Traffic Planning Models for Single-Point and Tight Diamond Interchanges

Although the single-point interchange (SPI) has been around for approximately three decades, users do not yet have a simplified procedure to evaluate its traffic performance with a similar configuration, the tight diamond interchange (TDI). Several studies have derived general results without decisive conclusions or with partial coverage to assist the users in the selection process. Simulation modeling is used to compare operational traffic performances of isolated SPI and TDI without frontage roads. Modeling was conducted on similar geometries over a wide distribution of traffic flow conditions (101 scenarios). The 101 scenarios were run for both SPI and TDI to derive control delay, stop time, and percent stops from CORSIM. The multivariate statistical comparison of all three variables showed a combined significant difference between the two interchange types, favoring SPI. TDI created more delay, stop time, and percent stops when the left-turn off-ramp flows were very high. Regression models were derived to estimate the three measures for both SPI and TDI. The models are robust and are functions of the highest flows on the cross street and off-ramps. These models are tools that can help planners in the evaluation and selection process of these interchange configurations.