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11 or far side to near side) can allow another preferential treat- The usual TSP treatment is a minor adjustment in signal ment to be applied or improve the performance of another phase split times. The green phase serving an approaching preferential treatment. For TSP, having the transit stop far side transit vehicle may stay longer or start sooner, so that delay at a signalized intersection will optimize the effectiveness of for a transit vehicle approaching an intersection can be reduced TSP, because transit vehicles can move through an intersection or eliminated. This is referred to as the "green extension/red without stopping to pick up or discharge passengers near side. truncation" concept. The expanded transit phase split time is Far-side stops are also preferred where buses use a bypass lane recovered during the following signal cycle so that a corridor near side to go through an intersection into a far-side stop, with signal timing coordination plan can be maintained. This con- or without supplemental signal priority. To implement a queue cept is illustrated in Figure 11. jump signal, having a near-side transit stop allows passengers to board and deboard before the signal is triggered. TSP can be activated either manually by the transit oper- ator or automatically using on-board technology. The auto- mated procedure is preferred because it eliminates the TRANSIT SIGNAL PRIORITY requirement for an operator to activate the emitter on the vehicle. In many cases, the automated TSP will be tied to an TSP alters traffic signal timing at intersections to give prior- automatic vehicle location (AVL) or automatic passenger ity to transit operating in a median transitway, in exclusive counter (APC) system that can determine if priority should bus lanes, or in mixed traffic. TSP modifies the normal sig- only be given if a certain condition in the transit operation nal operation to better accommodate transit vehicles while is being met--such as if the vehicle/train is behind sched- maintaining the coordinated operation and overall signal ule or if there are a certain number of passengers on board cycle length. TSP is different from signal preemption (typi- the vehicle/train. cally applied for emergency vehicles), where the normal sig- nal operation is interrupted through changing of the signal TSP detection can be identified by different means. In past cycle length, thus taking the general traffic progression out of years, many U.S. and Canadian agencies used optical detec- coordination associated with the preemption call. Signal pre- tion for transit priority requests from buses to signal con- emption is used by LRT trains when they operate in a sepa- trollers (see Figure 12). Inductive loop systems have also rate ROW and cross an urban street; however, the priority been applied, involving the use of an inductive loop embed- concept is applied when LRT trains travel along a street and ded in the pavement and a transponder mounted on the cross an intersecting street. underside of the transit vehicle. Another system includes use RED TRUNCATION GREEN EXTENSION Bus approaches red signal Bus approaches green signal SIGNAL CONTROLLER Signal controller detects bus; Signal controller detects bus; terminates side street green phase early extends current green phase Bus proceeds on green signal Bus proceeds on extended green signal FIGURE 11 Red truncation/green extension TSP concept [Source: TCRP Report 118 (5 )].
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12 FIGURE 12 TSP bus detection concept--Optical system (Source: Kittelson & Associates, Inc.). of radio frequency tags mounted on the vehicles that interact timings in the field to respond to current traffic conditions and with wayside reader stations (see Figure 13). New detection transit vehicle location. systems involving the use of global positioning systems (GPS) and wireless technology are emerging. TSP can be activated at either a distributed or centralized level. At the distributed level, decisions on TSP activation at There are three types of TSP strategies: passive, active, and an intersection are dependent on both the transit vehicle and real-time priority. Passive strategies provide some level of signal controller. At the centralized level, the decision to acti- transit priority through the use of pre-timed modifications to vate TSP is made by a centralized traffic management system. the signal system that occur whether or not a bus is present. Applications could range from just one signal to an entire TSP is typically applied when there is significant traffic signal system in a corridor. Active strategies adjust the sig- congestion and hence bus delays along a roadway. Studies nal timing after a transit vehicle is detected approaching an have found that TSP is most effective at signalized intersec- intersection. Either unconditional or conditional priority can tions operating under level of service "F" conditions, with be applied as an active strategy. Unconditional priority pro- a volume-to-capacity of ratio between 0.80 and 1.00. A vides priority for all transit vehicles equipped with detection, basic guideline is to apply TSP when there is an estimated whereas conditional priority only provides priority if a transit reduction in bus delay with negligible change in general vehicle meets some condition based on AVL and/or APC traffic delay. Given this condition, the total person delay data--such as if the transit vehicle is behind schedule or there (on both buses and general traffic) would decrease with the are a certain number of passengers on-board. Real-time or application of TSP at a particular intersection or along an adaptive strategies account for both transit vehicle and general extended corridor. TSP also has a positive impact in reduc- traffic arrivals at an intersection or system of intersections and ing travel time variability and hence keeping transit vehi- require specialized equipment capable of optimizing signal cles on schedule. Connection to Checkout Unit Antenna Transponder Reader Traffic Signal Controller FIGURE 13 TSP bus detection concept--Wayside reader system (Source: King County Metro).