Traffic Signal Preemption at Intersections Near Highway–Rail Grade Crossings (2017)

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13 chapter three RAIL SYSTEMS The primary pieces of the railway system (see Figure 3) are the detection system (i.e., track circuits), the warning time system, and the warning devices (i.e., lights and gates). A state-of-the-art detection system like that at the Los Angeles Department of Transportation (LADOT), described in chapter seven, can provide data to the railway warning time system on the speed and location of a train, whereas older systems can only provide data on the presence of a train anywhere in the track circuit. Using the data from the detection, the railway warning time system evaluates the train’s location over time and predicts its arrival at the crossing. The railway warning time system then notifies other systems, including the grade crossing warning devices and the traffic signal system, when the train’s predicted arrival time is equal to the design value programmed in the railway warning system. For the purposes of this synthesis, this chapter focuses on railway warning time systems with lights and two-quadrant gates as well as constant warning time track circuits that predict the arrival of trains (typically used by mainline railway agencies). It is important to note that some transit railway cannot use the CWT systems described in this synthesis. Although complex systems exist (e.g., four-quadrant gates), they are specialized applications requiring specialized skills. This synthesis focuses on the operational capabilities of typical mainline railway systems, not the many design details necessary for specialized applications (e.g., detection systems not based on track circuits or intersections requiring preemption on multiple approaches). TYPES OF DETECTION SYSTEMS Monitoring the track approaches to a highway–rail grade crossing is the typical means of detecting the approach of trains. Historically, railway agencies have used simple direct current (DC) circuits in three regions—the approach from each direction and the island (as shown in Figure 8). Simple motion-sensitive DC circuits are still used at many crossings today (Ogden 2007), but they only measure a train’s presence in the track circuit and if it is moving. The time it takes a train to arrive at the crossing from when it is detected depends on its speed, but simple motion-sensitive presence detection has to activate the grade crossing warning devices and preemption at the traffic signal based on an assumed travel speed. In order to be conservative, the circuit length is constructed based on the fastest train so that there is enough time to meet MWT requirements. That means that if there is a situation where a track circuit has been constructed to provide 20 seconds of warning time for a train traveling at 60 mph, the slowest train traveling at 30 mph may arrive at the crossing 20 seconds after it was predicted to arrive; however, this variability could be significantly greater. Although there are several alternative circuits that may meet special needs, a CWT system or grade crossing predictor currently provides information about both train speed and distance from the crossing. A CWT system measures speed at a single point and activates the appropriate railway out- puts based on the predicted arrival time (calculated from the location and speed measurement). How- ever, even though a CWT system bases the train arrival on a measured speed, if the train accelerates or decelerates after activation, the arrival time will still be sooner or later than the calculated value. Variability in train arrival time caused by speed variation is addressed by railway agencies through buffer time (BT). The railway may indicate to the highway agency that the required warning time is 20 seconds—i.e., vehicles should be cleared from the track(s) within 20 seconds. However, the

14 railway may add five seconds of BT to assure the 20 seconds is generally met. This means that under typical field conditions, a train may be moving at a constant speed and the highway agency may observe that it takes the train 25 seconds to reach the crossing after preemption activates instead of the designed 20 seconds of warning time. Although it might appear that there are five seconds of unused time, there may not be 25 seconds of warning time available to the highway agency because trains may, on some occasions, be speeding up as they approach the crossing. The highway agency should verify preemption design times with the railway agency, not from field observation. TYPES OF INTERCONNECT CIRCUITS While the many details of interconnect circuits are beyond discussion in this synthesis, the basic principles and important alternative concepts are discussed relative to traffic signal operations. Pre- emption at the traffic signal can be operated differently depending on the types and number of out- puts provided by the railway warning time system (options described in chapter four). Note that not all of these outputs may be available at a particular location; the outputs will depend on the specific location, highway agency requests, and railway agency policies. Simultaneous Preemption Circuit The simplest railway systems predict the time at which the train is at the design MWT point on the approach (actual value may vary owing to BT and train speed variability). When this event occurs, the railway system activates the railway warning system using a relay, known in the railway industry as the “XR” or “XC.” This is the same process that activates the simultaneous preempt relay for the traffic signal, that is, railway output 2 (RO2) in Figure 3. Note that it may either be the only railway output or one of several. Advance Preemption Circuit As discussed in chapter two, simultaneous preemption may not provide adequate RTT to meet the required TCGI in many conditions based on highway and railway agency practices, particularly con- sidering pedestrian clearance intervals and vehicle queue clearance times. To provide more time for the traffic signal to finish timing these clearances, advance preemption time (APT) can be provided. APT is obtained by extending track circuits to detect trains at a greater distance than required for the MWT. APT is railway output 1 (RO1) in Figure 3. FIGURE 8 Track circuits. Source: Kittelson & Associates, Inc.

15 The APT is provided to the traffic signal before the railway warning system is activated, so that the traffic signal system can begin an immediate preemption to clear vehicles off the track(s) (described in more detail in chapter four). The railway warning time system then uses a second prediction to determine when to activate the warning devices (RO2). In simple APT systems, RO2 is only sent to the railway warning devices; in some systems, RO2 is also sent to the traffic signal system. In theory, the time between the APT and when the warning devices activate is to be equal to the APT in the system design, but the warning devices may activate sooner (if the train speeds up) or later (if the train slows down). This variability can be addressed in several ways, as discussed in detail in chapter five. Gate-Down Circuit A concept that can be used to address the preempt trap (discussed in chapter four) and the variability in train speeds is called gate-down (GD) confirmation. Although there are other methods (NCHRP Report 812), they are less efficient from an operational standpoint. The GD concept allows the TCGI to dwell in green until the gates are down and then time the final portion of the TCGI. Use of the GD output (RO3) creates an additional complication if the gates are not confirmed to be down (e.g., broken gate). If the GD output is not given as a result of a broken gate, the dwell in TCGI will continue until the preempt is lifted by the railway warning time system. This gate issue can be addressed in two ways. The first way is to “wrap” the island circuit with GD (i.e., parallel outputs) so that the GD output is provided by the island circuit output if GD is not con- firmed. A simpler solution is to dwell in TCGI until the railway warning system is active (RO2) and then time the final portion of the TCGI using the time required for the gates to drop (10–15 seconds) plus the amount of time for the TCGI to continue after the gates are down (also discussed in chapter four). This approach provides the same desired TCGI regardless if the gates are broken or not. Island Circuit The railway uses the island circuit to know when a train is in the crossing and when it leaves, so it can terminate the railway warning system. The island circuit (RO4 in Figure 3), whether used with GD or alone, can provide an important piece of data for operational understanding. First, it can be noted that the island circuit is larger than the roadway width, so the circuit is active a few seconds before the train enters the crossing. This provides a margin of safety for railway system operations, as the railway agency uses the island-circuit output to turn off the warning devices once the train leaves the island circuit. The highway authority may request the island-circuit output, but the railway agency may or may not be willing to provide it because it is optional in the AREMA Communications & Signals Manual. If provided, the highway authority can use the island-circuit output to better understand the actual time relationships of various railway outputs. This is helpful in understanding railway “train-handling” issues resulting from variable speeds or switching activities. It can also provide information on false preemption calls. The Portland case example in chapter seven demonstrates how multiple outputs (including GD and island-occupied systems), along with traffic signal controller data, can be used to understand train-handling issues.