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6 chapter two OVERVIEW OF THE HIgHWayâRaIl gRaDE CROSSINg SySTEM Preemption of traffic signals near highwayârail grade crossings fundamentally involves interconnec- tion of the railway warning time system and the highway traffic signal system. The point of connecting these two systems is to ensure that vehicles are not on the track(s) when a train arrives at the crossing. There are many ways that a traffic signal can clear vehicles, but how efficiently that can be achieved depends heavily on the timeliness and accuracy of information provided by the railway agency. Historically, preemption has involved simple one-way communication from the railway system to the highway system. In recent years, however, AREMA has upgraded its Communications & Signals Manual to provide guidance on improved interconnection, including information about the use of multiple railway warning type system events. Traffic signal manufacturers have also improved their controller features to provide additional functionality, giving agencies better options for imple- menting traffic signal preemption. Although some agencies have implemented these more advanced concepts (as will be discussed in chapter seven, Case Examples), current practice for most agencies has remained unchanged and is widely varied. CONCEPT OF OPERaTIONS This âconcept of operations,â which is a non-technical description, explains the different ways a highwayârail grade crossing system can work. The following description focuses on how a state-of- the-art highway-preemption system could operate, so that a practitioner can better understand current practice. Figure 2 illustrates the major components of the railway system and the highway system. The interconnection between them represents the communication that can or does take place between the two systems. The technical details will be detailed in chapters three through five. The preemption process begins with the railway system detecting the approach of a train. There are a variety of detection technologies ranging from simple detection of presence (i.e., whether or not a train is anywhere in the detection zone) to more sophisticated systems that predict the arrival of a train at the crossing based on its speed in the detection zone. Once a train has been detected and its arrival time at the crossing predicted (which is the design value), the railway warning time system sends a preemption output through the interconnection to the traffic signal system. The actual railway output may vary from the design value as discussed here. The output from the railway system is often very simple (i.e., âtrain comingâ). If requested by the highway agency and agreed upon by the railway agency, outputs can be more complex, giving advance notification time, status of the railway warning devices, status of the gates, and presence of a train in the crossing area. Typically, the railway system guarantees a minimum amount of time for preemption, but in practice, outputs from the railway system generally assume the same designed amount of time every time, regardless of whether there is any time variance. For example, most sys- tems are unable to account for variable train speeds, which can cause significant variation in when a train arrives at the crossing (as discussed in chapter three). The traffic signal system takes the railway output (or in some cases multiple outputs) and imple- ments a preprogrammed sequence of events, depending on the nature of the railway output(s) and the capabilities of the traffic signal system. The response is typically simple: Abruptly stop the current operations and move to the track clearance green interval (TCGI). The TCGI is the time that is pro- vided to clear stopped vehicles from the track(s) on the approach to a traffic signalized intersection.
7 The TCGI calculation involves many considerations including minimum green, pedestrian clear- ance, vehicle clearance, clear storage distance, minimum track clearance distance, design vehicle, vehicle-gate interaction, and others; details of the calculation are not included in this synthesis. More sophisticated systems provide longer advance notice and allow more orderly termination of opera- tions, including, for example, not leaving phases sooner than necessary and providing full pedestrian clearance time (CT). IMPORTaNT CONCEPTS aND TERMINOlOgy Before providing a more technical description of how these processes can and do work, it is neces- sary to define terminology and explain conventions used throughout this synthesis. In most cases, railway operations are kept separate from roadway operations, but the two must interact to allow the crossing to be clear of vehicles when a train arrives. Railway Outputs Circuits can be used to monitor the track approaches and send information to the railway warning time system and highway traffic signal system. Figure 3 illustrates the different outputs that can be used for railway preemption; note that times are shown for example purposes only. Throughout this synthesis, the railway outputs (RO) will be numbered in chronological order as shown in Figure 3 and used consistently, regardless of the actual number of railway outputs provided to the highway system. The use of a railway output numbering system is necessary because some implementations use more than one railway output, adding complexity beyond the more traditional single railway output. Minimum Warning Time In order to know how much time is available for a signal to clear vehicular queues that may be pres- ent over the track(s), a practitioner must first know the minimum warning time (MWT) required to activate the railway warning devices; it also known as prescribed warning time in Part 234 of Title 49 of the 2016 Code of Federal Regulations (49 CFR 234). MWT is defined as the least amount of time that warning devices shall operate prior to the arrival of a train at a highwayârail grade crossing, and occurs at the same time as railway output 2 (RO2, as per Figure 3). 49 CFR 234 requires that warning time be tested annually by railway agencies or when the warning system is modified. MWT is the sum of a minimum time (MT) and a clearance time (CT), as shown in Eq. 1. According to the AREMA Communications & Signals Manual, the MT is a set value of no less than 20 seconds FIGURE 2 Highwayârail preemption system major components. Source: Kittelson & Associates, Inc.
