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

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22 Interconnection is the process by which the railway system communicates with the traffic signal system, as shown in Figure 3. Historically, the interconnection has been a simple two-wire preempt. Even this simple circuit comes in at least two types, which will be discussed in more detail in the following section. Conversely, the most sophisticated communication involves serial communica- tion using IEEE 1570-2002, which allows for two-way communication, multiple data items, and the ability to update the data over time. SIMPLE SINGLE-PREEMPT CIRCUITS The simplest preemption circuits basically call for “preempt” or “don’t preempt.” However, this simple circuit has been implemented in two ways: normally open or normally closed. The MUTCD standard (Section 8C.09) calls for the circuit to be normally closed (i.e., the relay has power applied to keep the circuit closed), indicating no train. In that case, the loss of power (for instance, if the cable is cut) opens the circuit and sends the traffic signal into preempt. The power source can be either AC (typically 120 volts) or DC (typically 24 volts). Because 120 VAC is readily available, it is commonly used. However, DC control circuits pose less risk for maintenance personnel. In a typical design, the traffic signal cabinet provides 120 VAC to the railway cabinet to power the preempt relay. The normal state of the preempt relay would be closed until the railway warning time system detects a train and decides when to preempt the traffic signal by opening the preempt relay. Opening the railway-preempt relay removes power from the traffic-signal-preempt relay, which subsequently opens the traffic-signal-preempt relay. Figure 15 shows the operation of these two-wire circuits and their associated relays. This is the most basic and common interconnection. The primary limitation of this circuit is that the circuit can be compromised if the two wires are shorted together, preventing the traffic-signal relay from recognizing a call. A supervised circuit monitors the health of the electrical interconnection between the railway warning system and the traffic signal system, and is an alternative to a simple normally closed circuit, as indicated in the MUTCD standard. There are a number of supervised interconnect circuit designs that can address most of the failure modes. Discussion of the many alternative designs is beyond the scope of this synthesis, as there are no typical implementations. Supervised circuits are discussed further in chapter five. MULTIPLE-PREEMPT CIRCUITS To improve traffic signal operation during preemption, some agencies have upgraded their prac- tice to receive multiple preempts from the railway warning time system. These additional railway output circuits do not require train-detection circuits beyond those needed for advance preemp- tion. The advance preemption circuit (RO1) can provide additional time to address the largest design RTT when simultaneous preempt cannot meet agency requirements. Because the railway agency has to activate the railway warning devices, a second preemption circuit (RO2) can read- ily provide a second preemption to the traffic signal system when the railway warning system becomes active. chapter five INTERCONNECTION

23 The railway can also provide a gate-down output (RO3), but this requires additional railway logic to determine when all the gates are down and provide a backup output (i.e., gates broken) when the island is occupied. As noted in chapter four, a simultaneous preempt output (RO2) can be substituted for the GD circuit, with the only change being added time for the gates to reach a horizontal position (10–15 seconds). The last railway output that can be used by the traffic signal system is the island circuit. This provides train arrival information at the crossing. SUPERVISION To provide a fail-safe design, the preempt interconnect should be held closed, or energized. A circuit failure (for example, a simple break) results in a preempt request. However, because the interconnect cable can be compromised in several ways (shorts, loose connections, or open circuits), supervision is desirable. Double-break relays offer the most protection because energy switches between positive and negative energy. Because of the large number of different solutions available, specific supervised circuit designs will not be provided in this synthesis. The practitioner should understand that although supervision improves safety, relay-based designs get more complicated as the number of railway interconnection circuits increases. FIGURE 15 Simple two-wire interconnection (normally closed). Source: Adapted from Mansel et al. (1999).

24 ADVANCED SYSTEMS Typical interconnection systems are based on relays in the railway warning time system and the high- way traffic signal system. Although relay-based systems are extremely reliable, there are more state- of-the-art approaches to interconnection that increase functionality. The IEEE 1570-2002 interface is designed based on fail-safe and closed-loop principles, and is flexible enough to accommodate both current practice and additional functionality. The only significant implementation is the LADOT sys- tem’s serial communication to the traffic signal controller using RS-232, RS-422, or RS-485 systems, which is discussed in chapter seven. A second advanced system, also designed based on fail-safe and closed-loop principles, operates on the traffic signal side, residing in the traffic signal cabinet. It is a parallel interconnect with separate circuits for each railway output, using typical 14 AWG traffic signal cable in conduit. The interface in the traffic signal receives the railway outputs and intelligently processes them, sending several pro- cessed preempts to the traffic signal controller (typically preempts 1-6). The interface includes intelli- gent supervision, control of auxiliary devices such as preempt confirmation lights, and data recording options. The system can also provide traffic signal health to the railway, allowing earlier activation of the railway warning time system in the case of traffic signal failure.