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1 S U M M A R Y The typical purpose of geometric design process is to provide the necessary three- dimensional features for a roadway to address stakeholder-identified problems or needs by providing the appropriate level of mobility and/or safety improvements for all road users. Historically, the highway design process has followed AASHO, and then AASHTO, policies starting in the 1940s. Geometric design involves the application of tools, methods, dimen- sions, and criteria; dimensional design standards and criteria are a means to an end. The tools used in the current process have been dimensionally based and designers typically follow the values in tables and equations in the AASHTO or agency policies. The traditional philosophi- cal approach to design has been to treat minimum design criteria as adequate to produce acceptable performance. The goal has been to meet these standards or criteria rather than to specifically provide sustainable traffic operations and safety. Some of the dimensions or models included in the AASHTO policy have not changed much over the decades. The typical updates of the AASHTO policies and guides have been focused on various individual design elements and not on the overall design process. An assessment of the existing design process was undertaken to suggest changes to ensure that recent advances in knowledge and emerging issues are incorporated in the design process. In recent years strong stakeholder interest has emerged. A geometric design process that is responsive to these issues requires measurable transportation or environmental impact terms. This dialogue between design professionals and stakeholders is essential for developing an optimal solution, balanced with transportation and community goals. The transportation knowledge base has grown over the years. Research findings propose that the end goal of all the geometric designs needs to be measured in the metrics of trans- portation performance, including mobility, accessibility, safety, maintenance & operations (M&O), and state-of-good repair. Every phase, methodology, or model developed and applied to conducting the highway design and establishing the highway design criteria will ideally be objectively related to one or more measures of transportation performance. A change to the current AASHTO A Policy on Geometric Design of Highways and Streets (Green Book) as defined by location and functional classification is proposed. The revised geometric design process as part of this report provides guidelines based on the project type and the problem or need being addressed. A potential outline to a new policy is developed and provides direction to use the categories of new construction, reconstruction of an existing route, or rehabilitation of an existing facility as the basis of geometric design. The geometric design criteria for any given project is recommended to be based on the context of the project location, and not limited to the facility type. This revised highway design process is intended for further development to become fully implementable. A Performance-Based Highway Geometric Design Process
2 A Performance-Based Highway Geometric Design Process Additional knowledge gaps are identified as part of this research. A more robust range of definitions of land uses will address the requirements of the users and surrounding development that the roadway will serve. The AASHTO policy serves as the basis for the design manuals of the state departments of transportation (DOTs). These documents will need to be updated to make them consistent with the revised geometric design process. This could include a supplemental recommendation process guide as a companion to the design policies, minor changes to the policies, or a completely new structure to the policies.
