TSP-Friendly Underlying Traffic Signal Control, An Essential Complement of Transit Signal Priority

In principle, transit signal priority (TSP) should be able to reduce bus delay to near zero; however, in U.S. practice, bus delay reductions from TSP are often meager. This may be because in the U.S., active TSP (green extension and early green) is often applied within an underlying traffic signal control framework that is not TSP-friendly. TSP-friendly signal control encompasses passive transit signal priority techniques that minimize the bus phase red as well as control logic that provides flexibility in shifting the bus phase’s green to match the bus arrival time and compensation mechanisms for phases with overflow queues caused by priority actions, which is necessary to have aggressive active priority settings. Practically, TSP-friendly signal control is defined in this study as (1) control logic that is either fully actuated or coordinated to bus trajectories, and (2) phasing and timing plans that avoid long red intervals for the bus phase by (a) minimizing the cycle length, (b) allowing phase rotation, and (c) including multiple bus phases within the cycle, where the bus movement is a left turn or queue jump. Simulation tests at four sites in Boston find that applying active TSP together with TSP-friendly underlying control reduces bus delay 2.0 to 3.1 times as much as applying active TSP is applied on top of existing traffic signal control, with bus delay falling below 5 s per intersection at two of the sites and below 9 s per intersection at the other two sites.