Comprehensive Human Factors Guidelines for Road Systems (Web-Only Document) (2005) / Chapter Skim
Currently Skimming:
Pages 54-133
The Chapter Skim interface presents what we've algorithmically identified as the most significant single chunk of text within every page in the chapter.
Select key terms on the right to highlight them within pages of the chapter.
From page 54... ...
APPENDIX C HUMAN FACTORS GUIDELINES FOR ROAD SYSTEMS: DESIGN AND OPERATIONAL CONSIDERATIONS FOR THE ROAD USER DRAFT EXAMPLE CHAPTER CHAPTER 5 From Driver Reaction Time, Maneuver Time, and Speed to Design Distances: General Guidelines C-1
|
From page 56... ...
5.4 DIAGNOSING SIGHT DISTANCE PROBLEMS ............................................................... C-50 5.4.1 The Six-step Process..........................................................................................
|
From page 57... ...
5. From Driver Reaction Time, Maneuver Time, and Speed to Design Distances: General Guidelines This section of the document, preceding Section 5.1, provides an introduction to a draft sample chapter of the Human Factors Guidelines ("HFG")
|
From page 58... ...
Table 1. Proposed Parts and Chapters for the HFG (from Lerner et al., 2002)
|
From page 59... ...
Based on the preliminary outline of the planned HFG, this chapter on sight distance is labeled Chapter 5. Chapter numbering is likely to change as the actual HFG evolves.
|
From page 60... ...
curves, transition zones, work zones)
|
From page 61... ...
5.1.2.2 COMPONENT DETERMINANTS OF SIGHT DISTANCE REQUIREMENTS There are several major components that together determine the sight distance that the driver requires to safely execute a maneuver. Perception-reaction time (PRT)
|
From page 62... ...
In summary, sight distance requirements are jointly determined by PRT, MT, and speed selection. All of these are sensitive to a range of human factors considerations.
|
From page 63... ...
Sight distance formulas for various maneuvers (presented in Section 5.2) differ from one another, but they share a common simple behavioral model as part of the process.
|
From page 64... ...
Figure 5.2. Diagrammatic version of the basic sight distance model The figure shows the linear chain of steps that comprise the PRT, and after the PRT is complete, the execution of the selected driving maneuver.
|
From page 65... ...
Figure 5.3 Added components to basic sight distance behavioral model In the figure, two additional components to the model are shown prior to the event becoming visible. One component is labeled "cognitive preparation." This is a general term to encompass various active mental activities that can influence response times and decisions.
|
From page 66... ...
• Information processing [Section 4.3]
|
From page 67... ...
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.
|
From page 68... ...
• Empirical findings. The values used in design equations may or may not derive from good empirical sources.
|
From page 69... ...
ITE Traffic Control Devices Handbook (2001) • Chapter 2, Human Factors, has sections on driver perception reaction time, maneuver time • Chapter 11, Highway-Rail Grade Crossings, contains discussion of sight distance requirements for at-grade crossings Highway Safety Manual (under development)
|
From page 70... ...
o Under unfavorable conditions • Guideline for MT o Under baseline conditions o Under unfavorable conditions • Rationale for Guideline • Summary Table Definition: Each sight distance definition is taken from the AASHTO Policy on Geometric Design of Highways and Streets (2001)
|
From page 71... ...
SSD is defined as follows: Metric US Customary V2 SSD = 0.278 VtPRT + 0.039 A V2 SSD = 1.47 VtPRT + 1.075 A where: tPRT = brake reaction time, 2.5 s V = design speed km/h A = deceleration rate, m/s2 where: tPRT = brake reaction time, 2.5 s V = design speed, mph A = deceleration rate, ft/s2 The current AASHTO value for PRT is 2.5 seconds. MT assumes that drivers are 100% efficient in braking, i.e., locked wheel braking, and that pavement friction is very poor.
|
From page 72... ...
STOPPING SIGHT DISTANCE: PERCEPTION-REACTION TIME GUIDELINE Under baseline conditions: Most reasonably alert drivers (95%) will be able to initiate braking within PRT of 1.6 s.
|
From page 73... ...
5.2.1.4 BASIS/RATIONALE FOR SSD GUIDELINE SSD PRT Stopping sight distance PRT has been addressed in a variety of experimental studies. The principal studies include the following: • Daytime PRT for clearly visible hazard placed on the road (Olson, Cleveland, Fancher, & Schneider, 1984)
|
From page 74... ...
