A Study on the Delay Model of Left Turn Bus on Left-Forbidden Signalized Intersection

Abstract

Left-forbidden management is widely used at signalized intersections at arterials in order to improve the efficiency by reducing conflicts between different traffic movements, bringing good order, and cutting down clearance and start-up time. This paper analyzes the delay of bus which is caused by conflicting with opposing through vehicles. A typical left-forbidden signalized intersection with bus lane of each approach in Kunming, southwest of China, is selected for the study. According to the field data, the bus headways fit the logarithmic normal distribution very well. A left turn bus conflict delay model is developed with four distribution types of the opposing through vehicles' headways and the left turn buses' critical gap is also given. 1. INTRODUCTION Intersections are bottlenecks of the urban road network. In order to reduce the delay and improve the level of service, left-forbidden management is widely used at signalized intersections at arterials in Kunming, southwest of China. It's obvious that this management method can reduce conflicts between different traffic movements at intersections and bring good order, lower start-up lost time and raise capacity. At left-forbidden signalized intersections, vehicles can fulfill a left turn by a right turn plus a U-turn or a U-turn plus a right turn. However, it also causes a problem that it is too long for buses to make U-turn at arterials. For the purposes of keeping public transit priority, buses are allowed to make left-turns at left-forbidden signalized intersections in the condition of safety, but are not provided with a left exclusive phase. So the Left Turn Buses (LTB) conflicting with the Opposing Through Vehicles (OTV) impact left-forbidden signalized intersections' operation. Left turn buses need to wait for an acceptable gap before the conflict point with the opposite through movement and then make a left turn (James 1999). While the opposing through volume is small, left turn buses can wait for an acceptance gap, and then cross the conflict point alternatively. With the opposing through volume increasing, left turn buses have no gap until the green time is over and then make left turn at clearance time. The bus headway distributions are tested to study LTB effect on bus headways which include subjective and opposing ones. This paper containing a model considers a variety of different distributions of OTV headways for analyzing LTB conflict delay. Headway is evaluated on the basis of the field data, and the conflict delay is evaluated in terms of the conflict delay model.