Relationship Between Geometric Design Features and Performance

The purpose of geometric design is to provide the necessary three-dimensional framework for a road or highway to address the identified problems by providing the appropriate level of mobility or safety improvements for the road users. Geometric design involves the application of tools, methods, dimensions, and criteria. Dimensional and other design standards and criteria are a means to an end. The end is transportation performance, including mobility, accessibility, safety, and state of good repair. Every phase, methodology, and model developed and applied to highway design and highway design criteria should be objectively related to one or more measures of transportation performance. The implications of this guiding principle are threefold. First, the mentality of the designer must shift from a dimensional-based approach to a performance-based approach. Second, the traditional philosophical approach to design has been to treat minimum design criteria as adequate to produce the performance desired. Third, in traditional design practice, the goal has been to meet these standards or criteria rather than specifically to provide a given level of service and safety. This paper discusses an approach to design that focuses on the performance of the facility rather than on traditional dimensional-based standards.