Performance-Based Analysis of Geometric Design of Highways and Streets (2014)

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31 C H A P T E R 4 4.1 Introduction This chapter presents information regarding the relationships between geometric design ele- ments and performance measures for the categories described in Chapter 3. The information presented in this chapter focuses on the established and known relationships between geometric design elements and the performance of highways and streets. In some cases, surrogate perfor- mance measures are presented where knowledge is limited (e.g., accessibility) or where the surro- gate provides a meaningful design assessment (e.g., inferred design speed). There is a wide range of relationships the broader transportation profession continues to research to be able to better quantify and describe those relationships. While future applied research will continue to docu- ment and summarize relationships between geometric design decisions and associated perfor- mance measures, the information in this chapter mainly highlights the current state of practice. This information and the process framework outlined in Chapter 5 are intended to set a pattern for the future, with flexibility to adapt to new research findings for maximum, long-term utility. In summary, while this report is static, the geometric design and performance relationships will continue to evolve. The process framework is intended to be adaptive to future research findings, methodologies, and tools. The Supplemental Research Materials Report (1) documents opportunities for additional research to better support performance-based analysis of geometric design. This chapter focuses on key established and direct relationships between performance measures and geometric design elements so practitioners are able to direct their attention to geometric elements and design deci- sions most likely to affect the performance characteristics that are most applicable to a given project or design. The early chapters of this report emphasized understanding and defining over- all project outcomes, project performance, and geometric design performance. This chapter is intended to support geometric design evaluations and decision making by summarizing infor- mation about the relationships between geometric design decisions and performance measures related to accessibility, mobility, quality of service, reliability, and safety. The intent of this chapter is to summarize the critical or high-priority known relationships between design elements and performance, document the general relationship, identify pos- sible performance tradeoffs, and present resources and tools that can be used to analyze a given design decision’s impact on performance measures in greater detail. Chapter 5 presents an application process for integrating this information into a performance-based analysis frame- work to inform geometric design decisions on projects within various stages of development. Chapter 6 presents project examples that (1) reinforce the background and foundational con- cepts of Chapters 1, 2, and 3; (2) apply the process framework described in Chapter 5; and (3) integrate specific example applications of the geometric design performance relationships presented in this chapter. Geometric Design Elements

32 Performance-Based Analysis of Geometric Design of Highways and Streets 4.2 Geometric Sensitivity A key fundamental concept in performance-based analysis to inform design decisions is geo- metric sensitivity. Geometric sensitivity refers to the degree to which varying the dimensions related to a geometric element has an impact on performance. It is at the core of what performance-based analysis is intended to communicate to practitioners. For example, geometric sensitivity is at the heart of being able to answer: In terms of crash expectancy, how much of a difference is there between a horizontal curve with a 1,000 ft radius and a horizontal curve with 1,100 ft radius? The degree to which crash frequency is sensitive to changes in curve radius for a given context is the relationship that will help practitioners accurately answer that question and make informed decisions regarding design tradeoffs. More precisely, geometric design sensitivity refers to a relationship that shows an expected impact on some aspect of transportation performance as a direct result of a geometric design decision. The level of sensitivity refers to the amount of the impact. Some relationships are highly sensitive (e.g., number of travel lanes versus passenger car mobility); others are less sensitive (e.g., lane width and average travel speed). Certain relationships are sensitive only for certain ranges of geometric dimen- sions. An example is provided in Exhibit 4-1, which shows the relationship between horizontal curve radius, operating speed, and the change in expected crash frequency. Horizontal curve radius influences vehicle operating speeds; however, operating speed is relatively insensitive to curve radius until the radius falls below approximately 1,000 ft. Similarly, the expected crash frequency is insensi- tive to curve radius until the horizontal curve radius falls below 1,000 ft. NCHRP Report 687: Guidelines for Ramp and Interchange Spacing (9) provides a similar exam- ple of the geometric design sensitivity of ramp spacing on predicted safety. Exhibit 4-2, taken from NCHRP Report 687 (9) for entrance-exit configurations (EN-EX), shows there is little change to relative crash risk as ramp spacing values increase beyond 2,600 ft. Similarly, it shows