Effects of Left-Turn Waiting Areas on Capacity and Level of Service of Signalized Intersections

Transportation professionals in China have started using an unconventional left-turn waiting area design as an innovative approach to mitigate traffic congestion at signalized intersections. The waiting area is set up beyond the stop bar at single or dual left-turn lanes at signalized intersections with lagging protected left-turn phases. This paper evaluated the effects of left-turn waiting areas on capacity and level of service of exclusive left-turn lanes at signalized intersections using empirical data. Capacity and delay models were developed by analyzing the arrival and discharge patterns of left-turning vehicles at single and dual left-turn lanes with differently sized left-turn waiting areas. The saturation headway, start-up lost time, and clearance time for left-turning passenger cars were estimated with field data measured from 22 sites. The capacity and delay models were calibrated and validated against both field-measured and simulated data. The results of sensitivity analyses showed that left-turn waiting areas increase the capacity of left-turn lanes at signalized intersections, and the capacity gains will increase with an increase in the storage capacity of the left-turn waiting areas. A procedure was proposed to determine the level of service of left-turn lanes with left-turn waiting areas at signalized intersections.