plans and evaluate each option in preparation for Step 4- Develop and Prioritize Countermeasures. While it may be difficult to conduct, an additional field visit may be an opportunity to verify traffic and behavioral conditions noted in the initial field visit, to collect and video or photos that may have been missed initially, and to generally assess the feasibility of implementing candidate countermeasures. This optional field visit may be conducted in coordination of Step 4. Step 4- Develop and Prioritize Countermeasures Objective of Step 4: To develop a prioritized list of potential countermeasures to the safety challenges identified in Step 2 of the process. As noted above, using the HSM and the HFG together gives designers and traffic engineers a means to improve diagnostic assessment and to address the contributing factors to crashes. This should increase the effectiveness of project treatment and 8 This will be especially useful for Column 4 of the HFIM – the Interactions column. However, applying this step to the factors list for the road user, vehicle, and environmental factors individually may yield useful information as well. Anticipated Outcome of Step 3b: A succinct summary of key road user issues, specific recommendations, countermeasures, or design options from the HFG, and any related information that will support a robust safety solution.

HFG TUTORIALS VERSION 2.1 22-68 selection and improve the level of safety performance for new or upgraded roadways. However, to get the most value out of the joint use of the HSM and HFG, the options and countermeasures provided by each document must be collated, compared, and considered as part of the prioritization process. It is anticipated that the HSM portion of Step 3 will yield several countermeasure options, as well as a number of general issues to consider. When selecting countermeasures, it is important to understand the road, crash types, volume restraints, and other factors under which the CMF was developed. In some cases, looking through these factors alone may eliminate CMFs from selection. In most cases, there will still be multiple CMFs, which appear to satisfy the site conditions and project requirements. It is critical for the user to understand the circumstances under which a CMF was developed. Applying CMFs to situations different from those under which the CMF was created may render the results of the analysis inconclusive or not applicable. Sources of CMFs, like the CMF Clearinghouse and the HSM contain valuable information defining how/ where the CMF may be applied. While these sources of information are useful for initial searches, there may be added benefit in reviewing the research studies behind the CMF. The study information will often contain all the necessary information to determine how, exactly, the CMF applies to the site under investigation. When the study does not include the necessary data, it is an indication that the CMF may be less reliable than other options. Evaluation of the CMF associated with a countermeasure is important when considering the prioritization and implementation of different projects. When considering the quantitative crash potential difference of two project alternatives, which are otherwise very similar, it may be prudent to give priority to the project with the most applicable CMF. When this is done, the highest priority project represents the most realistic potential for reducing crashes and offers the higher chance for success in terms of lowering the frequency or severity of crashes. While only applicable CMFs should be used in the project analysis of Step 3, it is understood that not every CMF will be a perfect match. For more information on CMF selection and application, refer to the list of recommended trainings from the CMF Clearinghouse (www.cmfclearinghouse.org/resources_trainings.cfm). CMF applicability is also not the only measure to consider in the project prioritization process. Countermeasure options from the HSM are evaluated on several factors in addition to estimated performance. The options need to be prioritized to justify future project selection. To achieve a vetted and reliable project priority list, the following characteristics are considered for each of the previously identified potential countermeasures.  Degree of confidence- Many potential countermeasures have multiple associated crash reductions. The selection of a particular CMF or calibration factor may play an important part in determining the overall crash reduction of the treatment. In particular, it is important to consider the quality and applicability of individual research studies supporting CMFs. It is strongly recommended that each of the treatments be evaluated for degree of confidence based on the assumptions and engineering judgement used in the analysis.  Benefits- What are the benefits of the solution? Is the potential crash reduction ample enough to justify the project or project changes?  Change- Will there be a change in traffic, pedestrian, or bicycle volume at the site, which could lead to additional crashes in that mode because of increased exposure (e.g., will pedestrian traffic increase due to more reliable crossing signals or are there no expected changes at all?).

HFG TUTORIALS VERSION 2.1 22-69  Conflicts- Does the project create any built, operational, or road user problems with the existing area. E.g., will driveway entries/exits become additional elements for residents/businesses/bikes/pedestrians/less mobile individuals using the road?  Feasibility- Is the potential solution cost and time appropriate, and does it provide benefits for the expenditure? This does not necessarily include a full cost benefit analysis of each potential solution. The prioritization of the potential countermeasures from the HSM should be thorough enough to capture specific local issues and may lead to a more extensive analysis of each alternative. It may be prudent to include a wide array of stakeholders in the prioritization process to capture key issues or concerns. Once the HSM countermeasures have been prioritized, the countermeasures from both the HSM and the HFG can be combined and summarized, perhaps in a table, as an aid to the broader process of prioritizing and selecting countermeasures. There is no defined process for synthesizing the results from the both the HSM and the HFG into a set of perfect countermeasures. Prioritizing the identified countermeasures requires comparing the potential countermeasures offered by each of the sources to the details of the crash sites/situation at hand. Specific questions might include the following:  What is the agreement between the engineering countermeasures(s), the crash trends, and contributing factors identified in the diagnosis, and the human factors assessment? o Do the identified countermeasures/solutions seem to be reasonable ones given the crash data? o Do the identified countermeasures/solutions seem to be reasonable to address observed human factor driver actions that contribute to safety risk? o Do they seem to be appropriate for the site and the general driving conditions? o Will the countermeasures affect other modes of transportation negatively (transit, bicyclist, pedestrians, motorcyclist, etc.)?  How do the recommendations from each source (the HSM and the HFG) relate to or complement one another? o Is there consistency across the recommendations form the HSM and the HFG– well- supported recommendations from both sources can provide a strong argument for solutions? o Is more than a single countermeasure appropriate for the situation? o How well would multiple countermeasures address the identified factors that seem to be contributing to crashes?  Given the overall situation, what recommendations from the two sources seem the most feasible from a safety, implementation, and cost perspective user acceptance and overall benefit?

