Impact of Exit Ramp Geometric Treatments at Diverging Diamond Interchanges on Queue Spillback

Diverging diamond interchanges (DDIs) have swept the country as an innovative interchange system since the original U.S. installation in Springfield, Missouri, in 2009. Although the DDI provides many benefits, prior research identified some operational challenges found at some of the first DDIs built around the country. This study explored the challenge of limited capacity for the right turn at the DDI ramp terminal intersection and associated queue spillback concerns that have been observed at several sites. The study investigated the impact of five geometric treatments at a DDI’s right-turn exit ramp, including a channelized turn lane with an auxiliary lane, dual right-turn lanes, right turn on red (RTOR), dual right-turn lanes with RTOR, and a slip lane. The results showed that any treatment that decreased the green-to-cycle length ratio for the right turn from the exit ramp simultaneously reduced queues at the exit ramp and the outbound through movement at the crossover. However, treatments that did not also increase the capacity of the outbound through movement at the downstream adjacent intersection resulted in increased queues at that movement. The study concluded that the design of the DDI must be considered in the context of the entire corridor. To provide the full extent of congestion relief that is possible with a DDI, consideration of the geometrics of downstream, closely spaced, adjacent intersections must be included from the earliest planning stages.