In recent years, with the advance in telecommunication systems and sensor technologies, there is a renewed interest in providing signal priority systems to transit buses. Signal priority systems are considered one of the important components in Intelligent Transportation Systems (ITS). It is often conceived that such systems would maximize the number of people passing through an intersection each cycle, since transit vehicles generally carry more people than the average automobile. By providing priority to buses at the intersection, it is hoped that the bus movement would be uninterrupted at the intersection and the schedule adherence of buses would be improved substantially. The authors show in this paper that the benefits to buses in terms of delay reduction resulting from a signal priority system depend primarily on the traffic control parameters and the detector location. The actual delay reduction would slightly decrease with the increase in the saturation flow level. As a result, the fractional reduction in total delay resulting from a signal priority system would steadily decrease as the intersection becomes congested. The remainder of the paper is organized as follows. The authors will first provide an overview of the basic signal priority strategies for a transit signal priority system and state the assumptions made in this analysis. They will derive expressions for the expectation and variance of delay reduction for buses at an intersection with a signal priority system. This will be done without considering the physical queues and the detector location. The queuing restriction will be relaxed and the detector location will be considered explicitly. They will show that the mathematical expression derived for average reduction in delay with priority treatment is a good approximation under the queuing condition. Finally, the analytical result from the mathematical expression derived in this paper is compared with the simulation results over a wide range of vehicle/cycle length (v/c) ratios.
Abstract