HFG TUTORIALS VERSION 2.1 22-70 Step 5- Conduct Safety Effectiveness Evaluation Objective of Step 5: To quantitatively assess the safety performance change of the completed project. The purpose of a safety effectiveness evaluation is to determine the actual impacts of a project after it has been completed at a site. This process typically includes a statistical comparison of crashes for a period before and after the project is completed using the Empirical Bayes approach. Often, the effectiveness evaluation must be conducted several years after the project is completed to allow for possible impacts to occur and for sufficient data to be obtained. Conducting an effectiveness evaluation will help guide future projects and improve project prioritization. While an evaluation of effectiveness is strongly recommended, the process to do so is a field of its own and this tutorial will not describe in detail the procedures necessary to conduct a valid and reliable safety effectiveness evaluation. Practitioners should consult addition research for guidance; potential resources include:  The Crash Modification Factor Clearinghouse recommendations for developing CMFs. www.cmfclearinghouse.org/developing_cmfs.cfm  Observational Before-After Studies in Road Safety (Hauer, 1997)  The FHWA Highway Safety Improvement Program Manual has an entire section of how to go about such studies: https://safety.fhwa.dot.gov/hsip/resources/fhwasa09029/fhwasa09029.pdf Anticipate Outcome of Step 4: A prioritized list of potential countermeasures that may be developed into actionable projects. Anticipated Outcome of Step 5: A quantitative analysis of the effectiveness of roadway changes based on recommendations from the HSM and HFG.

HFG TUTORIALS VERSION 2.1 22-71 TUTORIAL 8: USING THE HFG TO SUPPORT A ROAD SAFETY AUDIT (RSA) A valuable way to use the HFG is to use it to incorporate human factors issues and solutions into the Road Safety Audit (RSA) process. The HFG has been used in this manner in the past by a number of state DOTS and has provided a number of valuable insights and helpful solutions to improve safety solutions (see Campbell et al., 2016). The RSA is a formal safety performance examination of an existing or future road or intersection by an independent audit team (Synectics Transportation Consultants Inc., et al., 2006). An RSA provides a formal safety performance examination that qualitatively estimates and reports on potential road safety issues opportunities for safety improvements. The FHWA provides a number of helpful resources and tools for conducting RSAs here: https://safety.fhwa.dot.gov/rsa/resources/ Key issues assessed as part of an RSA include those elements of the road that may present safety concerns and the opportunities that exist to eliminate or mitigate those safety concerns. An RSA is very different from the process associated with traditional safety reviews; Table 1 below highlights some of the differences between an RSA and a traditional safety review (from Synectics Transportation Consultants Inc., et al., 2006). Table 1. Key Differences between an RSA and Traditional Safety Reviews Road Safety Audit Traditional Safety Review Performed by a team independent of the project The safety review team is usually not completely independent of the design team Performed by a multidisciplinary team Typically performed by a team with only design and/or safety expertise Considers all potential road users Often concentrates on motorized traffic Accounting for road user capabilities and limitations is an essential element of an RSA Safety reviews do not normally consider human factor issues Always generates a formal RSA report Often does not generate a formal report A formal response report is an essential element of an RSA Often does not generate a formal response report From Synectics Transportation Consultants Inc., et al. (2006), the steps in an RSA are:  Step 1: Identify Project or Existing Road to be Audited  Step 2: Select an RSA Team

HFG TUTORIALS VERSION 2.1 22-72  Step 3: Conduct a Pre-audit Meeting to Review Project Information and Drawings  Step 4: Conduct Review of Project Data and Conduct Field Review  Step 5: Conduct Audit Analysis and Prepare Report of Findings  Step 6: Present Audit Findings to Project Owner/Design Team  Step 7: Prepare Formal Response  Step 8: Incorporate Findings into the Project when Appropriate Step 1 is identifying the road or project to be audited and Step 2 involves picking the RSA team.  In Step 1, the greatest benefits to using the HFG will be obtained on RSA projects where there is some a priori reasons for believing that driver errors related to the roadway design are taking place and impacting safety performance  In Step 2, including someone with human factors experience or someone with experience using human factors design tools like the HFG is a good idea, especially if there are any specific concerns regarding road user behavior, capabilities, or limitations. Steps 3, 4, and 5 of the RSA process are where the HFG will most clearly benefit RSAs, and our discussion below focuses on those activities.  Step 3 is a pre-audit review to set the scope of the review and to share key information and documents among the RSA team. From the HFG perspective, the goal in this step is to review the relevant design information and crash history associated the facility being audited and to create an initial list of HFG topics and materials that reflects the facilities’ design features and possible safety concerns. Table 2 below provides an overview of this activity. Table 2. Using the HFG to Conduct Step 3 of the RSA – Conduct Pre-audit Review. Step in the RSA Process Ways to use the HFG Step 3: Conduct Pre-audit Review Meeting to Review Project Information and Drawings. Goal: Create initial list of HFG contents corresponding to design characteristics or safety issues within the road system. List roadway and traffic engineering elements contained within the road to be audited. Using the Glossary and the TOC, identify Chapters, Guidelines, and Tutorials that seem most relevant. Review crash data or previous safety evaluations and summarize findings. List relevant road user performance issues and corresponding sections within the HFG that might provide useful information.

