Comparison of the Operational and Safety Characteristics of the Single Point Urban and Diamond Interchanges

About the report:

The purpose of this study was to evaluate and compare the safety and operational characteristics of the single point urban interchange (SPUI) and the diamond interchange (DI) and develop guidelines that identify traffic and/or geometric conditions that favor one type of interchange over the other. State traffic engineers across the country were surveyed as to their experiences and opinions of the two types of interchanges in terms of operational performance and safety. Accident and operational data were also obtained for nine SPUIs and eight DIs. The interchange operations were studied using both field data and the computer simulation program TRAF-NETSIM. In addition, for each DI simulated, an SPUI counterpart was designed with the same geometric and traffic conditions for further comparison. No significant differences were found in the off-ramp, cross road, and overall interchange average delay. Ten volume scenarios were then developed and simulated for both interchanges at low- and high-volume conditions to analyze the effect of various traffic patterns on the relative operational performance of each interchange type. The accident data for the SPUIs and DIs were compared according to severity, collision type, and location on the interchange. No significant differences were found between the severity distribution and rates of the two interchange types. However, the proportion of on-ramp and off-ramp accidents was greater at the SPUI, and the proportion of accidents occurring in the center of the signalized intersection was greater at the DI. The proportion of angle accidents was greater at DIs than at SPUIs, whereas the proportions of rearend on-ramp, sideswipe, and fixed object accidents were greater at SPUIs. Vehicle conflicts at four interchanges in Virginia were also investigated and used along with the accident analysis results. The operational results, safety analyses, literature review, and survey of state engineers were used to develop guidelines to aid traffic engineers in the selection and design of the appropriate interchange type.

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