Analysis and evaluation of the impact of the length of left-turn lane on signalized intersection delays

The most common guidelines for determining the lengths of left-turn lanes are based on probability of accommodating a left-turn traffic at least traffic 95 percent of the time. These guidelines do not directly take into account the delays caused by through traffic for potentially blocking left-turn lanes. In this research the impact of the lengths of left-turn lanes on intersection delays are considered to optimize the lengths of the left-turn lanes. Data for traffic counts, queue lengths and signal timing are collected from an intersection in Las Vegas. The methodology involves development of simulation model using Corridor Simulation (CORSIM) and simulating various scenarios by varying traffic parameters to evaluate delays caused by varying lengths of the left-turn lane. Optimal lengths are computed and are compared to the 95 percent guidelines. Significant differences in lengths of the left turn lane are found for protected-permitted phasing. For protected left-turn phasing, the difference was not significant. The corresponding delays to these lengths are compared. The difference between control delays for protected-permitted phasing are found to be significant whereas for protected left phasing are found to be similar.