HFG TUTORIALS VERSION 2.1 22-73  Step 4 includes a more detailed review of design drawings and project data, as well as a field review of the site. From the HFG perspective that seem most relevant., the goal in this step is to review the relevant design information and crash history associated the facility being audited and to create a list of HFG topics and materials that reflects the facilities’ design features and safety concerns. Table 3 below provides a sample worksheet for this activity. Table 3. Using the HFG to Conduct Step 4 of the RSA – Review Project Data and Conduct Field Review. Step in the RSA Process Ways to Use the HFG Step 4: Conduct Review of Project Data and Field Review. Goal: Identify HFG guidance corresponding to the roadway characteristics. For each roadway and traffic engineering element contained within the road to be audited, list or describe the “as-built” specification. Using the checklist of HFG materials generated previously, list HFG recommendations corresponding to each “as- built” specification. During the field review, consider driving scenarios and driver/road user behaviors, especially in light of the site-specific crash and safety data. List relevant HFG recommendations contained within individual guidelines, as well as relevant Discussion, Design Issues, and Cross References subsections. List relevant information from the Tutorials.  Step 5 includes a more detailed review of design drawings and project data, as well as a field review of the site. From the HFG perspective, the goal in this step is to review the relevant design information and crash history associated the facility being audited and to create a list of HFG topics and materials that reflects the facilities’ design features and safety concerns. Table 4 below provides a sample worksheet for this activity Table 4. Using the HFG to Conduct Step 5 of the RSA – Conduct Audit Analysis and Prepare Report. Step in the RSA Process Ways to Use the HFG Step 5: Conduct Audit Analysis and Prepare Report of Findings Goal: Assess risks between any differences between the “as-built” specifications and the HFG recommendations

HFG TUTORIALS VERSION 2.1 22-74 Step in the RSA Process Ways to Use the HFG Evaluate risks and prioritize safety concerns Are the differences between the “as-built” specifications and the HFG recommendations likely to result in safety benefits? Do the HFG materials provide other insights or countermeasures into known or correctable safety issues? Prepare report For each safety issue/risk, use the HFG to identify relevant:  Road user capabilities or limitations  Key perceptual or correctable error issues  Known trade-offs  Countermeasures or design options  Data sources or relevant studies Steps 6, 7, and 8 are the reporting and follow-up activities in the RSA (see list on page 22-72 above).

HFG TUTORIALS VERSION 2.1 22-75 TUTORIAL 9: SUMMARY OF HFG TOPICS Part III HF Guidance for Roadway Location Elements 5-1 Chapter 5: Sight Distance Guidelines 5-2 Key Components of Sight Distance The required sight distance is the sum of the distance traveled during Perception-Reaction Time (PRT) while the driver notices a hazard or situation, decides what to do, and begins a response plus the Maneuver Time (MT), during which the driver completes the response. This guideline reviews factors that affect PRT and MT. 5-4 Determining Stopping Sight Distance Stopping sight distance (SSD) is the sum of the time required for a driver to notice a need to stop (such as an object in the road), to decide to stop, and to bring the vehicle to a stop. This guideline provides standard formulas for calculating stopping sight distance, and discusses how human factors reduces typical performance below what is optimally possible. 5-6 Determining Intersection Sight Distance This guideline explains how the sight distance required at an intersection depends on how the intersection is controlled (e.g., by signals or signs) and by the intended maneuver. It provides references to formulas in the AASHTO green book for selecting the proper sight distance. 5-8 Determining When to Use Decision Sight Distance Decision sight distance (DSD) represents a longer sight distance than is usually necessary. This guideline provides examples and formulas for complicated or non-standard highway situations in which drivers might require extra distance to make a decision. 5-10 Determining Passing Sight Distance Passing sight distance (PSD) is how far ahead a driver must be able to see to successfully complete a passing maneuver. This guideline provides the design values for passes made at different speeds provided in AASHTO and summarizes the research behind them. 5-12 Influence of Speed on Sight Distance The design of a road affects drivers’ speeds, which in turn affect required sight distance. This guideline quantitatively discusses how features such as lane width and pavement surface affect the operating speed. 5-14 Key References for Sight Distance Information Sight distance issues have been covered extensively in a range of standard references. This guideline points to chapters in six standards where traffic engineers can find primary sources of sight distance information. 5-16 Where to Find Sight Distance Information for Specific Roadway Features This guideline cites references for calculating the sight distance for eight specific non-intersection features. 5-18 Where to Find Sight Distance Information for Intersections This guideline cites references for calculating the sight distance for nine types of intersections. 6-1 Chapter 6: Curves (Horizontal Alignment) 6-2 Task Analysis of Curve Driving This guideline identifies the basic activities that drivers typically perform while navigating a single horizontal curve. It explains how design aspects such as consistency, curvature, and lane width affect the driver’s workload, and where and how driving tasks should be made easier to perform. 6-4 The Influence of Perceptual Factors on Curve Driving Visual information around a horizontal curve can cause drivers to perceive the radius to be different from the actual radius. This guideline provides graphs of recommended combinations of horizontal curvature and vertical sag curvature, and discusses the effects of vertical crests, cross slope, and other features. 6-6 Speed Selection on Horizontal Curves Drivers’ speed selection on horizontal curves reflects a variety of vehicle, driver, and roadway factors. Use this guideline in combination with the previous one to learn about factors that affect how drivers select their speed.

