Crash Comparison of Single Point and Tight Diamond Interchanges

Abstract

Although the single point interchange (SPI) has been around for approximately 3 decades, very few attempts have been made to evaluate its safety performance with a similar configuration, the tight diamond interchange (TDI). This paper provides a crash comparison between the TDI and the SPI interchanges for intersection related crashes on the cross road only. The SPIs considered in this study are without frontage roads. Data from 27 diamond interchanges (DIs) in Washington were used in building a negative binomial model to predict total crashes and injury/fatality based on the off-ramp flow, cross-road flow, and separation distance between left and right ramp terminals. Crash analysis of 13 SPI sites was used to compare with newly developed crash models of diamond interchanges. Wilcoxon signed-rank test was applied for safety comparisons between the SPI and TDI. The safety comparison did not reveal a significant difference between the two types of interchanges neither for total crash. However, the single point urban interchanges were found to be safer than the comparable DIs for injury/fatality frequencies.