Impacts of Changing the Transit Signal Priority Requesting Threshold on Bus Performance and General Traffic: A Sensitivity Analysis

US Transportation Collection2020PDFTech ReportSchultz, Grant G.Sheffield, Michael HBassett, DavidEggett, Dennis L.Brigham Young University. Department of Civil and Environmental EngineeringUtah. Department of Transportation. Research and Innovation DivisionUnited States. Department of Transportation. Federal Highway AdministrationUtah Transit AuthorityUtah. Dept. of Transportation. Division of ResearchUtahUnited StatesBus trafficDwell timeGreen interval (Traffic signal cycle)On time performanceSensitivity analysisStatistical analysisTraffic signal priorityTravel timeDedicated short range communicationsReliabilityVehicle to infrastructure communicationsLateness thresholdV2IDSRCOTPTSP:Requesting thresholdSchedule deviationSplit failureUT- 20.0619-8279Final Report Sept. 2018 to June 2020A sensitivity analysis was performed on the transit signal priority (TSP) requesting threshold to evaluate its impact on bus performance and general traffic. Route 217, a conventional bus route, and the Utah Valley Express (UVX), a bus rapid transit line, utilize a dedicated short-range communication (DSRC)-based TSP system as part of their normal, day-to-day operations. Using field-generated data exclusively, bus performance and general traffic were evaluated over a seven-month period from February through August 2019. Bus performance was evaluated through on-time performance (OTP), schedule deviation, travel time, and dwell time, while the traffic analysis was performed by evaluating split failure, change in green time, and the frequency at which TSP was served. The requesting thresholds evaluated for Route 217 were 5, 3, 2, and 0 minutes, which stipulate how far behind schedule the bus must be in order to request TSP. For UVX, 5 minutes and 2 minutes, as well as always ON and always OFF scenarios were evaluated. A combination of observational and statistical analyses concluded with convincing evidence that OTP, schedule deviation, and travel time improve as the requesting threshold approaches zero with negligible impacts to general traffic. For Route 217, as the requesting threshold changed from 3, to 2, to 0 minutes, OTP increased 2.0 and 2.5 percent, respectively; mean schedule deviation improved 15.9 and 20.9 seconds, respectively; and travel time decreased at 72 percent of timepoints. Meanwhile, negative impacts to traffic occurred if an increase in split failure was measured after TSP was served, a phenomenon observed a maximum of once every 43 minutes. For UVX, as the requesting threshold changed from 5, to 2 minutes, to ON, OTP increased 7.6 and 4.7 percent, respectively; mean schedule deviation improved 24.3 and 15.0 seconds, respectively; and travel time decreased between 72 percent of timepoints. Thus, it is concluded that under the TSP system as implemented, bus performance improves as the requesting threshold approaches zero, with inconsequential impacts to general traffic.109