Impact of Left-Turn Spillover on through Movement Discharge at Signalized Intersections

While the Highway Capacity Manual (HCM) 2000 does provide a separate procedure to estimate the back of queue for a lane group, its signalized intersection analysis procedure does not explicitly account for the impact to through movement traffic as a result of left-turn bay spillover. If the HCM methodology is used for an analysis of an intersection approach where left-turn spillover interferes with the discharge of through vehicles, the estimated delay results, and corresponding level of service, will be overly optimistic. Thus, the objective of this study was to determine the factors that significantly affect left-turn lane spillover and develop a model, or models, to predict the expected through movement capacity and discharge rate as a function of this spillover. Two approaches were used in this study: an analytical approach that developed a probability of left-turn spillover equation to incorporate into a capacity estimation equation, and an empirical approach incorporating the use of microsimulation to develop equations to estimate through movement discharge as a function of left-turn spillover. The developed probabilistic model was an enhancement to one from the literature. However, its accuracy of estimation was still limited due to the issue of intercycle queue-formation dependencies not being taken into account. The two models developed from regression analysis of simulation data—one for intersection approaches with only a single through lane and one for intersection approaches with multiple through lanes—replicate the simulation results quite reasonably.