HFG TUTORIALS VERSION 2.1 22-76 6-8 Countermeasures for Improving Steering and Vehicle Control Through Curves This guideline describes how to select curve geometries that help drivers maintain proper lane position, speed, and lateral control through curves. It includes quantitative guidance on curvature, spiral length, and reverse curves. 6-10 Countermeasures to Improve Pavement Delineation This guideline describes how pavement markings can help driver performance in curves maneuvers. It includes guidance on edge and center lines, raised reflective pavement markers, and markers on signs. 6-12 Signs on Horizontal Curves The key to effective curve negotiation is to notify the driver of the upcoming curve so that the driver can change the speed or path of the vehicle. This guideline summarizes research results on sign placement, chevrons, flashers, and dynamic message signs. 7-1 Chapter 7: Grades (Vertical Alignment) 7-2 Design Considerations for Turnouts on Grades Turnouts are widened, unobstructed shoulder areas that allow slow-moving vehicles to pull out of the through lane to give passing opportunities to following vehicles. This guideline provides design recommendations for use of signs, sight distance, entry speed, and exits. 7-4 Geometric and Signing Considerations to Support Effective Use of Truck Escape Ramps The AASHTO Green Book has comprehensive guidance for the design and location of emergency escape ramps. This guideline emphasizes its key aspects and adds guidance from additional sources. 7-6 Preview Sight Distance and Grade Perception at Vertical Curves Preview sight distance applies to horizontal curves near the top of crest vertical curves or at the bottom of sag vertical curves, where the horizontal curve is initially out of the driver’s line of sight. This guideline explains the need for preview sight distance and how to calculate it. 8-1 Chapter 8: Tangent Sections and Roadside (Cross Section) 8-2 Task Analysis of Lane Changes on Tangent Sections In this guideline, the perceptual, cognitive, and psychomotor tasks before and during a lane change are identified and analyzed; workload levels for various activities are estimated. 8-4 Overview of Driver Alertness on Long Tangent Sections Fatigue is a combination of boredom, sleep disruption, and other factors that reduce vigilance. Although data are not definitive on the relationship between length of a tangent and the fatigue-related crash risk, this guideline explains how to break the monotony, such as adding visual complexity, or provide countermeasures, such as shoulder rumble strips. 9-1 Chapter 9: Transition Zones Between Varying Road Designs 9-2 Perceptual and Physical Elements to Support Rural-Urban Transitions This guideline deals with transitions between rural and more densely settled areas. Use it to understand how to use treatments beyond signs to slow vehicles for a town along an otherwise rural highway. 10-1 Chapter 10: Non-Signalized Intersections 10-2 Acceptable Gap Distance The acceptable gap is the minimum vehicle-to-vehicle time typically accepted by drivers turning from a minor road to a major road. This guideline gives values and rationales for the gaps required by different vehicles for left and right turns. 10-4 Factors Affecting Acceptable Gap Driver age, wait time, direction of the turn, familiarity with the roadway, size of the oncoming vehicle, and headlight glare all affect acceptable gap size. This guideline discusses these factors and also includes a task analysis of the steps of performing a turn across traffic onto a four-lane highway. 10-6 Left-Turn Lanes at Non-Signalized Intersections This guideline explains how left-turn lane length, storage length, and lane offset can affect crash risk. It provides principles for when to install a left-turn lane, overall turn lane length, and storage lengths for reducing rear-end crash risk. It also provides information about how lane effective lane offset can improve sight distance to oncoming traffic.

HFG TUTORIALS VERSION 2.1 22-77 10-8 Sight Distance at Left-Skewed Intersections Drivers at these intersections need to look backwards over their right shoulder and past parts of their own vehicle. This guideline presents the geometry and tables to calculate the available sight distance from the skew angle, set back distance, and properties of typical vehicles. 10-10 Sight Distance at Right-Skewed Intersections Vision of drivers at these intersections is limited by their ability to turn their eyes and neck to look backwards over their left shoulder. This guideline presents the geometry and tables to calculate the available sight distance from the skew angle and set back distance, with special considerations for the restricted movement of older drivers. 10-12 Countermeasures for Improving Accessibility for Vision-Impaired Pedestrians at Roundabouts Roundabouts are a challenge for vision-impaired pedestrians, due to the absence of cues that they typically use for safe navigation. This guideline discusses the efficacy of several countermeasures that have been investigated to address this problem. 11-1 Chapter 11: Signalized Intersections 11-2 Engineering Countermeasures to Reduce Red Light Running This guideline lists countermeasures regarding traffic characteristics, signal operations, and motorist information that have been shown to reduce red light running. 11-4 Restricting Right Turns on Red to Address Pedestrian Safety This guideline describes when right turn on red movements need to be restricted to protect pedestrians. It summarizes research on signage and geometry treatments improve safety. 11-6 Heuristics for Selecting the Yellow Timing Interval This guideline contains formulas for calculating the duration of the yellow interval and the red clearance time. It also discusses the decisions and actions of drivers faced with the “dilemma zone.” 11-8 Countermeasures for Improving Accessibility for Vision-Impaired Pedestrians at Signalized Intersections Accessible pedestrian signals and curb treatments improve the ability of vision-impaired pedestrians crossing at signalized intersections. Use this guideline to identify recommendations for the location and design of curb ramps, signal timing, and push buttons. 12-1 Chapter 12: Interchanges 12-2 Task Analysis of Driver Merging Behavior at Freeway Entrance Ramps This guideline lists the steps followed by a driver while merging to a freeway and discusses the challenges of each step. Differences between light and heavy traffic and between older and younger drivers are noted. 12-4 Reducing Wrong-Way Entries onto Freeway Exit Ramps This guideline covers treatments to reduce the frequency of drivers entering freeways by using the exit ramps. It can be used to identify appropriate countermeasures for visibility, signing, and road geometry. 12-6 Driver Expectations at Freeway Lane Drops and Lane Reductions Lane drops may violate driver expectations and cause confusion when the driver expects the lane to continue. This guideline describes visual, geometric, and signing principles to prepare drivers for lane drops and reductions. 12-8 Driver Information Needs at Complex Interchanges Complex interchanges should be designed to give drivers the information they need and expect. Use this guideline to identify specific features of geometric elements, signing, and sight distance that can be used to minimize violations of driver expectations. 12-10 Arrow-per-Lane Sign Design to Support Driver Navigation Destination information and sign design must allow drivers to pair the destination with an arrow that points to a particular lane. This guideline helps designers avoid pitfalls associated with sign placement and how information is presented on a sign. 12-12 Driver Behavioral Trends Based on Exit Ramp Geometry Well-designed exit ramps support the intended behaviors of an exiting driver. This guideline itemizes the steps a driver performs in a safe exit and discusses how drivers use the available taper and deceleration lane.

