Comprehensive Evaluation on Transit Signal Priority System Impacts Using Field Observed Traffic Data

To improve the level of service for Community Transit (CT) buses, the South Snohomish Regional Transit Signal Priority (SS-RTSP) project has been launched. To understand the overall benefit of this project, the SS-RTSP system was tested and evaluated after the completion of the hardware and software installations on the 164th Street SW street 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 based on field observed data. Simulation models were also built and calibrated to compute measures of effectiveness that cannot 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 introduced remarkable benefits to transit vehicles, with insignificant negative impacts to local traffic on cross streets. The overall impact of the SS-RTSP system on local traffic of 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. Considering that negative impacts of the SS-RTSP on local traffic were not significant, more transit trips can be granted with proper TSP treatments to generate more benefits from the SS-RTSP system. Also, near-side bus stops were found to introduce extra transit delays when TSP was provided under certain conditions. Our recommendation is that the TSP treatment of extended green be disabled at intersections with near-side bus stops to avoid introducing negative impacts on transit vehicles.

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