Estimating Sustained Service Rates at Signalized Intersections with Short Left-Turn Pockets

A computational approach for incorporating the effects of short left-turn pockets on sustained service rates in a mesoscopic modeling environment is presented. Mesoscopic models, intended to handle fairly large networks while maintaining individual vehicle identity, provide a detailed yet efficient alternative to estimate sustained service rates. However, mesoscopic models typically ignore midlink perturbations and queuing, thus limiting their reliability in the presence of short left-turn pockets at signalized intersections. The model presented here relies on a gating mechanism situated at the entry point to the left-turn pocket. Through a series of logical triggers, the gating mechanism allows for the formation of a (vertical) queue of vehicles upstream of the pocket when arrivals exceed storage capacity. The method satisfies all assumed requirements for integrating the effects of short left-turn bays, which include pocket spillback, pocket starvation, and sensitivity to signal timing and phase sequence. The approach has been implemented within the mesoscopic modeling platform DYNASMART for both single and multiple left-turn pockets, with the use of varying pocket lengths and demand volumes. The resulting sustained service rates favorably compare to those generated by a representative microsimulation model (VISSIM). Further comparisons of the proposed approach against empirical observations are planned.