Guidelines for Selecting Single-Point Urban and Diamond Interchanges: Nationwide Survey and Literature Review

Abstract

Metropolitan areas in the United States are experiencing a steep increase in traffic growth and traffic congestion on their urban freeways. In the past, the most prevalent type of urban interchange has been the diamond interchange (DI). In more recent years, however, to overcome the need for significant additional right-of-way that is generally required for this type of interchange and still respond to increased traffic demand, a new type of interchange has emerged: the single-point urban interchange (SPUI). Traffic engineers do not agree about the relative operational efficiency of the SPUI compared with the DI. Also, the unfamiliar geometry and operation of the SPUI and its large, uncontrolled conflict area have raised several concerns about the safety of motorists who travel through it. The purpose of this study was to evaluate and compare the safety and operational characteristics of the SPUI and DI and to develop guidelines that identify traffic and geometric conditions that favor one over the other. We surveyed state traffic engineers across the nation about their experiences and opinions of the two interchanges in terms of operational performance and safety. A complete literature review on the subject revealed significant findings as well. We used both of these sources of information to develop guidelines to aid traffic engineers in selecting and designing the appropriate type of interchange.

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