A Performance-Based Highway Geometric Design Process (2016) / Chapter Skim
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Pages 25-48
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25 3.1 Introduction This research addresses highway geometric design and the need for a new geometric design process. The first major change should be to acknowledge that "geometric design" as conventionally considered has little meaning absent the context in which the design is being completed.
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26 a performance-Based highway Geometric Design process the cost of access to businesses. A footprint that supports better operation through an intersection may require right-of-way acquisition and come at increased cost.
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highway Geometric Design and project Development 27 Design Study Reports are typically prepared in conjunction with environmental documents, and describe what is often referred to as 30% design in which the three-dimensional footprint of the road is set. Bridge studies (referred to as "type, size, and location," or "TS&L")
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28 a performance-Based highway Geometric Design process 3.1.2.1 Core Values Reflected in Transportation Projects The transportation values that create the priorities and ultimately define and shape the program and each project involve one or more of the following three basic needs: • Maintaining transportation infrastructure in a state-of-good repair. • Providing or enhancing a transportation LOS, which may include multiple modes, such LOS characterized by mobility, accessibility, or both; and such service attending to both persons as well as goods.
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highway Geometric Design and project Development 29 established methods and practices and apply the agency's design standards and technical guidance. The following are typical technical disciplines involved in road design projects: • Transportation planners, • Traffic engineers, • Environmental planners, • Environmental scientists, • Cultural resource experts, • Pavement and materials specialists, • Road safety engineers, • Highway engineers and geometric designers, • Drainage and hydraulic engineers, • Bridge and retaining wall engineers, • Public involvement and facilitation specialists, and • Construction engineers.
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30 a performance-Based highway Geometric Design process for which they work. Failure to fully perform their duties correctly can leave the agency open to a potential tort action should a crash occur that can be attributed to a problem associated with the road's design.
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highway Geometric Design and project Development 31 Roadway design decisions inherently involve trade-offs and judgments, with many of these involving environmental issues. The level of detail associated with quantifying environmental effects is increasingly greater.
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32 a performance-Based highway Geometric Design process 3.2.2 Finding 2: Context Matters -- And It Varies A project's context as broadly defined matters greatly with respect to what is physically possible to construct, what is reasonable to expect in terms of both operational and safety performance, what performance in fact will occur, what direct implementation costs are incurred, and what socioeconomic and environmental effects may result. The AASHTO highway context framework is limited in that it only reflects general considerations of the cost or difficulty of construction, and the traffic operational trade-off associated with mobility vs.
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highway Geometric Design and project Development 33 Seven context zones are described by both type and intensity of land use. Guidance for appropriate design controls that reflect sensitivity to vulnerable users, including specifically speed, is presented.
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34 a performance-Based highway Geometric Design process or disruption to property owners (businesses, residents, government agencies) who invested in their land based on the right-of-way and footprint of the existing road.
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highway Geometric Design and project Development 35 3.2.2.2.3 Construction Costs and Constructability. Design criteria per AASHTO are based in part on the notion of cost effectiveness, which is based on cost elements of the quantities of new pavement, structures, and earthwork balance.
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36 a performance-Based highway Geometric Design process width design values -- has a footnote that addresses the safety and operational performance of existing roads within the context of establishing a need for widening. Such process improvements would represent an appropriate emphasis on performance rather than roadway geometry as a surrogate for performance.
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highway Geometric Design and project Development 37 3.2.5 Finding 5: Speed Is an Essential Input to Determination of Design Values and Dimensions The design process is significantly reliant on speed as a central input or control. Current AASHTO processes incorporate the concept of "design speed" and others have suggested the use of "target speed." Regardless of the term used, development of design dimensions and details will to a great extent require the setting of speed control or variable.
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38 a performance-Based highway Geometric Design process The design process and AASHTO guidance will also by necessity require the identification of those contexts and circumstances in which high-speed operations should not be encouraged because of the increased risk to vulnerable road users. 3.2.6 Finding 6: AASHTO Design Criteria Produce Uneven Outcomes Re: Performance AASHTO design criteria have evolved over the years.
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highway Geometric Design and project Development 39 Table 7. AASHTO criteria evaluation.
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40 a performance-Based highway Geometric Design process Empirical models of safety performance based on HSM research confirm that the risk of a crash varies by road type. For both segments and intersections, the following is known: • The frequency of crashes by type varies widely for two-lane versus multilane roads and roads in urban vs rural areas.
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highway Geometric Design and project Development 41 In particular, the research team has concern over the simple models for horizontal curve design and SSD, which have not been challenged nor substantially changed over the years. As both of these are core criteria that heavily influence geometric design, they warrant substantial further study.
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42 a performance-Based highway Geometric Design process of allowing an exception. Moreover, setting a very low bar for minimum criteria carries with it a responsibility for agencies and their design staff to be more knowledgeable and exercise judgment in ways that they currently do not typically do.
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highway Geometric Design and project Development 43 The design process, while requiring nominal safety thresholds, should be focused not on producing minimum designs but rather on the optimization of substantive safety (and substantive performance) within an overall framework of implementation cost effectiveness.
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44 a performance-Based highway Geometric Design process width are evident. Indeed, there is a striking disparity between the known importance of access control and management on both traffic operations as well safety performance, and the extent to which it is referenced in the development of other geometric elements, most notably urban cross section design.
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highway Geometric Design and project Development 45 integrating simulation into design include turning lanes at intersections (based on, for example, 95th percentile queues from simulation rather than fixed dimensions)
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46 a performance-Based highway Geometric Design process Figure 11. Pre-automated drafting environment.
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highway Geometric Design and project Development 47 DOTs are just now beginning to learn the methods and approaches. Typical applications are in design exception evaluations, and to a limited extent alternatives analysis.
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48 a performance-Based highway Geometric Design process made do with somewhat limited technical knowledge of operations and safety. The primary value to be met was speed.
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