Impacts of Signal Phasing Sequence on Left-Turn Operation

The sequence of signal phasing (the order and combination of movements that make up the signal phasing) can have great impacts on the safety and efficiency of the left-turn operation at an intersection. Generally, there are two critical problems in left turn operation: (1) left-turn lane overflow and (2) the blockage of left-turn lane entrance by through traffics. Increasing the length of the left-turn lane is a direct solution for these two problems. However, the length of the left-turn lane was limited by various local factors and could not have been increased as desired in some intersections. Improving the sequence of the left-turn signal phasing provides another solution to these problems. In this study, the traffic simulation model (SIMTRAFFIC) was used to investigate the impacts of the signal phase sequence on left-turn operation under various traffic and roadway geometric conditions. The results revealed that choosing the appropriate signal phase sequence can significantly reduce the vehicle delay caused by the left-turn lane overflow and blockage problems. In addition, criteria for selecting the sequence of the signal phasing that best suits different traffic and roadway geometric conditions were recommended in this paper.