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16 chapter four TRAFFIC SIGNAL SYSTEMS This chapter focuses on how traffic signals near highwayârail grade crossings use railway outputs to clear vehicles from the track(s). It is important to understand that over the past six decades, the âarea near grade crossingsâ has had various definitions. The 1948 MUTCD suggested that preemption be considered for traffic signals within 500 to 1,000 feet of a railway crossing. In the 1961 MUTCD, the distance was reduced, for reasons that are not documented but may have reflected the complexity of dealing with larger distances, to 200 feet. The 1988 version indicated that preemption be limited to 200 feet except under unusual circumstances. However, the 2000 MUTCD added language that stated coordination with the flashing-light signal system be considered for traffic control signals located farther than 200 feet from the highwayârail grade crossing. Factors to be considered include traffic volumes, vehicle mix, vehicle and train approach speeds, frequency of trains, and queue lengths. Figure 9 illustrates the case where there is an immediate simultaneous preemption (RO2) occur- ring when the warning devices become active and the traffic signal is already in the TCGI. If pre- emption is activated when the controller is in the same phase as the TCGI, the actual right-of-way transfer time is zero (Urbanik et al. 2015), as shown in Figure 9. Although a RTT of zero seconds may give a first impression of an acceptable scenario, Figure 10 illustrates that for immediate advance preemption (RO1), the TCGI can terminate before the lights start to flash and the gates start to come down (Urbanik et al. 2015). In what is referred to as the âpreempt trap,â a queue may form over the track before the gates descend. Once the gates lower, the queue between the gates and the intersection will not be served by the controller until preemption is removed, potentially leaving the vehicles stranded on the track(s). In addition, actual advance preemption time may be longer than its design value as a result of decreased speeds as the train approaches the crossing. This extended APT furthers the potential negative effects of the preempt trap if the TCGI is shorter than the actual APT. ADDRESSING THE PREEMPT TRAP In order to eliminate the preempt trap and reduce the resultant safety concerns, several treatments can be implemented using existing railway and traffic signal technology (see NCHRP Report 812 for additional information). One such treatment is the installation of a not-to-exceed timer by the railway operation that forces the railway warning devices to activate no later than the end of the design APT (Communications & Signals Manual 2016). Implementation of the not-to-exceed timer should be coupled with a TCGI duration in the traffic signal controller that is at least equal to the APT. This strategy ensures that irrespective of both the variability in train speed and the corresponding APT, the railway warning devices will be activated before the TCGI ends, reducing the potential for vehicle queues over the track(s). This approach can result in much longer than necessary TCGIs. Another approach incorporates the gate-down output from the railway system into the traffic signal controller as a second preempt (RO3), as shown in Figure 11 (Yohe and Urbanik 2007; Sun et al. 2008). With this treatment, the controller receives the conventional advance preemption call, times RTT if any (in this example, actual RTT = 0), and holds (i.e., dwells in) the TCGI until a second preempt is received from the railway informing the controller that the warning gates are down. The GD preempt, assigned a higher preempt priority, releases the green hold resulting from the advance preempt and enters a timed TCGI (Engelbrecht et al. 2002).
17 A simpler alternative to the GD preempt is the use of a simultaneous preempt (i.e., railway warning system active) output (RO2) as a second preempt in addition to the advance preempt (RO1) to time the final design TCGI, as shown in Figure 12. This alternative begins the design TCGI when the warning devices are activated. Additional time for the gates to come down can be added to the timed TCGI in Figure 11 to account for the time required for the gates to descend. Note that Figure 12 illustrates a RTT of zero when the advance preempt occurs. FIGURE 9 Simultaneous preempt operation with no right-of-way transfer time. Source: Kittelson & Associates, Inc. FIGURE 10 Example of preempt trap when RTT is zero and there is no mitigation. Source: Kittelson & Associates, Inc.
18 Another approach is a âsmartâ traffic signal controller sequence (described in chapter two) following the advance preempt (Sun et al. 2008). This âsmartâ preemption approach is discussed in the California case example in chapter seven. The city of Portland, Oregon, also has a complex inter section with a smart implementation of multiple railway outputs. The operation includes two railway outputs before the warning system is active as well as gate-down, island-occupied, and traffic signal health. The concept, which has a number of practical benefits, is detailed in chapter seven. FIGURE 11 Gate-down preempt operation. Source: Kittelson & Associates, Inc. FIGURE 12 Two-preempt operation [Preempt 1 (RO1) and Preempt 2 (RO2)]. Source: Kittelson & Associates, Inc.
19 QUEUE MANAGEMENT Queue management is a proactive approach to reducing the potential for vehicles stopping on the railway track(s). This is often addressed using pre-signals or queue cutters. Both techniques have strengths and weaknesses. The Utah Department of Transportation (UDOT) has provided guidance on this topic in Preempting Traffic Signals near Railroad Crossings in Utah, A UDOT Manual. Pre-Signals A pre-signal is a supplemental traffic signal head that is operated as part of the nearby intersection traffic signal, located in a position that controls traffic approaching the highwayârail grade crossing and signalized intersection (Korve 1999). Figures 13 and 14 demonstrate the operation of a pre-signal at an at-grade crossing when it is used in combination with two preempts (RO1 and RO2). MUTCD guidance suggests that if the traffic signal is located within 50 feet (or 75 feet if multi-unit vehicles regularly cross at the grade crossing), consideration should be given to use of a pre-signal. If a pre-signal is used, it should display steady red during the TCGI. The pre-signal operation may also include a timed offset from the downstream intersection in order to keep the area between the track(s) and the intersection clear of stopped vehicles every traffic signal cycle. When appropriately timed, the space between the crossing and the downstream stop bar would be cleared every cycle. In addition, detection can be added for unusual conditions to further extend the green time for the downstream intersection if needed. An extensive discussion of pre-signals can be found in the Railroad-Highway Grade Crossing Handbook (Ogden 2007). Several states, including Illinois, Michigan, South Carolina, and Ohio, have standards for pre-signals when crossings are close to signalized intersections. Michigan uses pre-signals without a TCGI (Alroth et al. 1999). FIGURE 13 Example of a pre-signal at an at-grade crossing. Source: Kittelson & Associates, Inc.
FIGURE 14 Example of a pre-signal with two-preempt operation [Preempt 1 (RO1) and Preempt 2 (RO2)]. Source: Kittelson & Associates, Inc.
21 Queue Cutters The use of queue cutters is less common. Definitions of queue cutters include a separate traffic control signal upstream of a grade crossing that is: ⢠Intended to prevent vehicular queuing across the track(s) at a highwayârail grade crossing where traffic queuing occurs; ⢠Activated for one direction of travel by either an approaching train or actuation from down- stream queue detection; or ⢠Activated independently of or coordinated with an adjacent intersection traffic control signal. The intention is to react to building queues from a downstream signal in order to prevent queuing across the track(s) at locations that are at distances that make conventional preemption problematic. Either proactively, or in concert with preemption, queue cutters aim to prevent queuing across the track(s) by essentially storing excess vehicular demand upstream of the railway crossing. They can also be used upstream of roundabouts near grade crossings.
