Case Study Conducted in Utah Tested Short-Range Communication-Based Transit Signal Priority System to Improve Bus Performance and Traffic Flow.
Impacts of Changing the Transit Signal Priority Requesting Threshold on Bus Performance and General Traffic: A Sensitivity Analysis
Transit Signal Priority (TSP) is an operational strategy that facilitates the movement of transit vehicles through traffic-signal controlled intersections with the objective of reducing travel time and the variability in travel time of transit vehicles. The Utah Department of Transportation (UDOT) and the Utah Transit Authority (UTA) sought to implement TSP to reduce transit travel time and travel time variability. This study assessed the impacts of TSP on bus performance and general traffic conditions. The tested TSP system utilized dedicated short-range communications (DSRC) technologies and a modified Multi-Modal Intelligent Traffic Signal System (MMITSS or MMITSS-UT for Utah’s system). DSRC allowed buses to communicate with traffic signals along Route 217 in Salt Lake County and the Utah Valley Express (UVX) in Utah County; the former was a conventional bus route while the latter was a bus rapid transit (BRT) route. Field generated data from February to August 2019 were used to evaluate bus performance and general traffic conditions.
Bus performance in this study was evaluated based on (i) on-time performance (OTP), (ii) schedule deviation, (iii) travel time, and (iv) dwell time; and the general traffic analysis was performed by evaluating (i) split failure, (ii) change in green time, and the (iii) frequency at which TSP was served. For OTP, the acceptable range of lateness was five minutes on Route 217 and two minutes for UVX. The requesting threshold was how far behind schedule that buses must be to request TSP. For Route 217, the thresholds were five, three, two, and zero minutes. For UVX, the thresholds were five and two minutes; two additional scenarios were ON (buses could request TSP regardless of lateness) and OFF (TSP was not active). General traffic was primarily evaluated by the occurrence of split failure 10 minutes before and after TSP system activation. Split failure occurs when green time is unable to meet traffic demand, but for this study, it occurred when occupancy exceeded 85 percent of green time and the first five seconds of red time. System performance and TSP impact on the system were quantified by the frequency at which TSP was requested.
- The results showed that as the requesting threshold approached zero, OTP, schedule deviation, and travel time improved, while general traffic was only negligibly impacted.
- As the requesting threshold changed from three to two minutes and from two minutes to zero minutes, OTP improved 2.0 percent and 2.5 percent, respectively.
- As the requesting threshold changed from three minutes to two minutes and from two minutes to zero, mean schedule deviation improved 15.9 seconds and 20.9 seconds, respectively.
- The data showed that median travel time between timepoints decreased an average of 2.1 seconds each time the requesting threshold changed from five minutes, to three, to two, to zero. Median travel time decreased as the requesting threshold approached zero at 26 of the 36 timepoints evaluated (72 percent of timepoints).
- Dwell time at individual timepoints for the zero-minute threshold was generally higher than it was for the other thresholds, often with little difference in dwell time among the five, three, and two-minute requesting thresholds. This indicated that the zero-minute threshold generally led to higher dwell time than the other requesting thresholds, which was due to receiving TSP when the bus was only slightly behind schedule.
- The traffic analysis showed that negative impacts to general traffic rarely follow the serving of TSP. Even though serving TSP at the zero-minute threshold impacted general traffic three to four times as often as the two-minute threshold, negative impacts were infrequent and were observed a maximum of once every 43 minutes.
- As the requesting threshold approached zero, OTP and schedule deviation improved, while travel time achieved negligible improvements.
- For OTP, as the requesting threshold changed from five minutes to two minutes and from two minutes to ON, improvements of 7.6 percent and 4.7 percent, respectively, were realized.
- For schedule deviation, as the requesting threshold changed from five to two minutes and from two minutes to ON, mean schedule deviation improved 24.3 seconds and 15.0 seconds, respectively.