Concurrent Optimization of Signal Progression and Crossover Spacing for Diverging Diamond Interchanges

Diverging diamond interchanges (DDIs) are widely recognized to be capable of reducing conflict points, number of stops, and consequently average traffic delay. However, the design of their crossover spacing and signal offsets, which is critical to the capacity and efficient operations of DDIs, have not been addressed in most design guidelines. These two critical design components are actually interdependent in nature, because the estimated travel time between a DDI’s two subintersections for all movement paths is essential for the design of signal offsets. Also, the crossover spacing should be designed to accommodate queues comprised mostly of those vehicles not moving within the signal progression band, which is often designed with a given crossover spacing. Considering such an interdependent relation between signal offsets and the crossover spacing, this study presents a model that can concurrently optimize these two vital DDI design elements at the planning level. A case study at a DDI site with the proposed model has also been conducted to justify the necessity to perform the concurrent optimization under different operational conditions. The results of extensive numerical experiments confirm that the design with the optimized crossover spacing and offset can yield the shortest total delay and the least number of stops for vehicles over the entire network, especially under near-saturated conditions. The optimized crossover spacing can also prevent the formation of the queue spillover over the crossovers in a DDI.