• Nighttime on-road study comparing detection distance for alerted and unalerted drivers (Roper & Howard, 1938) In a daytime study conducted for the purposes of assessing the established AASHTO value for PRT in a stopping sight distance situation, Olson et al.
|
From page 75... ...
constant deceleration was 0.55 g, and the 85th percentile, 0.48 g. When research participants used test vehicles, they decelerated more rapidly than they did in their own vehicles, with an equivalent constant deceleration that was about 14% higher.
|
From page 76... ...
5.2.2 Intersection Sight Distance 5.2.2.1 DEFINITION: ISD Intersection sight distances (ISD) are the minimum sight distances required for drivers to safely negotiate intersections, including those with no control, stop control and signals, and including those for drivers turning left, right and going straight through.
|
From page 77... ...
Metric US Customary ISD = 0.278 Vmajortg ISD = 1.47 Vmajortg where: ISD = intersection sight distance (length of the leg of sight triangle along the major road (m) Vmajor = design speed of major road (km/h)
|
From page 78... ...
• Complex pavement markings (multiple turn lanes) • Complex or atypical intersection geometry • Visual clutter in urban areas due to commercial lighting • A high percentage of older drivers.
|
From page 79... ...
INTERSECTION SIGHT DISTANCE: PERCEPTION REACTION TIME GUIDELINES Under baseline conditions (based on Lerner et al., 1995) the median PRT is about 1.3 sec, and the 85th percentile PRT is about 2.0 sec.
|
From page 81... ...
INTERSECTION SIGHT DISTANCE: TIME GAP GUIDELINE Under baseline conditions: The 85th percentile gap accepted by left turning passenger car drivers, including substantial numbers of older drivers, is 11 sec. This is more than a 50th percentile gap for single unit trucks and close to a 50th percentile gap for double unit trucks.
|
From page 82... ...
Basis/Rationale ISD PRT, MT and Critical Gap The separate components of driver behavior on which ISD depends are difficult to define precisely because drivers generally start the search process while stopping at an intersection and continue their search as they move forward, ready to abandon the maneuver. Thus PRT overlaps MT; it is not a serial process when the driver is stopped as had been traditionally defined by AASHTO.
|
From page 83... ...
ISD MT MT for turning movements was determined to have ended at the point where the driver's vehicle was oriented parallel to the major roadway (as opposed to the AASHTO definition of the end of the maneuver being when the driver has reached 85% of the major road speed)
|
From page 84... ...
• By female drivers (accept gaps that are 1 sec longer than those accepted by male drivers, but difference is mainly in daytime) • For left and right maneuvers as compared with through maneuvers • Under daytime conditions (by about 1.5 sec)
|
From page 85... ...
distance is set at the value that is accepted 50% of the time by a large group of drivers, it may not be that desired by a group of older drivers, whose maneuver times are longer than those for young drivers, and who would willing have pull out onto a major road with a gap of only 7.5 sec. Furthermore, the critical gaps accepted by drivers of single or double unit left turning trucks are substantially longer (by 1.6-1.8 sec for single unit trucks, and by 2 sec for double unit trucks)
|
From page 86... ...
SUMMARY: ISD Cases C1 and C2 PRT Factors PRT MT Factors MT CRIT GAP Factors 85th Accepted GAP AASHTO Driver age, gender 2.0 sec Driver age 6.3 sec Driver age 11 sec (left) 8.0 (left)
|
From page 87... ...
The decision sight distance for avoidance maneuvers C, D and E are determined as: Metric US Customary d = 0.278VtPRT+MT d = 1.47VtPRT+MT where: tPRT+MT = pre-speed/path/direction change maneuver time: 10.2 – 11.2 sec rural 12.1 – 12.9 sec suburban 14.0 – 14.5 sec urban V = design speed, km/h where: tPRT+MT = pre-speed/path/direction change maneuver time: 10.2 – 11.2 sec rural 12.1 – 12.9 sec suburban 14.0 – 14.5 sec urban V = design speed, mph In computing and measuring DSD the same 1.08 m (3.5 ft.) eye-height and 0.6 m (2.0 ft)
|
From page 88... ...
speed reduction for turns. Where lane changes are required, PRT includes time for drivers to search for a gap in traffic.
|
From page 89... ...
