Signal priority near a major bus terminal in Boston, Massachusetts

Giving priority to buses at traffic signals helps minimize person delay and promotes use of public transportation. However, near major bus terminals, bus volume may be so high that it is impractical to interrupt the signal cycle for every bus requesting green. This study looks at priority strategies where bus traffic is high, and where buses are turning into and out of arterials with coordinated traffic signals. The objective is to minimize initial delay for buses, and then to dynamically provide them green waves through multiple intersections. Two challenges besides the very high bus volume are needed to maintain signal coordination for arterial non-bus traffic to prevent queue spillback, and resolving requests from conflicting streams of buses.;The study site is a network with four signalized intersections surrounding Ruggles Station in Boston. During the A.M. peak hour, 77 buses arrive and depart in mixed traffic on intersecting arterials that are near saturation. At the busiest intersection, only 3% of the vehicles are buses, yet 37% of the people passing through are bus passengers. Existing signal control has a coordination plan that favors through traffic, resulting in long delays to buses that have to turn into and out of the main arterials. Numerous priority tactics are tested using a traffic microsimulation model (VISSIM), with signal control logic programmed using VISSIM's internal language Vehicle Actuated Programming (VAP). They can be grouped into four general strategies: passive priority (an approach that does not rely on bus detection): adjusting cycle splits, phase sequence, and offsets to favor bus flows while still serving the main traffic flows; active priority: green extension, early green, phase insertion, and phase rotation; flexible control logic with natural compensation: cycle-constrained full actuation, coordination point floating, and dynamic coordination (where changes to the cycle at one intersection trigger corresponding changes at the next intersection in order to give buses a green signal at each intersection); capacity enhancement: modifications in signal timing plans to increase the capacity of intersections.;Simulation results indicate that during the morning peak hour, by using multiple advanced priority strategies that combine aggressive priority with compensation to affected traffic streams, proposed improvements will halve average bus delay from 90 seconds to 44 seconds, for an average delay reduction of 22 seconds per bus per intersection with no detectable impact on non-transit traffic vehicle delay. Each of the priority strategies tested makes a substantial contribution to this overall improvement. The greatest benefit comes from passive priority, which creates a coordination pattern favoring transit that eliminates much of the need for buses to request priority. Substantial benefit is also gained from the active and advanced strategies tested except for dynamic phase rotation.