HFG TUTORIALS VERSION 2.1 22-78 13-1 Chapter 13: Construction and Work Zones 13-2 Overview of Work Zone Crashes This guideline characterizes work zone crashes and provides a framework for work zone design. It specifies the need for additional driver guidance in work zones based on the number, type and severity of crashes occurring in work zones. 13-4 Procedures to Ensure Proper Arrow Panel Visibility Arrow panels are used ahead of work zones to inform drivers of the need to move out of a closed lane. This guideline has values for the use, intensity, and position of flashing arrow panels. 13-6 Caution Mode Configuration for Arrow Panels This guideline provides recommendations for how to use the non-directional Caution Mode of an arrow panel configuration during temporary traffic control. It summarizes research on the shape of the flashing symbol. 13-8 Changeable Message Signs This guideline has principles for keeping messages terse and legible so their information can be quickly processed by drivers. 13-10 Sign Legibility A number of design characteristics of work zone signs contribute to drivers’ ability to perceive and understand a sign’s message. This guideline explains factors that determine signs’ legibility including retroreflectivity (sheeting type), color, letter font, and location (roadside or overhead). 13-12 Determining Work Zone Speed Limits Speed limits are reduced in work zones to maintain safe traffic flow. This guideline provides results from crash studies on what speed limit reductions are most desirable. It also discusses how lane width, speed displays, and the amount of reduction itself affect the mean and variance of actual traffic speed. 14-1 Chapter 14: Rail-Highway Grade Crossings 14-2 Task Analysis of Rail-Highway Grade Crossings This guideline addresses the key factors found to affect driver decisions whether to obey control devices at grade crossings. Many of the factors are covered in more detail in the following guidelines. 14-4 Driver Information Needs at Passive Rail-Highway Grade Crossings This guideline covers the information that drivers need at grade crossings not protected by gates or lights. It explains what is required beyond the traditional crossbuck. 14-6 Timing of Active Traffic Control Devices at Rail-Highway Grade Crossings This guideline has quantitative recommendations on the time that should elapse between the initiation of the flashing light and the arrival of the train. It discusses driver expectations and behaviors observed at crossings for a range of delays. 14-8 Four-Quadrant Gate Timing at Rail-Highway Grade Crossings This guideline discusses the time interval between the entrance gate’s beginning to descend and the exit gate’s beginning to descend. Use it to find formulas and standards to calculate the interval. 14-10 Countermeasures to Reduce Gate-Rushing at Crossings with Two-Quadrant Gates This guideline discusses countermeasures against driving under or around gates at grade crossings. They include physical barriers (four-quadrant gates or centerline barriers prior to the gate), engineering changes to improve the credibility of warning devices, and wayside horns. 14-12 Human Factors Considerations in Traffic Control Device Selection at Rail-Highway Grade Crossings This guideline discusses factors to consider when selecting a yield sign, stop sign, or active control at a grade crossing. 15-1 Chapter 15: Special Considerations for Urban Environments 15-2 Methods to Increase Driver Yielding at Uncontrolled Crosswalks Uncontrolled crosswalks are those without regular signals to control traffic, though they might have a pedestrian- actuated half signal or Pedestrian Hybrid Beacon. Use this guideline to learn how to improve sight lines and convey to drivers the need to look for pedestrians.