increasing relative crash risk as ramp spacing values decrease from 1,200 ft. In many cases, geometric design sensitivity is expected but has not been uncovered by research to date. For example, research conducted to create the HSM chapter on urban and suburban arterials began with the expectation that “an understanding of the relationship between lane width and safety is central to design decision making concerning urban and suburban arteri- als.” However, the research concluded that “No consistent relationship was found between lane width and safety. Therefore, lane width was not included in the model [predictive method].” This does not necessarily mean that safety is insensitive to lane width on urban and suburban roads, only that the relationship was not uncovered by this particular research. Exhibit 4-1. Relationship between horizontal curve radius and speed (11). 0 1 2 3 4 5 6 7 8 9 10 0 10 20 30 40 50 60 70 80 0 500 1000 1500 2000 2500 3000 Sp ee d (m ph ) Horizontal Curve Radius (ft) CMF, rural two-lane C rash M odification Operating speed, rural two-lane

Geometric Design Elements 33 4.3 Relationships between Geometric Design Elements and Performance Categories The information presented in this section has been assembled from a number of published documents and research findings. A full list and description of resources consulted is provided in the Supplemental Research Materials Report (1). Key resources included AASHTO’s High- way Safety Manual (2); Highway Capacity Manual 2010 (HCM2010) (3); Transit Capacity and Quality of Service Manual, Second Edition (4); FHWA’s Speed Concepts: Informational Guide (5); draft HSM chapters for freeways and interchanges developed as part of NCHRP Project 17-45, “Enhanced Safety Prediction Methodology and Analysis Tool for Freeways and Interchanges” (6); Interactive Highway Safety Design Model (7); procedures in macroscopic, mesoscopic, and microscopic simulation tools; and published and ongoing research that includes significant aspects of geometric design and performance relationships. The information in this section mainly focuses on presenting high-priority, well-established, and direct relationships between geometric design decisions and performance. Practitioners should also be aware of the broader range of expected relationships, since there are likely rela- tionships that exist but have not yet been clearly defined, quantified, and documented because of limitations in data, analysis techniques, and other similar challenges. The remainder of this section identifies expected or likely relationships between performance and geometric elements, even if they have not been uncovered by research or published findings. The information on “expected” or “likely” relationships is summarized in Exhibits 4-3 through 4-5, which provide the expected relationships between geometric design elements and performance categories for segments, intersections, and interchanges, respectively. The process used to arrive at these exhib- its is described in the Supplemental Research Materials Report (1) associated with these guide- lines and is also summarized in the following paragraphs. The research team used three possible notations to classify each geometric characteristic or design decision and performance category combination as either “expected direct effect,” 1 Relative crash risk is measured by the percent difference in crashes, of all types and severities, at some ramp spacing value compared to a ramp spacing of 1,600 ft. Exhibit 4-2. Preliminary safety assessment tool for ramp spacing: entrance ramp followed by exit ramp (9, Exhibit 5-5).

34 Performance-Based Analysis of Geometric Design of Highways and Streets “expected indirect effect,” or “no expected effect.” These classifications are based on the research team’s professional opinion drawing from members’ knowledge of the state of related research. A definition of each classification is as follows: • Expected direct effects are performance effects caused by the geometric design decision that occur at the same time and place (e.g., a given horizontal curve radii affects expected crash frequency at that location immediately). Segment Geometric Elements/Characteristics Accessibility Mobility Quality of Service Reliability Safety Access points and density ●* ●* ●* □◊ ●* Design speed and target speed — □◊ □◊ □◊ □* Horizontal alignment — ●◊ ●◊ □◊ ●* Number of travel lanes ●* ●* ●* □* ●* Sidewalk and pedestrian facilities (including ADA) ● ● * ●* □x ●x Bicycle accommodation features ● ●* ●* □x ●x Median provisions ●◊ ●* ●* □◊ ●* Travel lane width(s) ●◊ ●* ●* □* ●* Auxiliary lane width(s) ●x ●x ●x □x ●x Type and location of auxiliary lanes ● ◊ ●* ●* □◊ ●* Shoulder width(s) and composition ● ◊ ●* ●* □* ●* Shoulder type(s) ●◊ ●x ●x □◊ ●* Lane & shoulder cross slopes — — — □x ●x Superelevation — ●x ●x □◊ ●* Roadside design features ●x ●x ●x □x ●* Roadside barriers ●◊ ●* ●* □◊ ●* Minimum horizontal clearances ●◊ ●* ●* □◊ ●* Minimum sight distance ●x ●x ●x □x ●x Maximum grade(s) □◊ □* □* □◊ □* Minimum vertical clearances ●◊ □x □x □x □x Vertical alignment(s) — ●* ●* □* ●* Bridge cross section ●◊ ●* ●* □* ●* Bridge length/termini — — — □◊ ●* Rumble strips ●◊ — — □x ●* ● = expected direct effect □ = expected indirect effect — = expected not to have an effect * = relationship can be directly estimated by existing performance prediction tools ◊ = relationship can be indirectly estimated using more than one existing tool x = relationship cannot be estimated by existing tools Exhibit 4-3. Segments: expected geometric elements and performance relationships.

