Effects of ITS-Based Left Turn Policies on Network Performance

Increasing transportation network capacity is very important, particularly in congested traffic condition. In certain conditions, providing left turn green time could reduce intersection capacity because of: a) larger lost time due to added phase, and b) shorter green time for the through traffic movements. In this study, we will compare the effects of three ITS-based left turn policies on the performance of a congested transportation network. The base policy is allowing left turns in all intersections along a corridor of the network with high traffic demand. The second policy is prohibiting them in some intersections of that corridor, while the third policy is removing these turning movements from all intersections along that corridor. When a left turn is removed, the left turners are rerouted in the network. To make sure that in each policy the network is working at its optimal condition, we used Genetic Algorithms (GAs) to determine optimal signal timing parameters for each policy. The results showed that prohibiting left turns in every other intersection along an arterial of the case study network increased the total number of vehicles processed by the network by 6.6% and resulted in a total of 2550 processed vehicles and reduced average delay per vehicle by 10.7% resulting in delay of 196.3 seconds per vehicle. Removing left turns from all intersections along an arterial of the network increased the total number of vehicles processed by the network by 9.1% and resulted in 2607 processed vehicles. This policy resulted in a decrease in average delay per vehicle by 3.1% and resulted in 213.0 seconds of delay per vehicle. Thus, in periods of heavy traffic demand for through movement and low traffic demand for left turns, prohibiting left turns in all or some intersections of a network could result in a significant increase in the performance of the network as well as a significant decrease in delay per vehicle.