Study on Capacity for Exclusive Triple Left-Turn Lanes with Outside Trap Lane at Signalized Intersections

Based on the survey data of three signalized intersections with exclusive triple left-turn lane and with outside trap lane in Beijing, traffic flow characteristics of left-turns are studied by using statistical methods. It shows that the discharge headway and saturation flow rate of exclusive triple left-turn lane with outside trap lane divides into two stages when the discharge time of vehicles in the left-turn bay is less than the left-turn green time. This problem is uncharted in the Highway Capacity Manual (HCM). This research analyzes the effects of relationship between the length of left-turn bay and green time on the left-turn capacity. A theoretic model for left-turn capacity is developed by considering this characteristic. The developed capacity model will be validated on the basis of VISSIM simulations of a real-world signalized intersection. And the results demonstrate that the proposed capacity models well estimate the left-turn capacity at signalized intersections in the case, which the length of left-turn bay and green time is not well optimized and well reflect the capacity loss due to the transitional area bottleneck.