Performance-Based Analysis of Geometric Design of Highways and Streets (2014) / Chapter Skim
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Pages 31-43
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From page 31... ...
31 C H A P T E R 4 4.1 Introduction This chapter presents information regarding the relationships between geometric design elements 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.
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From page 32... ...
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 geometric sensitivity. Geometric sensitivity refers to the degree to which varying the dimensions related to a geometric element has an impact on performance.
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From page 33... ...
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)
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From page 34... ...
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)
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From page 35... ...
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.
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From page 36... ...
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 relationship has been uncovered by research and is included as part of a performance prediction tool, an accepted publication, or other knowledge base.
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From page 37... ...
Geometric Design Elements 37 4.4 Performance Categories and Measures This section presents information about design elements/decisions related to segments, intersections, and interchanges and their relationship to performance measures from each of the transportation performance categories identified and defined in Chapter 3. In some cases, surrogates for transportation performance are presented where knowledge is limited (e.g., accessibility)
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From page 38... ...
38 Performance-Based Analysis of Geometric Design of Highways and Streets The performance measures shown in Exhibit 4-6 are intended to document critical considerations 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 identification and evaluation, preliminary design, and final design stages of the project development process.
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From page 39... ...
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 TwoLane Highways, Chapter 16 Urban Streets (3 ) Segment Inferred speed The maximum speed for which all critical designspeed-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)
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From page 40... ...
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 )
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From page 41... ...
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 suburban 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.
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From page 42... ...
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 )
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From page 43... ...
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.
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