Trade-Off Considerations in Highway Geometric Design (2011) / Chapter Skim
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8 CHAPTER TWO LITERATURE REVIEW This chapter presents the information gathered through the synthesis literature search. It starts with a focus on the conventional process of evaluating trade-offs using the Green Book, explores the concept of nominal and substantive safety, how the design exception process affects the analysis of trade-offs and examines the impact of flexibility in highway design.
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9 Updates to the Green Book have attempted to provide guidance regarding the necessity to consider the needs of nonhighway users and the environment as well during benefit-cost analysis. The authors acknowledge that this adds to the complexity of the analysis, but emphasize that this broader approach will allow for both the need for a given project and the relative priorities among various projects to be taken into account.
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10 Although lane widths of 3.6 m [12 ft.] are desirable on both rural and urban facilities, there are circumstances where lanes less than 3.6 m [12 ft.]
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11 FIGURE 2 Venn diagram relating crash causes. Source: Rumar (21)
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12 However, there is no formal priority when examining the trade-offs associated with respect to nominal safety of each of the 13 controlling criteria. There is no clear, quantifiable means of determining which controlling criteria are most important and how crash frequency varies with variation in each controlling criterion.
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13 CSD is defined as the project development process (including geometric design) that attempts to address safety and efficiency while being responsive to or consistent with the road's natural and human environment.
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14 efficiency, safety, cost, serving multiple users, and achieving environmental sensitivity. CSS/CSD principles and practices provide a framework for designers to document the rationale for adjustments to guidelines or criteria to best satisfy the needs of the working environment.
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15 tation functions (for entertainment, retail, public gathering, and recreation)
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16 Communities (37)
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17 alignment are also by default defined. In addition, the functional classification essentially establishes the basic roadway cross section in terms of lane width, shoulder width, type and width of median area, and other major design features (18)
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18 • Driver performance (age, reaction time, driving task, guidance, etc.) • Traffic characteristics (volume and composition)
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19 TABLE 4 COMPARISON OF DESIGN SPEED RANGES BETWEEN THE 1997 AND 2006 DESIGN GUIDES Roadway Type (Based on 1977) 1997 Manual 2006 Guidebook Rural Arterial (Level Terrain)
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20 – Most important measures needing to be balanced are usually "apples and oranges" and are impossible to collapse to a single common measure. Although disparate measures cannot be directly compared in common terms, simply computing and comparing them represents an improvement.
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21 The ITE Context Sensitive Solution in Designing Major Urban Thoroughfares for Walkable Communities (40) outlines the alternative screening process (Figure 7)
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22 Livable Communities (37) , the Pennsylvania and New Jersey DOTs jointly developed a generalized checklist to help ensure that alternatives are inclusive (Table 6)
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23 TABLE 6 CHECKLIST FOR EXPLORING ALTERNATIVES Source: Smart Transportation Guidebook: Planning and Designing Highways and Streets that Support Sustainable and Livable Communities (37)
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24 Measures of Effectiveness It is important that MOEs be directly related to the stated project needs. These needs often focus on the overarching categories of physical condition, operational performance, safety, access, environment, and social and historical.
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25 are finally selected for project evaluation, consideration of a wide range of measures at the beginning of a project can help identify important community values that may otherwise be overlooked. Further, it is critical to develop MOEs before alternatives have been formulated to help prevent stakeholders from attempting to steer the analysis toward certain types of alternative solutions instead of focusing on establishing MOEs to capture what is most important.
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26 time savings for a representative trip may be used to better understand the return on a proposed investment (37)
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27 CSD training, for instance, includes a session by the attorney general's office that discusses the need to document the use of sound engineering judgment. Such practices offer more protection than blind adherence to the maximum values in design manuals (50)
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28 mental gains from the existing conditions. Simple selection of the alternative that has been judged to be "safer" misses the opportunity to evaluate the safety gains based on the marginal rate of return.
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29 • Establish project scope • Fast track project development • Improve interagency communications • Bridge institutional borders • Better balance the needs of road users and those of the community or the environment • Reach consensus on difficult issues (54)
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30 attempts at this quantification. The Victoria Transportation Policy Institute has produced a number of publications that begin to monetize environmental and community quality of life factors related to transportation investments.
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31 the purpose or mission of the subject under study. (Promote Street Life)
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32 FIGURE 16 FAST diagram for enhancing social ties. Source: Osman et al.
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33 being compared. Unfortunately, participants who have a hidden agenda have the opportunity to attempt to "game" the outcome of this decision-making process.
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34 Step 1: Summarize the Attributes of Each Alternative The attributes for each alternative are identified in the spreadsheet by factor on the line titled "attributes." No value judgment is made regarding these attributes. For example, for Site 8 and the factor addressing water, the attribute is that water is "60 feet away." Step 2: Decide the Advantages of Each Alternative To determine the advantages, it is important that the group share an understanding of what attribute provides an advantage.
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35 tages is determined, keeping in mind that the score must be less than the score assigned to the most important advantage. The least important advantage (identified by the underline)
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36 Project risk analysis is a four-step process (Figure 20)
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37 Several risk identification techniques have been developed for project planning: • Brainstorming. An interdisciplinary team of experts who are not part of the project team generate a comprehensive list of potential project risks under the leadership of a facilitator.
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38 ensures that all participants address risk management from a common perspective (63)
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39 Each of the methods for qualitatively prioritizing risk has advantages and disadvantages, as summarized in Table 12. A further drawback to the assessing probability and consequence method is that it equates risk of a high-probability, low-damage event with that of a low-probability, high-damage event.
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40 Hazard Identification Hazard identification depends on knowledge, experience, forecasting, engineering judgment, and imagination to adequately identify potential hazards. The key is to include all hazards, however unlikely they may be.
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41 and mitigation of risk. Research into the psychology of risk perceptions by U.S.
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42 ply monitoring the system to ensure that it functions within acceptable limits. By comparison, as much of the underlying rationale for design standards is removed from design manuals, designers who do not understand the fundamentals underlying the criteria are acting in a similar manner.
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43 Overview One of the key takeaways from the work done in the organizational accident area is that many disasters occur when a combination of active and latent failures align to create an accident. When dealing with trade-offs in roadway design, the designer and decision makers must be careful to step back and look at the trade-off decisions holistically once they are made, as individual trade-off decisions that may be perfectly safe can in combination create conditions that compromise safety.
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44 Safety-Conscious Planning Although safety lies at the core of all transportation planning, the utility and role of safety measures in the planning process has been difficult to manifest in subsequent analysis, evaluation, prioritization, and system performance monitoring. The concept of incorporating safety issues into the transportation planning process in a more comprehensive way arose around the same time that CSS/CSD began to gain increasing consideration.
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45 cedures for conducting RSAs. The guide outlines the typical steps associated with undertaking an RSA (see Figure 31)
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46 Interactive Highway Safety Design Model The IHSDM is a suite of software analysis tools for evaluating the safety and operational effects of geometric design decisions. Under its current configuration, it can assist designers in evaluating design alternative for two-lane rural highways against relevant design policy values and provide estimates of a design's expected safety and operational performance.
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47 FIGURE 33 Organization of the Highway Safety Manual. Source: Highway Safety Manual (71)
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48 • Network Screening Tool. Identify sites with potential for safety improvements.
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