Priority Strategies. 7 CHAPTER 4 METHODS 10 4.1 Measures of Effectiveness 10
Priority Strategies. The SS-RTSP system applies active priority strategies, which are dynamic signal timing enhancements that modify the signal phases upon detection of a transit vehicle. These strategies provide efficient operation of traffic signals by responding to a transit TSP call and then returning to normal operation after the call has been serviced or has expired. Although several active TSP strategies are available, such as phase insert and phase suppression (Xxxxx et al. 2002), only two active transit signal priority strategies are used in the SS-RTSP system: • early green (early start or red truncation of priority phase) • extended green (or phase extension of priority phase). Early green and extended green are the most common TSP treatments for transit vehicles. The early green strategy indicates a green light before the normal start of a priority movement phase. This process is implemented by shortening the green time of the conflicting phase(s), without violating the minimum green time and clearance intervals, so that the green time for the priority phase can start early. The extended green strategy is typically used when a transit vehicle arrives near the end of the green indication of a priority phase. When extended green is applied, the traffic signal holds the green signal of the priority phase for additional seconds to allow eligible vehicles to pass through the intersection without further delay. Depending on the signal control policy, green times for conflicting phases may or may not be shortened to compensate for the extended green for the priority phase. In the latter case, a constant signal control cycle length is not enforced. Both the early green and extended green strategies are intended to decrease transit vehicle delays at TSP-enabled intersections. Depending upon the arrival time of a TSP-eligible transit vehicle, early green or extended green may be used to provide an appropriate TSP treatment to the transit vehicle. The basic priority logic flowchart of the TPRG is shown in Figure 3-2. Some intersections may have additional logic or may conduct the eligibility tests in the readers. For the SS-RTSP evaluation, the TPRG sent a transit priority request to the traffic controller only for an eligible bus and only when the bus was • operating on one of the three test routes (114, 115, and 116) • equipped with Keypad • 0 to 30 minutes behind its scheduled time. Keypad is a device installed beside the bus driver’s seat to input the route number and tri...