DECISION SIGHT DISTANCE: PERCEPTION REACTION TIME – MANUEVERS C, D AND E GUIDELINES Under baseline conditions it can be assumed that the driver is responding to either signs or markings. Under baseline conditions most drivers (85%)
|
From page 90... ...
5.2.3.4 BASIS/RATIONALE FOR DSD AVOIDANCE MANEUVERS C, D & E GUIDELINE DSD PRT and MT are difficult to define exactly because drivers may respond to one of several cues (signs, markings or site geometry) , and may find it difficult to report recognizing the situation before they start responding to it.
|
From page 91... ...
• Measurement of PRT and MT for 98 drivers (28 aged 20 – 40, 35 aged 65 – 69 and 35 aged 70+) at 11 sites: 2 freeway lane drop exits, 3 mainline lane drops, 4 arterial turn lanes, one arterial lane drop due to parking, and one complex intersection (Lerner et al., 1995)
|
From page 92... ...
cues which are visible from a greater distance, as well as being further from the physical gore than are markings, allowing more time for the response. At freeway sites, the 85th percentile values for daytime PRTs were similar for all three age groups (7.8 for 20 – 40 year olds, 7.6 for 65 – 69 year olds, and 7.8 for 70+ year olds)
|
From page 93... ...
sec longer than the overall mean. MTs were measured separately for left (4.7 sec)
|
From page 94... ...
5.2.4 Passing Sight Distance 5.2.4.1 DEFINITION Passing sight distance is the length of the highway ahead necessary for one vehicle to pass another before meeting an opposing vehicle that might appear after the pass begins (ITS Traffic Engineering Handbook, Pline, 1999)
|
From page 95... ...
5.2.4.2 RELATED DESIGN/OPERATIONAL ISSUE Required passing sight distance relates to vehicle characteristics, road grade, and vehicle speeds. It also relates to whether or not the pass is aborted.
|
From page 96... ...
5.2.4.4 BASIS/RATIONALE FOR GUIDELINE The AASHTO model for PSD is based on data for single passenger vehicles passing single passenger vehicles. It is further based on the assumption that once drivers begin to pass, they have no opportunity to abort the pass.
|
From page 97... ...
The primary cue that a driver uses to determine whether or not it is safe to initiate a pass is the size of the image of the oncoming vehicle. In a series of experiments on a road not open to the public, Farber and Silver examined judgments in an overtaking situation (Farber & Silver, 1967)
|
From page 98... ...
In a study at five sites on a recreational two-lane highway in Wisconsin, Kaub (1990) used field observers to record time in the opposing lane and type of pass (Kaub, 1990)
|
From page 99... ...
Multiple passes were found to occur during 6.4 to 21.4% of passes, depending on the direction and on the site. The likelihood of a multiple pass did not appear to be related to the length of the passing zone.
|
From page 100... ...
5.3 Influence of Design on Speed 5.3.1 Background The design of a road affects drivers' speeds through two major mechanisms. First, the design creates the driving task.
|
From page 101... ...
A study of the effects of various geometric and environmental factors on the speeds for 2-lane rural highways (Yagar & Van Aerde, 1983) collected data for over 5000 5-minute periods at 35 locations.
|
From page 102... ...
general terrain and sight distance. Models to predict operating speeds on tangent sections of two-lane rural highways were developed by Polus et al.
|
From page 103... ...
Results of one study of 30 road sections posted at 50 km/h show that 85th percentile speeds were 62 km/h in road sections with little side friction (that is wide clear zones) , but were 50 km/h in road sections with side friction due to the presence of pedestrians, bicyclists, parked vehicles etc.
|
From page 104... ...
Step 1. Collect Field Data Prepare site diagram Collect speed data Observe erratic vehicle maneuvers Inventory traffic control devices Measure geometric sight distances Record features affecting flow speeds Record visual distractions at hazard and approach Add specified labels to site diagram Step 2.
|
From page 105... ...
Step #1: Collect Field Data This step involves making specific field measurements and observations. Data are to be gathered both at the location of a designated or possible hazard as well the approach roadway section immediately in advance of the hazard.
|
From page 106... ...
Step # 1C Observe erratic vehicle maneuvers on approach Procedure Product/Application Observations of vehicle movements should be considered in situations of sufficiently high traffic volumes to justify this type of study, e.g., 100 vph and above. Typical target vehicle behaviors indicative of a sight distance problem are sudden slowing (e.g., observable break light activation)
|
From page 107... ...