Geometric Design Elements 35 • Expected indirect effects are performance effects caused by the geometric design decision that occur later in time (e.g., providing additional auto capacity induces more auto travel) or farther removed in distance. Indirect effects may include growth-inducing effects and other effects related to induced changes in the pattern of land use and traffic patterns from the geo- metric change. For example, a new interchange providing access to a freeway may result in travel pattern changes on the freeway and surrounding surface streets, thus impacting mobil- ity and safety on those facilities. This would be noted as an indirect effect. In some instances, indirect effects may influence the intended project outcomes and so, to the extent possible, the potential implications of indirect effects should be considered. In this example, the new interchange would increase vehicle traffic on a street or connecting streets that did not have access to the freeway before. This new access could increase network connectivity and achieve goals of increasing economic competitiveness and vitality. However, the increase in motor- ized traffic could affect quality of service for pedestrians and bicyclists. It may also influence businesses along the street that now have access and increased exposure to potential patrons. There would obviously also be direct safety and operational effects due to the presence of the new ramp terminals immediately at the terminal locations. Intersection Geometric Elements/Characteristics Accessibility Mobility Quality of Service Reliability Safety Intersection form, control type, and features ● ◊ ●* ●* □x ●* Number and types of lanes ●◊ ●* ●* □x ●* Sidewalk and pedestrian facilities (including ADA) ● * ●* ●* □x ●x Bicycle accommodation facilities ●* ●* ●* □x ●x Design vehicle accommodations □x □x □x □x □x Traffic islands ●x ●x ●x □x ●x Lane widths ●x ●x ●x □x ●x Auxiliary lane terminals and transitions ● ◊ ●* ●* □x ●x Shoulder width and composition ●x ●x ●x □x ●x Horizontal alignment of approaches ● x ●x ●x □x ●* Vertical alignment of approaches ●◊ ●* ●* □x ●* Pavement cross slope and superelevation — — — □ x ●x Intersection sight distance ●x ●x ●x □x ●x Median opening configuration ●◊ ●◊ ●◊ □x ●x Curve tapers and radii ●x ●x ●x □x ●x ● = expected direct effect □ = expected indirect effect — = expected not to have an effect * = relationship can be directly estimated by existing performance prediction tools ◊ = relationship can be indirectly estimated using more than one existing tool x = relationship cannot be estimated by existing tools Exhibit 4-4. Intersections: expected geometric elements and performance relationships.

