Evaluation of Transit Signal Priority strategies and the impact of background signal timing

Abstract: A Transit Cooperative Research Program (TCRP) report of 2020 assesses the current state of practice of transit signal priority (TSP) for 31 surveyed transit agencies in US and Canada. The study found that despite the wide spread adoption of TSP, there was no universal understanding of the selection of TSP strategies and parameters, the quantification of TSP benefits and impacts, and identified the need to further research advanced signaling approaches integrating TSP among other recommendations. A review of previous studies reveals that the effectiveness of Transit Signal Priority (TSP) is mixed, having been shown to reduce transit vehicle delays in some instances, while having minimal effect in others. Similarly, regarding the general traffic, some studies report minimal impacts while others report significant deterioration in the level of service experience by conflicting movements. In coordinated arterial systems, signal timing parameters including cycle lengths, splits, and offsets are typically selected to provide optimal service to the general traffic without consideration of transit vehicle operations. This potentially limits the flexibility of the signal control to accommodate TSP. Considering these gaps in research and practice, the current effort performs a series of simulation experiments to (1) compare the performance of different TSP strategies, (2) evaluate TSP performance under different levels of congestion, and (3) assess the impact of cycle length on TSP performance. Performance measures are determined by fusing high resolution signal controller data with traffic performance data.

On comparing the two TSP strategies of green extension (GE) and early green (EG) it was found that (a) GE tends to provide more significant bus travel time improvements than EG, (b) GE tends to have a lesser impact on conflicting movements than EG, and (c) the conflicting movement delay for the same number of seconds of GE versus EG tends to be higher and last for more cycles when EG is granted. It is seen that TSP performance is most favorable in lower to medium v/c conditions. As congestion increases the effectiveness of TSP decreases. On a highly congested corridor, with v/c ratios approaching or exceeding 1.0, it is possible that TSP may become infeasible as the conflicting non-TSP movements may have insufficient slack in available capacity to recover from the TSP related green truncation. As cycle length increases (for the same demand) the proportion of buses requesting TSP (GE) decreases, while the effectiveness of GE requests improves. In addition, bus travel time increases slightly and cross street delay due to TSP is lowest when the cycle is moderately higher than optimal (where optimal is calculated to optimize non-transit traffic demands). Cross street delay not in the presence of TSP increases as cycle length increases beyond optimal; however, the number of cycles required to dissipate the effect to the side street reduces as well as the impact per cycle when a TSP call is served.