To improve the level of service for Community Transit (CT) buses, the South Snohomish Regional Transit Signal Priority (SS-RTSP) project was launched. To understand the overall benefit of this project, the SS-RTSP system was tested and evaluated after the hardware and software had been installed on the 164th Street SW corridor (Phase One) and the SR 99 corridor (Phase Two) in Snohomish County, Washington State.
In this study, impacts of the SS-RTSP system on both transit and local traffic operations were quantitatively evaluated on the basis of field-observed data. Simulation models were also built and calibrated to compute measures of effectiveness that could not be obtained from field-observed data. With simulation models and field observed data, the impacts of the SS-RTSP system on both transit and local traffic operations were quantitatively evaluated. Our evaluation results showed that the SS-RTSP system provided remarkable benefits to transit vehicles, with insignificant negative impacts to local traffic on cross-streets under the current coordinated control strategy. The overall impact of the SS-RTSP system on local traffic at each entire intersection was not statistically significant at the p=0.05 level.
To improve the performance of the current SS-RTSP system, more transit vehicles can be made TSP eligible. The average number of granted TSP trips was only 16.96 per day per intersection during the Phase One test and 14.40 during Phase Two test. Given that negative impacts of the SS-RTSP project on local traffic were not significant, more transit trips can be granted with proper TSP treatment to generate more benefits from the SS-RTSP system.
Further simulation-based investigations on TSP system operations and optimization were conducted. The research findings indicated that to achieve the best operation efficiency, the compatibility between TSP control schemes and signal control coordination should be strengthened to minimize transit disruption to signal coordination. TSP systems must be fully tested under different coordinated control plans prior to implementation.