36 Performance-Based Analysis of Geometric Design of Highways and Streets • No expected effect expresses that the geometric characteristic or design decision is not expected to impact the respective aspect of performance, either directly or indirectly. A second set of notations in Exhibits 4-3 through 4-5 indicates whether the expected relation- ship has been uncovered by research and is included as part of a performance prediction tool, an accepted publication, or other knowledge base. The secondary notation classifies each relationship as one of the following: • The relationship can be directly estimated by existing performance prediction tools. • The relationship can be indirectly estimated using more than one existing tool or supplemental calculations. • The relationship cannot be estimated by existing tools. • Not applicable (i.e., the relationship is not expected to exist). Exhibits 4-3 through 4-5 presented what direct and indirect relationships are expected to exist as well as related, current performance prediction capabilities. The many gaps in the profession’s knowledge base highlight the importance of additional research to better understand the effect our design decisions have on different performance categories. The following section, Section 4.4, presents the critical performance measures for each performance category that are expected to influence or are influenced by geometric elements. Section 4.4 is limited in scope to allow prac- titioners to focus attention on the performance measures and geometric elements most likely to substantially influence the degree to which a project is able to meet its intended project outcomes from a transportation performance perspective. Interchange Geometric Elements/Characteristics Accessibility Mobility Quality of Service Reliability Safety Interchange form and features ● ◊ ●◊ ●x □x ●* Sidewalk and pedestrian facilities (including ADA) ● x ●x ●x □x ●x Bicycle accommodation facilities ● x ●x ●x □x ●x Auxiliary lane lengths ●◊ ●* ●* □x ●* Horizontal alignment of ramp ● ◊ ●◊ ●x □x ●* Vertical alignment of ramp ●x ●x ●x □x ●x Pavement cross slope and superelevation ● x ●x — □x ●x Ramp cross section ●◊ ●* ●* □x ●* Mainline ramp gores and terminals ● ◊ ●* ●* □x ●* Ramp roadside ●x ●x — □x ●x Ramp barriers ●x ●x ●x □x ●* Cross road ramp terminals ●◊ ●* ●* □x ●* ● = expected direct effect □ = expected indirect effect — = expected not to have an effect * = relationship can be directly estimated by existing performance prediction tools ◊ = relationship can be indirectly estimated using more than one existing tool x = relationship cannot be estimated by existing tools Exhibit 4-5. Interchanges: expected geometric elements and performance relationships.

Geometric Design Elements 37 4.4 Performance Categories and Measures This section presents information about design elements/decisions related to segments, inter- sections, and interchanges and their relationship to performance measures from each of the trans- portation performance categories identified and defined in Chapter 3. In some cases, surrogates for transportation performance are presented where knowledge is limited (e.g., accessibility) or where the surrogate provides a meaningful design assessment (e.g., inferred design speed). Infor- mation is organized by basic facility type (i.e., segment and node—interchange or at-grade inter- section) and performance category. Interchanges can, in general, be treated as a series of segments and intersections. In a few instances, they are identified as a specific facility type. Performance measures associated with segments and intersections can also typically be applied to interchanges. For example, the design of an interchange ramp proper closely mimics the design process and elements of roadway segments. Ramp terminal intersection treatments at service interchanges follow the design considerations of at-grade intersections. 4.4.1 Accessibility Accessibility is defined as the ability to approach a desired destination or potential opportu- nity for activity using highways and streets (including sidewalks and/or bicycle lanes). Exhibit 4-6 summarizes, by facility type, the performance measures specific to access and acces- sibility, the sensitive geometric design elements influencing those performance measures, the basic relationship between the design element and the performance measure, potential tradeoffs between the design element and the performance of other transportation elements, and resources that can be used to evaluate the sensitivity of that geometric relationship in greater detail. Facility Type Performance Measure Definition Geometric Design Elements Basic Relationship Potential Performance Tradeoffs Evaluation Resources Segment Driveway density Number of driveways per mile Access points and density Higher density of driveways associated with higher motor vehicle access Degrades bicycle LOS, increases crash likelihood, increases average travel speed HCM2010 Chapters 16 and 17 (3), HSM Part C Chapters (2) Urban/ Suburban Segment Transit stop spacing Distance between transit stops along a roadway segment Transit accommodation features Higher frequency increases access for transit riders Increases transit travel time and may degrade mobility for other vehicle modes Transit Capacity and Quality of Service Manual (4) Segment Presence of pedestrian facility Presence of a sidewalk, multiuse path, or shoulder Sidewalk and pedestrian facilities Greater connectivity and continuity of pedestrian network increases access for pedestrians Implementing pedestrian facilities in a constrained environment may require removing capacity or parking for vehicle mode HCM2010 Chapters 16 and 17 (3) Segment Presence of bicycle facility Presence of bicycle lanes, multiuse path, or shoulder Bicycle accommodation features Greater connectivity and continuity of bicycle network increases access for bicyclists Implementing bicycle facilities in a constrained environment may require removing capacity or parking for vehicle mode HCM2010 Chapters 16 and 17 (3) Exhibit 4-6. Access and accessibility performance measures.

