TSP-Friendly Underlying Traffic Signal Control: An Essential Complement to Transit Signal Priority

In principle, transit signal priority (TSP) should be able to reduce bus delays 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 means control that minimizes the bus phase’s scheduled red period, offers flexibility to shift the bus phase’s green to match the bus arrival time, and includes compensation mechanisms that allow phases interrupted by priority actions to quickly recover, which in turn allows TSP to be more aggressive. Simulation tests at four sites in Boston find that applying active TSP together with TSP-friendly underlying control reduces bus delay 2 to 3 times as much as applying active TSP on top of existing traffic signal control without negatively impacting other vehicles or pedestrians. Aspects of TSP-friendly signal control demonstrated in the case studies include fully actuated control, reservice for minor bus phases, coordination that follows bus trajectories, phase rotation, and coordination following bus trajectories.