HFG TUTORIALS VERSION 2.1 22-79 15-4 Methods to Increase Compliance at Uncontrolled Crosswalks This guideline discusses several treatments available for uncontrolled crosswalks and provides statistics for driver and pedestrian compliance with each. 15-6 Methods to Reduce Driver Speeds in School Zones Traffic control devices and pavement markings are used to encourage drivers to slow for school zones. This guideline summarizes research on a variety of methods for reducing speeds. 15-8 Signage and Markings for High Occupancy Vehicle (HOV) Lanes Signage is necessary to inform drivers of lanes in congested areas that are reserved for HOVs. This guideline summarizes research on how to word and place signs and markings to maximize understanding by drivers and minimize crashes. 15-10 Sight Distance Considerations for Urban Bus Stop Locations Bus stops can be located immediately before an intersection (near-side stop), after an intersection (far-side stop) or mid-block. This guideline discusses relative advantages and disadvantages of each location with respect to sight distance and pedestrian traffic conflicts. 16-1 Chapter 16: Special Considerations for Rural Environments 16-2 Passing Lanes A passing lane is a lane added in one or both directions of travel on a two-lane, two-way highway to improve passing opportunities. Recommended lengths and spacing of passing lanes are provided for various conditions, and signage is discussed. 16-4 Countermeasures for Pavement/Shoulder Drop-offs This guideline provides maximum vertical drop-off heights as a function of lane width. If vertical pavement edges are less than the recommended heights or if they are beveled as recommended, then drivers who inadvertently drift over the edge and on to the shoulder can more safely return to the roadway. 16-6 Rumble Strips Shoulder rumble strips are a narrow line of indentations in the pavement immediately outside the traveled way, alerting drowsy drivers who drift out of their lane. This guideline lists levels of various sounds in a vehicle and recommends values for the shape and location of rumble strips. 16-8 Design Consistency in Rural Driving Drivers generally make fewer errors when geometric features are predictable and consistent with their expectations. A list of design factors that should be considered during a design consistency review is presented. 17-1 Chapter 17: Speed Perception, Speed Choice, and Speed Control 17-2 Behavioral Framework for Speeding This guideline provides an overview of the key factors relevant to speed selection. It includes a comprehensive list of the many situational, demographic, and environmental factors that can contribute to drivers’ decisions about speeding, and identifies the many studies that have aided our understanding of speeding. 17-4 Speed Perception and Driving Speed While a direct measure of speed is available from the speedometer, drivers rely on a number of cues for their sense of speed. This guideline discusses how various cues can cause drivers to overestimate or underestimate their own or another vehicle’s speed. 17-6 Effects of Roadway Factors on Speed Geometric factors affect the free-flow speed. Use this guideline to learn which factors have proven effects on speed on rural highways and urban streets. 17-8 Effects of Posted Speed Limits on Speed Decisions Advisory speeds have at best a modest effect on speed, particularly for drivers familiar with the road. This guideline discusses light-vehicle driver compliance with posted speed limits on non-limited access rural and urban highways. 17-10 Speeding Countermeasures: Setting Appropriate Speed Limits This guideline discusses how to set an appropriate speed limit accounting for the unique traffic, design, and environmental aspects of a roadway. It includes standard conditions, variable speed limits, and heavy truck traffic.

HFG TUTORIALS VERSION 2.1 22-80 17-12 Speeding Countermeasures: Communicating Appropriate Speed Limits This guideline discusses best practices for communicating posted speed limits to drivers and explains when to use approaches such as redundant signs, active speed warning, and in-pavement measures. 17-14 Speeding Countermeasures: Using Roadway Design and Traffic Control Elements to Address Speeding Problems Geometric elements and traffic control devices both affect speed-related crashes. Like the others in this chapter, this guideline should be used in consultation with the original NCHRP reports for a full appreciation of the factors. Part III HF Guidance for Traffic Engineering Elements 18-1 Chapter 18: Signing 18-2 General Principles for Sign Legends Sign legends (the text and symbols on a sign) must be short and simple to maximize driver comprehension. Use this guideline for information on the content of common signs and on the distance ahead of a decision point where a sign should be placed. 18-4 Sign Design to Improve Legibility Signs must be designed so that the text is legible. This guideline summarizes research on characteristics including the color, size, and style of letters, with special considerations for older drivers and nighttime visibility. 18-6 Conspicuity of Diamond Warning Signs under Nighttime Conditions A critical factor in the driver’s ability to see, locate, and comprehend warning signs at night is to maximize sign conspicuity relative to surrounding background elements. This guideline provides desirable characteristics about the sign itself and its environment to improve its conspicuity. 18-8 Driver Comprehension of Signs There are three stages of comprehension—legibility, recognition, and interpretation. This guideline explains when to use text, icons, and a combination of the two, and other ways to maximize comprehension. 18-10 Complexity of Sign Information The complexity of sign information refers to the number of information units (words or numbers) in the message on a roadway sign. This guideline gives comprehension rates and reading times for messages of different complexity and explains when short (sometimes only one word) messages are important. 19-1 Chapter 19: Changeable Message Signs 19-2 When to Use Changeable Message Signs Changeable message signs (CMSs) allow for the display of time-sensitive or temporary information that affects travel and, in many cases, requires drivers to take an action. This guideline provides general principles for when to use CMSs and how to maintain their credibility. 19-4 Presentation to Maximize Visibility and Legibility This guideline discusses lighting and shape characteristics of changeable message signs to maximize their readability. It contains the results of research on the contrast ratio, luminance, character spacing, and dot matrix size, noting differences for older drivers and nighttime conditions. 19-6 Determining Appropriate Message Length Messages must be short enough that drivers can comprehend them in the limited time they have while passing the sign. This guideline includes recommendations and quantitative research on the length, content, and arrangement of messages and placement of the changeable message sign. 19-8 Composing a Message to Maximize Comprehension The way a message is worded and formatted can significantly affect the ability of a driver to comprehend it. Guidance on abbreviations, date format, and word choice is provided. 19-10 Displaying Messages with Dynamic Characteristics The guideline has formulas for the timing of messages and the blank intervals between them. It also explains how some attempts to draw drivers’ attention can increase reading time.