38 Performance-Based Analysis of Geometric Design of Highways and Streets The performance measures shown in Exhibit 4-6 are intended to document critical consid- erations related to access and accessibility and design considerations. The measures are focused on elements that would be considered and would be influential within the alternatives identi- fication and evaluation, preliminary design, and final design stages of the project development process. There are system-level accessibility and access metrics that would be more applicable within broader system-wide planning activities. System-wide access or accessibility metrics could consider broader access issues such as access to transit for transportation-disadvantaged populations and/or freeway or highway access to industrial areas that may serve as an attractor for potential employers. These system-level access considerations support identifying future transportation network needs. In the broadest sense, these considerations are often influenced by identifying and trying to attain overall project outcomes. The intent of the performance measures in Exhibit 4-6 is to identify access and accessibility considerations that are directly applicable at the project level. 4.4.2 Mobility Mobility is defined as the ability to move various users efficiently from one place to another using highways and streets. Exhibit 4-7 summarizes, by facility type, the performance measures specific to mobility, the sensitive geometric design elements influencing those performance measures, the basic relation- ship between the design element and the performance measure, potential tradeoffs between the design element and the performance of other transportation elements, and resources that can be used to evaluate the sensitivity of that geometric relationship in greater detail. Improving many of the mobility-oriented performance measures shown in Exhibit 4-7 for vehicles has the potential to negatively affect the quality of service for pedestrians, bicyclists, or transit users. The tradeoff that often occurs in providing additional vehicle capacity is increased motor vehicle speeds. Increased speeds are associated with lower quality of service (e.g., lower comfort and safety) for pedestrian, bicycle, and transit modes. Additional vehicle capacity can also come at the expense of providing pedestrian or bicycle facilities. However, in some cases, providing a bicycle lane can provide a de facto shoulder or a shoulder can serve as a de facto bicycle lane. The concept of using inferred speed as a performance measure that is able to inform geometric design decisions is illustrated in Chapter 6, Project Example 2. 4.4.3 Quality of Service Quality of service is defined as the perceived quality of travel by a road user. It is used in the HCM2010 to simultaneously assess LOS for motorists, pedestrians, bicyclists, and transit riders (i.e., MMLOS). It may also include the perceived quality of travel by users of larger vehicles such as trucks or transit vehicles. Exhibit 4-8 summarizes, by facility type, the performance measures specific to quality of service, the sensitive geometric design elements influencing those performance measures, the basic rela- tionship between the design element and the performance measure, potential tradeoffs between the design element and the performance of other transportation elements, and resources that can be used to evaluate the sensitivity of that geometric relationship in greater detail. The quality of service metrics summarized in Exhibit 4-8 represent a combination of recent advancements in how the transportation profession understands, evaluates, and attempts to quantify quality of travel experience for different road users and fundamental considerations related to critical design vehicles that need to be served within a project. Ongoing research on multimodal quality of service especially related to pedestrian and bicycle quality of service will

Geometric Design Elements 39 Facility Type Performance Measure Definition Geometric Design Elements Basic Relationship Potential Performance Tradeoffs Evaluation Resources Segment Average travel time The mean amount of time it takes a road user to travel from one point to another point along a roadway segment Number of travel lanes Increased vehicle lanes decrease average travel time for autos and increases vehicle speed Degrades quality of service for pedestrians and bicyclists Degrades mobility for pedestrians and bicyclists Higher vehicle speeds are associated with higher severity crashes HCM2010 Chapters 10 Freeway Facilities, Chapter 14 Multilane Highways, Chapter 15 Two- Lane Highways, Chapter 16 Urban Streets (3 ) Segment Inferred speed The maximum speed for which all critical design- speed-related criteria are met at a particular location Horizontal alignment, vertical alignment, and cross section Higher inferred speeds associated with higher free-flow speeds and higher mobility Higher vehicle speeds are also associated with higher severity crashes FHWA Speed Concepts: Informational Guide (5) Two-Lane Segment Average percent time spent following The average percentage of total travel time that vehicles must travel in platoons behind slower vehicles due to an inability to pass Horizontal and vertical alignment, sight distance, type and location of auxiliary lanes Increased opportunities to pass slow- moving vehicles reduces percent time spent following, providing a passing lane can reduce crashes Increases vehicle speeds, increases potential for higher severity crashes HSM Chapter 10 (2 ); HCM2010 Chapter 15 (3) Freeway Segment Freeway speed The freeway speed down- stream of an entrance ramp and before an exit ramp or another entrance ramp Ramp spacing dimensions as defined in NCHRP Report 687 Use of downstream auxiliary lane At relatively high exit ramp volumes, ramp spacing affects freeway speeds Decreased freeway speeds are possible with decreased ramp spacing An auxiliary lane may improve freeway speeds NCHRP Report 687 (9); HCM2010 Chapters 11, 12 and 13 (3 ) Intersection Delay Average control delay experienced by road users at an intersection Intersection form, control type, and features; number and types of lanes Lower control delay for any road user improves mobility for that mode Often tradeoffs occur between delay experienced by different modes depending on the type of traffic control present HCM2010 Chapters 18 through 22 (3); NCHRP Report 672 (8) Intersection Volume-to- capacity (v/c) ratio The ratio of volume present or forecasted and the available capacity at the intersection Intersection form, control type, and features; number and types of lanes Increased vehicle capacity associated with lower v/c ratios Degrades quality of service for pedestrians and bicyclists Degrades mobility for pedestrians and bicyclists HCM2010 Chapters 18 through 22 (3 ); NCHRP Report 672 (8) Exhibit 4-7. Mobility performance measures.

