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SUMMARY TRADE-OFF CONSIDERATIONS IN HIGHWAY GEOMETRIC DESIGN AASHTOâs A Policy on Geometric Design of Highways and Streets (commonly called the Green Book) is at the core of the conventional approach to highway design, and its design criteria are based on a large body of research and empirical data relating driver and vehicle performance to geometric characteristics. However, fewer new alignment roadways are being constructed and much more effort is being directed toward improving and recon- structing our existing infrastructure. Especially in urban areas, there is a much greater emphasis on the need for multimodal transportation solutions that fit as seamlessly as pos- sible into an established context. The 1990s began a greater focus on flexibility in highway design. As more designers begin to understand that the Green Book is not intended to impose rigid standards that artificially limit design options for a project, the use of appropriate flexibility in design has increased. To fully utilize that flexibility, the challenge to designers is to find solutions that balance often-competing objectives. Thus, more designers are beginning to see that design is a series of trade-offs and not simply a rigid application of design standards. The goal of this Synthesis was to discover what processes transportation agencies are currently using to evaluate design trade-offs between competing interests. The report also attempts to highlight any existing gaps in information or analysis processes available to support the design decision. The ability to adequately identify trade-offs associated with design decisions and strike a balance between competing factors is critical to developing transportation projects that maintain safety and mobility while preserving the scenic, aes- thetic, historic, social, and environmental resources of a community. This synthesis is based on a survey distributed to 52 state transportation agencies (STAs), which resulted in responses from 43 agencies: 41 STAs, the District of Columbia, and Puerto Rico. In addition, a literature review focused on key publications outlining the conventional approach to design, as well as the newer context-sensitive solutions/context- sensitive design (CSS/CSD) and performance-based planning approaches. It also presents publications that outline complimentary fields that can be used to evaluate trade-offs, such as value engineering, choosing by advantages, risk analysis, and management and safety. Rather than an exhaustive literature search on a single topic area, the synthesis attempts to present an overview of the wide range of techniques available from the highway design and related fields and how they relate to trade-off analysis. One of the key issues that the survey identified was that few STAs have codified proce- dures for evaluating trade-offs in highway geometric design. Based on the input received, the agencies surveyed generally had to rely on engineering judgment when conducting trade-off analyses. Most agencies evaluate trade-offs during preliminary engineering or environmental clearance. However, several agencies pointed out that frequently trade-offs are not raised until a design is nearly complete, often because of a lack of available design resources and decision makers in the predesign period. However, the later in the project development process trade-off decisions are made, the more limited the flexibility in deal- ing with them becomes.
2 Eleven typical categories of trade-offs were identified for inclusion in the survey instru- ment: (1) access management, (2) cost, (3) environmental issue, (4) historic impact, (5) human factors/driver expectancy, (6) operational efficiency, (7) right-of-way (ROW) avail- ability, (8) safety, (9) schedule, (10) social concerns, and (11) tort liability exposure. STAs overwhelmingly identified safety as the trade-off most used as justification for design deci- sions. Cost and environmental issues are also frequently used to justify design decisions. Approximately half of the STAs believed that there were gaps, problems, or missing com- ponents in their procedures and tools for evaluating design trade-offs. Some of the concerns identified were associated with a lack of formal guidance and procedures, which force STAs to rely on engineering judgment. Weaknesses of this approach are limitations associated with inexperienced staff, inconsistencies associated with informal practices, failure to ade- quately identify and consider appropriate trade-offs, and inconsistencies in documentation of decisions. Conversely, those agencies that did not believe there were gaps often pointed to processes and policies that, when followed, minimized gaps. Approximately three-quarters of the agencies did not have risk prediction tools or tech- niques to help balance competing interests in the design process. Those that did have tools in place almost all used ones that combine a mixture of qualitative and quantitative analyses. Approximately half of STAs have some tools and training to assist designers in evaluating trade-offs in the design selection process. Common tools identified are the Highway Safety Manual, the Interactive Highway Safety Design Model, Roadside Safety Analysis Program, value engineering, crash history, life-cycle cost analysis, and a design policy manual. How- ever, only five agencies have developed specific performance goals regarding the evaluation of trade-offs. Shoulder width was overwhelmingly the controlling criterion most often associated with a design exception request. Other controlling criteria are horizontal alignment, vertical alignment, and lane width. None of the respondents selected the controlling criterion of structural capacity, and several respondents added notes to the survey responses that this criterion would never be considered. Approximately three-quarters of the agencies had no plans to reevaluate how trade-offs are handled in the design selection process in the next 6 to 12 months. Approximately 90% of the agencies had no plans to reevaluate how design exceptions are handled over the same period. To evaluate the trade-offs associated with design, the designerâs understanding of the basic controls and criteria associated with each element of the design is important. Although the Green Book provides little guidance on evaluating these trade-offs, it does establish the framework from which most controls and criteria are derived. For many situations, there is sufficient flexibility within the design criteria to achieve a balanced design and still meet minimum values. CSS and CSD both consider the overall context within which a transportation project fits. The conventional approach to design does not emphasize an interdisciplinary approach, whereas the CSS/CSD approaches do. As the design process evolves, issues that do not center on design criteria become more important to determining the ultimate success of a design. This increases the need to identify trade-offs associated with design decisions accu- rately and completely and strike a balance between the competing factors in an interrelated decision-making process. CSS and CSD are excellent tools for providing structure to the compromise and trade-off process.
3 Several other closely related fields may offer insight into how to structure trade-offs to ensure the best decisions. Many of the tools, techniques, and processes currently utilized as part of the transportation field can be adapted for use in trade-off analysis, and some agen- cies have begun to incorporate them into everyday operations. These include performance- based planning, value engineering, choosing by advantages, project risk analysis, risk and reliability analysis, and risk management. Promoting safety and safe travel is at the core of all transportation planning and design. The basic principles and guidelines that influence much of what happens in project develop- ment are founded on professional principles of encouraging safe design. As such, trade-offs may influence the safety potential of an alternative. Several key areas provide insight into how to address these trade-offs: organizational accident analysis and prediction, safety- conscious planning, Road Safety Audits, the interactive highway safety design manual, and the new Highway Safety Manual. Preparation of this synthesis revealed that there are further research needs associated with evaluating trade-offs in highway geometric design. Several topics emerged as areas of interest for future study, including a formal process for evaluating trade-offs, risk pre- diction tools, tools for evaluating trade-offs, performance goals, online resources for the Green Book, impact of design consistency, the Highway Safety Manual, integration of project and system level trade-offs, and self-enforcing design.