HFG TUTORIALS VERSION 2.1 22-81 19-12 Changeable Message Signs for Speed Reduction Changeable message signs can be used to alert drivers to the need to reduce their speed for temporary conditions such as work zones, adverse weather, incidents, or heavy congestion. This guideline provides principles on the wording and placement of changeable message signs to achieve speed reductions. 19-14 Presentation of Bilingual Information Bilingual information may be required in areas with culturally diverse populations or heavy international tourism; however, bilingual signs must be used cautiously because they can increase the reading time of both monolingual and bilingual drivers. This guideline discusses ways to distinguish the two languages. 20-1 Chapter 20: Markings 20-2 Visibility of Lane Markings Lane markings help drivers align their vehicle with the lane. This guideline discusses preview time and the use of peripheral vision, the required luminance of the markings, and the width of the stripe. 20-4 Effectiveness of Symbolic Markings Horizontal signing is sign text painted on the roadway. This guideline explains how these markings can alert drivers to speed reductions for horizontal curves or to impending lane drops, and prevent wrong-way movements. 20-6 Markings for Pedestrian and Bicyclist Safety These markings encourage safe practices of motor vehicles, pedestrians, and bicyclists. The guideline has a table for which roads are candidates for marked crosswalks, and it summarizes research on markings for shared bicycle lane. 20-8 Post-Mounted Delineators Post-mounted delineators are a series of retroreflective marking devices above the pavement surface to indicate alignment. They are useful when the alignment might be confusing or unexpected. This guideline provides recommendations for delineator use, including spacing. 20-10 Markings for Roundabouts This guideline covers pavement markings at the entrances and exits of roundabouts. Refer to it for suggestions on lighting, pavement marking, spacing, stopping sight distances, and accommodations for bicyclists and pedestrians. 21-1 Chapter 21: Lighting 21-2 Countermeasures for Mitigating Headlamp Glare Glare occurs when the intensity of a light source is greater than the adaptation level to the surrounding view. This guideline discusses approaches to reducing glare from other vehicles’ headlamps. 21-4 Nighttime Driving Visibility at night in rural areas is limited by the lack of ambient light, the reach of headlamps, and the minimal contrast of persons and objects in the roadway. This guideline lists the respective benefits and suggested conditions for using seven treatments, ranging from continuous lighting to advance warning signs. 21-6 Daytime Lighting Requirements for Tunnel Entrance Lighting Lighting at tunnel entrances requires special consideration because of the contrast between illumination inside the tunnel and the surroundings. This guideline provides recommendations for minimum lighting, the spacing and direction of lights within the tunnel, and treatments outside the entrance. 21-8 Countermeasures for Improving Pedestrian Conspicuity at Crosswalks This guideline contains information on the effective use of flashing lights mounted to a crosswalk sign, the pole supporting the sign, or the pavement. 21-10 Characteristics of Lighting that Enhance Pedestrian Visibility This guideline addresses the positioning and spectral contentment (i.e., kind of light) of luminaires to enhance the visibility of pedestrians. The discussion includes colors of clothing and function of the eye. 21-12 Characteristics of Effective Lighting at Intersections This guideline provides principles for improving nighttime visibility of pedestrians, vehicles, roadway features, and obstacles at intersections. It recommends illuminance values and luminaire placement.

HFG TUTORIALS VERSION 2.1 22-82 28-1 Chapter 28: Pedestrians 28-2 Task Analysis of Pedestrian Crossing in a Multiple Threat Scenario In the multiple threat scenario, a pedestrian crossing in front of a stopped vehicle is at risk of being struck by a second vehicle traveling in the adjacent lane. This guideline shows the interactions that can take place between pedestrians and drivers and countermeasures for reducing conflicts in this scenario. 28-4 Countermeasures to Reduce Pedestrian Exposure to Vehicles at Crossings This guideline addresses road treatments that provide some level of physical protection from surrounding traffic and/or reduce the time required to cross the street. It summarizes the conditions of use and the effects that can be expected when using these countermeasures. 28-6 Speed-Calming Countermeasures at Crosswalks Excessive vehicle speed puts pedestrians at risk because it is harder for drivers to perceive, react, and stop for pedestrians. This guideline provides information about common countermeasures for reducing vehicle speeds when approaching crosswalks. 28-8 Improving Pedestrian Visibility and Conspicuity at Crosswalks This guideline provides countermeasures that increase driver awareness of the crosswalk, draw attention to pedestrians, and make pedestrians easier to detect. It emphasizes countermeasures that improve lines of sight between vehicles and pedestrians and focuses drivers’ attention on the crosswalk. 28-10 Selecting Beacons to Improve Pedestrian Conspicuity at Crosswalks Pedestrian Hybrid Beacons (PHB) or Rectangular Rapid Flashing Beacons (RRFB) are effective countermeasures for alerting drivers to the presence of pedestrians in the crosswalk. This guideline provides a heuristic for selecting between PHBs or RRFBs at an uncontrolled, marked mid-block crosswalk. 28-12 Influence of the Built Environment on Pedestrian Crossing Safety Characteristics of the built environment contribute to the likelihood of a pedestrian-vehicle crash. This guideline highlights factors in the surrounding environment that have the largest influence on safety and provides recommended countermeasures. 28-14 Design Challenges for Older Pedestrians This guideline contains special considerations for designing pedestrian crossings that help older pedestrians with physiological and/or cognitive, age-related impairments to cross the street in a timely manner. These pedestrians face a diverse set of challenges, in terms of roadway safety, that may not be fully addressed by standard design practices. 28-16 Pedestrian Rail Crossing Safety Though rare, pedestrian-rail crashes are generally caused by pedestrians ignoring warnings, possibly due to inattentiveness, lack of situational awareness, or direct disobedience in order to catch a train. This guideline provides potential countermeasures to address many different crossing situations. 28-18 Key References for Pedestrian Crossing Safety Countermeasures There are numerous resources available to aid in the selection of countermeasures at pedestrian crossing locations that experience an unacceptable number of crashes. This guideline summarizes key resources available for finding information about countermeasures that improve pedestrian safety. 29-1 Chapter 29: Bicyclists 29-2 Signals And Signal Timing For Bicycles at Intersections Design for traffic signals at intersections are generally determined by motor vehicle use, which creates problems for bicyclists because they have different operating characteristics and needs. This guideline discusses signal timing and design countermeasures that can minimize conflicts with motor vehicles. 29-4 Markings for Bicycles at Intersections This guideline discusses intersection lane marking countermeasures that can minimize conflicts with motor vehicles. Bicyclists are particularly vulnerable at intersections because they are less visible due to their size, and crashes can be severe because of lack of protective structures and low mass relative to motor vehicles. 29-6 Bicycle Lanes Designated bicycle lanes can encourage safety by separating motor vehicles from bicycles and increase motor vehicle drivers’ awareness by indicating where they can expect bicyclists to be traveling. This guideline discusses methods for maximizing safety and reducing potential traffic impacts when designing bicycle lanes.