Facility Type Performance Measure Definition Geometric Design Elements Basic Relationship Potential Performance Tradeoffs Evaluation Resources Urban/ Suburban Segment Pedestrian LOS A letter grade associated with the quality of travel experience for a pedestrian; based on HCM2010 methodology Sidewalk and pedestrian facilities, width of pedestrian lanes, buffer from vehicle traffic, driveway density, crossing frequency Increasing width of pedestrian facility, increasing distance from vehicle traffic, decreasing driveway density, and increasing opportunities to cross a street improve pedestrian LOS Meeting performance metrics for pedestrians may degrade travel quality for other modes – e.g., on-street parking improves pedestrian LOS and degrades bicycle LOS HCM2010 Chapters 16 and 17(3) Urban/ Suburban Intersections Pedestrian LOS A letter grade associated with the quality of travel experience for a pedestrian; based on HCM2010 methodology Crossing distance, traffic control delay Decreasing pedestrian crossing distance and delay to cross a street improves pedestrian LOS Meeting performance metrics for pedestrians may degrade travel quality for other modes HCM2010 Chapters 16 and 17 (3 ) Urban/ Suburban Segment Bicycle LOS A letter grade associated with the quality of travel experience for a bicyclist; based on HCM2010 methodology Bicycle accommodation features, physical separation from motor vehicle traffic, access points and density, on- street parking Increasing width of bicycle facility, decreasing driveway density, increasing separation from moving vehicle traffic, and removing on-street parking improves bicycle LOS Meeting performance metrics for bicyclists may degrade travel quality for other modes HCM2010 Chapters 16 and 17 (3 ) Urban/ Suburban Intersections Bicycle LOS A letter grade associated with the quality of travel experience for a bicyclist; based on HCM2010 methodology Traffic control delay Decreased delay for bicyclists increases quality of travel experience Meeting performance metrics for bicyclists may degrade travel quality for other modes HCM2010 Chapters 16 and 17 (3 ) Urban/ Suburban Segments and Intersections Transit LOS A letter grade associated with the quality of travel experience for a transit rider; based on HCM2010 methodology Transit accommodations facilities (presence of transit-only lane, bus pullout areas, bus merge/diverge lanes, bus queue jump lanes) Providing bus-only lane, queue jump lanes, merge/diverge lanes decreases bus travel time and improves transit rider quality of travel Incorporating transit-only features often comes at the expense of providing additional auto or bicycle capacity or treatments HCM2010 Chapters 16 and 17 (3 ) Urban/ Suburban Segments and Intersections Auto LOS Number and duration of stops along an urban/suburban corridor Number of travel lanes; intersection form, control type, and features Reducing the number of stops and duration of stops along a corridor improves auto LOS Increased vehicle lanes and speeds degrade pedestrian and bicycle MMLOS HCM2010 Chapters 16 and 17 (3 ) Intersections and Segments Large-vehicle turning and off-tracking characteristics Ability and ease with which large vehicles are able to physically move through an intersection or along a segment Curve radii, curb radii, lane width Generally larger curve radii, larger curb radii, and wider vehicle lanes enable easier navigation for larger vehicles Increasing curve radii, curb radii, and lane width often degrades pedestrian and bicycle MMLOS due to the longer crossing distances AutoTURN, truck turning templates Exhibit 4-8. Quality of service performance measures.

