Unlock DDI’s Capacity by Re-Routing Left-Turns at Nearby Intersections

Since the opening of the first Diverging Diamond Interchange (DDI) in the U.S. in June 2009, 34 DDIs have been completed as of March 2014. At least 40 more DDIs are being planned/designed/constructed. The sheer number of DDI projects indicates this innovative design is gaining wide acceptance. Field results demonstrate that this design is much more cost effective than the conventional improvement designs. By converting the 3-phase signal at traditional diamonds into 2-phase, DDI provides more effective green time for moving traffic. Most DDIs constructed to date are near large business, retail, and/or medical centers; and the intersections adjacent to DDI crossovers often have high traffic demands and require 6 or more phases and longer cycle length to serve the demands from conflicting movements. Such condition limits the DDI’s capacity utilization. This study explored three approaches to reduce the cycle length at nearby signals—relaxed Bowtie, Superstreet, and Quadrant Roadway intersections. They can all convert the adjacent intersections into 2-phase operation and enable the DDI to operate more efficiently. The ideas were tested in a simulated environment at a proposed DDI in Anchorage, Alaska. The simulation results indicate level-of-service at nearby intersections may be improved from D/E to A/B.