Step # 1F Note factors affecting flow speeds Procedure Product/Application Certain roadway environmental features are known to affect drivers' selection of speed. Examples are pavement defects, narrow shoulder widths and protruding bridge piers, abutments, guardrail, median barriers, etc.
|
From page 108... ...
Step # 1I Label the diagram with specified symbols. Procedure Product/Application Σ∆ΗΑΖ - Sight distance to a potential hazard – The point at which a location or object is first detectable to an approaching motorist.
|
From page 109... ...
Approach Distance to Hazard, ft. Estimated Operational Speed, mph Visually Cluttered Environment (A)
|
From page 110... ...
Step #2B. Examine Approach with respect to AASHTO Design Criteria Procedure Product/Application As with the procedure noted in Step 2A, to ensure the integrity of the overall sight distance diagnosis, it is necessary to assess whether the approach to the hazard location has any inherent design shortcomings.
|
From page 111... ...
Step #2E. Examine Traffic Control Devices with respect to MUTCD Criteria Procedure Product/Application The Manual of Uniform Traffic Control Devices (MUTCD)
|
From page 112... ...
Step #3C. Examine Potential Sight Distance Causation Effect Procedure Product/Application Certain patterns of accident behaviors (i.e., precollision maneuver)
|
From page 113... ...
Step # 4B. Establish and plot information sources and associated sight distances along approach segment.
|
From page 114... ...
Step # 4C. Define component driver response sections within approach segment.
|
From page 115... ...
Step #5: Analyze Component Driving Task Requirements In this step the practitioner applies human factors principles, i.e., comprising informationprocessing 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.
|
From page 116... ...
Step #5B. Determine driving task requirements within each component roadway segment.
|
From page 117... ...
Step #5C. Quantify the applicable PRT and MT requirements for each driving task component.
|
From page 118... ...
Step #5D. Assess the adequacy of the available sight distance components.
|
From page 119... ...
Step #6: Develop Engineering Strategies for Amelioration of Sight Distance Deficiencies. In this final step the practitioner recommends improvement, e.g., traffic control device applications or minor design modifications to correct deficiencies.
|
From page 120... ...
5.5 References AASHTO (2001)
|
From page 121... ...
Kaub, A
|
From page 122... ...
Olson, P
|
From page 123... ...
Wortman, R
|
From page 124... ...
The example driving situation consists of a 55-mph, two-lane rural roadway which approaches a 35-mph curve followed by a Stop-controlled intersection. The intersection approach is to a main highway, i.e., requiring application of destination guide signing.
|
From page 125... ...
2.1.1 Signed Intersection Approach Segment Steps 2A thru 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.
|
From page 126... ...
This roadway is diagrammed below. 3.2 Signed Intersection Approach Segment On this roadway section, 35-mph motorists are confronted with a stop-signed intersection and two guide signs containing destination names and route shields.
|
From page 127... ...
4. Step 5 – Analyze Component Driving Task Requirements 4.1 Curve Approach Segment The roadway section, requiring the driver to slow from 55 mph to a 35-mph curve, considers sight distance to the curve and legibility distance requirements posed by the advisory speed sign.
|
From page 128... ...
also known that most reasonably alert drivers are able to initiate braking within a PRT of 1.6 s [Section 5.2.1]
|
From page 129... ...
Step 5B – Determine the driving task requirements Considering the two possibilities in this case, 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-mph speed zone to intersection Σ∆ΗΑΖ -- -- -- -- Æ Α The sight distance requirement (to accommodate travel time)
|
From page 130... ...
term. This advance distance is not specified in the MUTCD.
|
From page 131... ...
Thus the total sign reading time is 7.0 seconds. 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 sufficient information-processing sight as some drivers may need the entire set of information.
|
From page 132... ...
The above sum of PRT requirements would apply to a serial task process. However, a realistic assessment of PRT requirements considers that many the above tasks are concurrent.
|
From page 133... ...
References Dewar, R.E., Kline, D.W., Schieber, F., Swanson, H.A., "Symbol Signing Design for Older Drivers", Federal Highway Administration, Report # FHWA-RD-94-069, Washington, D
|
Key Terms
This material may be derived from roughly machine-read images, and so is provided only to facilitate research.
More
information on Chapter Skim is available.