Geometric Design Elements 41 likely continue to evolve as the collective profession increases its focus and attention on creating and retrofitting existing roadways to “complete streets” that better serve a wide range of road users. While many of the performance measures in the exhibit are noted as applying to urban and sub- urban conditions, the same principles can be applied to more rural conditions when considering the design tradeoffs and multimodal implications or benefits in providing wider shoulders along a roadway segment and/or the physical footprint of an intersection. 4.4.4 Reliability Research is ongoing within the transportation profession to develop performance measures to be used to connect reliability to specific geometric design elements or decisions. Variation in travel time and variation in speed are two more common performance measures used to understand potential reliability of a facility. At the time of assembling this guidance document, there is no clear set of performance measures available for practitioners to easily integrate into design decisions. A number of design considerations can be applied to highways and streets: • Tradeoffs between mobility gained in implementing peak period hard shoulder running on a freeway segment and risk associated with a disabled vehicle during the peak period. • Tradeoffs between congestion pricing strategies on freeway segments to improve reliability and potential equity implications for lower-income households. • Tradeoffs between ramp metering strategies to preserve the quality of mainline traffic flow at the expense of degrading mobility on adjacent local streets. • Tradeoffs between implementing transit signal priority, bus-only lanes, and/or queue jumps for transit vehicles along an urban corridor to improve the reliability of bus service with the potential impact of degrading mobility for side street vehicle traffic. • Tradeoffs between implementing concrete median barriers with heights that eliminate dis- tractions from incidents on opposing roadway lanes (“rubbernecking”) and the potential safety performance degradation by introducing a fixed object. • Considerations of on- or off-facility incident or enforcement pull-off areas and over- all effectiveness compared to secondary effects such as delay and congestion caused by rubbernecking. The preceding considerations are a sampling of potential tradeoffs that may exist between implementing strategies for one or more performance measures and the corresponding potential tradeoff with reliability. For additional information regarding reliability, see the research materials and reports related to the following SHRP 2 projects: • L07, “Evaluation of Cost-Effectiveness of Highway Design Features” (http://apps.trb.org/ cmsfeed/TRBNetProjectDisplay.asp?ProjectID=2181) • L08, “Incorporation of Travel Time Reliability into the Highway Capacity Manual” (http:// apps.trb.org/cmsfeed/TRBNetProjectDisplay.asp?ProjectID=2197) • L09, “Incorporation of Non-recurrent Congestion Factors into the AASHTO Policy on Geomet- ric Design” (http://apps.trb.org/cmsfeed/TRBNetProjectDisplay.asp?ProjectID=2196) 4.4.5 Safety Safety is defined as the frequency and severity of crashes occurring on or expected to occur on highways or streets. Exhibit 4-9 summarizes, by facility type, the performance measures specific to safety, the sensi- tive geometric design elements that influence those performance measures, the basic relationship between the design element and the performance measure, and resources or tools that can be used to evaluate the sensitivity of that geometric relationship in greater detail.

42 Performance-Based Analysis of Geometric Design of Highways and Streets Facility Type Performance Measure Definition Geometric Design Elements Basic Relationship Potential Performance Tradeoffs Evaluation Resources Rural two-lane segments Crash frequency and severity Expected number and severity of crashes Horizontal alignment, shoulder width and composition, shoulder type, lane width, type and location of auxiliary lanes, rumble strips, roadside design features, lighting, two-way left-turn lane, grade See HSM Some safety improvements reduce mobility, reduce access (e.g., reducing driveway density), or negatively affect another performance measure HSM Chapter 10 (2 ) Rural two-lane intersection Intersection form, control type, and features, number and types of lanes, lighting, skew See HSM HSM Chapter 10 (2 ) Rural multilane segments Shoulder width and composition, shoulder type, lane width, lane and shoulder cross slopes, median provisions, lighting, two-way left-turn lane See HSM HSM Chapter 11 (2 ) Rural multilane intersection Intersection form, control type, and features; number and types of lanes; lighting; skew See HSM HSM Chapter 11 (2 ) Urban/ suburban segments Basic cross section, access points and density, fixed object density, median provisions, on-street parking See HSM HSM Chapter 12 (2 ) Urban/ suburban intersection Intersection form, control type, and features; number and types of lanes; signal phasing See HSM HSM Chapter 12 (2 ) Freeway Segments Lane width, shoulder width and composition, ramp spacing, use of auxiliary lanes, ramp entrance/exit configurations See final report for NCHRP 17-45 Final report for NCHRP 17-45 (6 ), NCHRP Report 687 (9) Interchange Interchange form and features, number and types of lanes, horizontal alignment, cross section, roadside See final report for NCHRP 17-45 Exhibit 4-9. Safety performance measures.

