NCHRP Report 785, Performance-Based Analysis of Geometric Design of Highways and Streets,
presents ways to incorporate performance-based analysis into the project development process. This process framework begins with setting desired project multimodal outcomes and design controls. Geometric design decisions that can influence those outcomes are identified as well as analysis tools that can be used to estimate the impacts of those decisions. The report includes six hypothetical case studies illustrating how this framework can be applied to actual projects. The report will be useful to geometric designers in making informed decisions about the tradeoffs inherent in design.
Most highway and street design processes rely on standards that set minimum values or ranges of values for design features. These standards are intended to provide operational safety, efficiency, and comfort for the traveler, but it is difficult or impossible for the designer to characterize quantitatively how the facility will perform. For both new construction and reconstruction of highways and streets, stakeholders and decision makers increasingly want reasonable measures of the effect of geometric design decisions on the facility's performance for all of its users.
Each agency has its own process for designing a highway or street. Three critical stages in the process are project initiation (i.e., setting the project's purpose, need, and scope), preliminary design (e.g., analyzing alternative designs and environmental impacts and setting design criteria), and final design (i.e., preparing the construction plans); these stages may have different names in different agencies. Although the expected performance of the facility is only one of the factors that must be considered in designing a highway or street, a better understanding of the expected performance should result in better decisions during these stages. Research was needed to provide the designer with the tools to evaluate the performance of different design alternatives objectively.
NCHRP Project 15-34A completed the work begun under NCHRP Project 15-34. In that project, Pennsylvania State University and Kittelson & Associates, Inc. described the geometric design decisions that occur throughout the project development process and identified performance metrics that are sensitive to those decisions. They also reviewed tools that are available for evaluating the performance of a particular design. This work culminated in the interim report that also presented a plan for developing a process framework.
In NCHRP Project 15-34A, Kittelson & Associates, Inc. and the University of Utah developed the process framework. The framework includes both an approach for integrating performance-based analysis into geometric design decisions and information on the effects different geometric elements have on project performance measures. It is expected that future research will build upon the latter to improve designers’ abilities to assess the performance of a design.
Supplemental material (including a summary of the work done in both projects, suggested future research, and draft text for AASHTO’s A Policy on Geometric Design of Highways and Streets) is also available at the link above.