HFG TUTORIALS VERSION 2.1 22-83 29-8 Separated Bicycle Lanes Separated bicycle lanes are one or two-way exclusive bikeways parallel to the roadway yet physically separated from moving traffic. They can provide increased safety and comfort for bicyclists in areas with higher traffic volumes and speeds. This guideline discusses strategies for implementing separated bike lanes. 29-10 Contraflow Bicycle Lanes Contraflow bicycle lanes are installed on the left side of a one-way street in situations where travel in a with-flow bike lane would result in substantial out-of-direction travel or reduced access. This guideline discusses methods for maximizing bicyclist safety and reducing potential traffic conflicts when designing contraflow bicycle lanes. 29-12 Shared Use Lanes This guideline discusses methods for maximizing safety and reducing potential traffic impacts when designing shared use bicycle lanes. When shared use lanes are supplemented with shared lane markings, bicycle traffic on the street is legitimized and motor vehicle drivers are alerted to the potential presence of bicyclists. 29-14 Shared Bus-Bicycle Lanes Increasingly, cities across the U.S. are implementing shared bus-bicycle lanes to improve multimodal mobility. This guideline provides recommendations for designing bus-bicycle shared lanes that can help mitigate safety challenges associated with differences and interactions between buses and bicycles, particularly at and around bus stops. 29-16 Mitigating Heavy Vehicle Conflicts with Bicycles Heavy vehicle blind zones and reliance on mirrors for visibility make it difficult for heavy vehicle drivers to see bicyclists, while bicyclists are often unaware of these visibility challenges and engage in unsafe maneuvers. This guideline provides recommendations for road designs that help to mitigate heavy vehicle conflicts with bicycles. 30-1 Chapter 30: Roundabouts 30-2 Reducing Vehicle Speeds Approaching Roundabouts Reducing vehicle speeds approaching roundabouts provides more time for drivers to react to the presence of pedestrians and bicyclists crossing at or traveling through the roundabout and may reduce the incidence of run-off- road crashes. This guideline provides countermeasures for reducing vehicle speeds at roundabouts. 30-4 Increasing Driver Yielding Rates for Pedestrians at Roundabouts Because of competing tasks, attentional demands, and limited sight distance, drivers are not always aware of crossing pedestrians when the vehicle is entering or exiting the roundabout. This guideline describes countermeasures that can increase driver yielding rates for pedestrians at roundabouts. 30-6 Guide Signing at Roundabouts Advance-destination guide signs assist drivers in determining, prior to entering the roundabout, which lane to travel in and where to exit the roundabout, allowing them to focus more attention on the driving task and less attention on wayfinding. This guideline describes designing for guide signs that effectively presents this information to drivers. 30-8 Accommodations for Bicyclists at Roundabouts Vehicle-bicycle crashes generally involve a vehicle entering or exiting a roundabout where cyclists are circulating. Reducing speeds and improving bicyclist conspicuity can reduce these crashes. This guideline describes strategies for implementing bicycle lanes and lane markings to improve visibility and conspicuity of bicyclists in roundabouts. 30-10 Countermeasures for Improving Accessibility for Visually Impaired Pedestrians at Roundabouts This guideline identifies countermeasures for improving accessibility for visually impaired pedestrians at roundabouts. It describes the implementation of auditory cues and tactile features that support traversing crosswalks at roundabouts without visual information. 30-12 Roundabout Lighting Lighting is used both to enhance the visibility of a roundabout from a distance and to improve the perception of the layout and visibility of other users for the driver during hours of low light and darkness. This guideline describes when, where, and how to effectively implement lighting strategies into roundabout designs.