Geometric Design Elements 43 The information in Exhibit 4-9 focuses on quantifying safety impacts using crash frequency and severity as the key performance measures. In some instances, the tools listed in the exhibit may not apply to a given project; in which case, it may be necessary to use surrogate measures for safety such as the number of conflict points or consideration of speed or speed differentials as surrogates for severity. Other resources that may be beneficial in considering safety performance include FHWA’s Crash Modification Factors (CMF) Clearinghouse (10). 4.4.6 Summary The prior subsections presented the key performance measures and related geometric ele- ments based on what is currently documented within the transportation profession. As high- lighted in Section 4.3, a number of other relationships are indirect in nature and therefore more difficult to quantify or clearly understand the geometric sensitivity a given element has on per- formance. Other relationships have yet to be explored extensively enough to understand what relationship may exist between a performance measure and geometric element. Chapter 5 presents an application framework for integrating the information presented in Section 4.4 into performance-based analysis to inform geometric design decisions. For example, Section 5.3.1 discusses how to identify the geometric features influencing the intended project outcome discussed in Chapter 3 as well as Section 5.2.2. Chapter 6 illustrates how to apply the framework and foregoing information to projects at different stages within the project devel- opment process. For example, Project Example 2 integrates safety and mobility performance measures for a rural two-lane roadway segment, focusing on the influence shoulder width and horizontal curve characteristics have on crash frequency (safety measure) and inferred speed (mobility measure). 4.5 References 1. Kittelson & Associates, Inc., and University of Utah. Supplemental Research Materials Report. NCHRP Project 15-34A. Portland, Oregon: Kittelson & Associates, Inc. http://apps.trb.org/cmsfeed/TRBNetProject Display.asp?ProjectID=3322. 2. American Association of State Highway and Transportation Officials. Highway Safety Manual. Washington, D.C.: 2010. 3. Transportation Research Board (TRB). Highway Capacity Manual. Washington, D.C.: Transportation Research Board of the National Academies, 2010. 4. Transportation Research Board (TRB). Transit Capacity and Quality of Service Manual, Second Edition. Washington D.C.: Transportation Research Board of the National Academies, 2003. 5. Federal Highway Administration. Speed Concepts: Informational Guide. Washington, D.C.: 2009. 6. Bonneson, J. A., S. Geedipally, M. P. Pratt, and D. Lord. Safety Prediction Methodology and Analysis Tool for Freeways and Interchanges. Final Report, NCHRP Project 17-45. College Station, Texas: Texas Transportation Institute, 2012. 7. Federal Highway Administration. Interactive Highway Safety and Design Model. Washington, D.C.: 2003. 8. Rodegerdts, L., J. Bansen, C. Tiesler, J. Knudsen, E. Myers, M. Johnson, M. Moule, B. Persaud, C. Lyon, S. Hallmark, H. Isebrands, et al. NCHRP Report 672: Roundabouts: An Informational Guide, Second Edition. Washington, D.C.: Transportation Research Board of the National Academies, 2010. 9. Ray, B. L., J. Schoen, P. Jenior, J. Knudsen, R. J. Porter, J. P. Leisch, J. Mason, R. Roess, and Traffic Research & Analysis, Inc. NCHRP Report 687: Guidance for Ramp and Interchange Spacing. Washington, D.C.: Transpor- tation Research Board of the National Academies, 2011. 10. Federal Highway Administration. Crash Modification Factors Clearinghouse. Website link: http://www. cmfclearinghouse.org/. 11. Porter, R. J., E. T. Donnell, and J. M. Mason. “Geometric Design, Speed, and Safety,” In Transportation Research Record, Journal of the Transportation Research Board No. 2309, 2012, pp. 39–47.