8 as prescribed by the 2009 FHWA Manual on Uniform Traffic Control Devices for Streets and High- ways (MUTCD), whereas the CT is variable depending on the minimum track clearance distance. Additional clearance time can also be requested by the highway agency, subject to approval by the railway agency. MWT MT CT20 seconds if required (1)( ) ( )= + where MWT = minimum warning time (seconds), MT = minimum time (20 seconds), and CT = clearance time (seconds). FIGURE 3 Railway outputs. Source: Kittelson & Associates, Inc.
9 Depending on highway and railway agency practices, an upper limit for MWT in the range of 35 to 45 seconds may be imposed. Some states have allowed longer times through the use of such tools as four-quadrant gates, median barriers, and traffic conversion to one-way streets. Constant warning time (CWT) railway detection systems; that is, grade crossing predictors, can generally provide reasonably consistent warning times for this notification if trains are moving at a relatively constant speed (see Figure 4). However, areas with nearby switching are not able to provide consistent warning times because of the variability in train speeds; trains often accelerate or decelerate in switching areas, or as they move into or out of stations or sidings. Railway Preemption actions The actions highlighted in Figure 5 will be used throughout the following section to describe pre- emption on both the railway and highway sides. Use of colors will be carried throughout to explain various concepts. For example, the time required for right-of-way transfer will be shown in red as highlighted in Figure 5. Simultaneous Preemption There are multiple ways that a traffic signal controller can transition a signalized intersection before a trainâs arrival, but they all depend on the available railway outputs. Most systems use a single pre- empt that is immediately implemented by the traffic signal controller as the railway warning system is activated (i.e., crossing lights begin to flash). This most basic form of preemption is called simul- taneous preemption; Figure 6 demonstrates the concept, with the simultaneous preempt labeled as RO2. Multiple preempt systems will be discussed in chapters three through five. Entry into traffic signal preemption is the most critical stage of the railway preemption process. In this stage, the right-of-way is transferred to the track clearance green interval (TCGI), which is the green interval associated with the signal phase(s) that clear vehicles queued on the track(s). The time required to transfer the right of way to the TCGI (from the time preemption is activated in the traffic signal controller) is known as the right-of-way transfer time (RTT). The components of RTT include the minimum allowable green for the current conflicting vehicle green phase, the FIGURE 4 Effects of constant train speed, acceleration, and deceleration. Source: Kittelson & Associates, Inc.
10 time required for the pedestrian clearance if active (if agency policy is to serve the pedestrian phase during preemption), and the time required for the yellow change and red clearance intervals of the active conflicting phase. The RTT plus the time to clear the vehicular queue must be less than the MWT under simultaneous preemption. Otherwise, vehicles could still be on the track(s) when the train arrives. Many agencies, as allowed by the MUTCD, choose to reduce RTT by not serving (i.e., truncating) pedestrian clear- ance and/or reducing minimum green to as little as zero seconds. When simultaneous pre emption cannot meet MWT needs, advance preemption can be used. Alternatively, some highway agencies will ask for additional CT to allow for more RTT; this is subject to the maximum value that is agree- able to the railway agency. advance Preemption Simultaneous preemption, while common, often requires the shortening of minimum green times and pedestrian clearance times, even if additional CT is provided by the railway agency. The situ- ations in which simultaneous preemption may not be adequate include when it is undesirable for a FIGURE 5 Railway preemption actions at an at-grade crossing. Source: Adapted from NCHRP Report 812: Signal Timing Manual, Second Edition (Urbanik et al. 2015).
11 pedestrian phase to be truncated; or when longer queue clearance times require longer TCGIs. Long TCGI queue clearance times may result from substantial space between the intersection and the crossing or when the queue has vehicles with longer start-up lost times (such as trucks). In such cases, a practitioner can request an advance preemption output from the railway agency (labeled as RO1 in Figure 7), in order to have additional time to transition traffic signal operations. In the example shown in Figure 7, the design RTT ends before the railway warning system is active, FIGURE 6 Design simultaneous preemption. Source: Kittelson & Associates, Inc. FIGURE 7 Design advance preemption. Source: Kittelson & Associates, Inc.
12 which may or may not be the case in all situations. This example design also provides a separation time after the design vehicle clears the crossing and the train is predicted to arrive. Although separa- tion time is desirable, it is not a requirement and difficult to achieve without a long RTT. It is important to note that advance preemption is not seen favorably by some agencies because of the potential for a preempt trap (more information provided in chapter four; reference NCHRP Report 812: Signal Timing Manual, Second Edition for a complete explanation (Urbanik et al. 2015). Although advance preemption can be avoided in some cases by increasing CT, simultaneous preemption cannot always accommodate longer RTTs required at some locations. It will be seen in the case examples in chapter seven that there are efficient ways to operate a traffic signal using mul- tiple railway outputs to prevent a preempt trap. However, even if a highway agency would like to use advance preemption, some railway agencies may be unwilling to provide multiple outputs (despite being included in the AREMA Communications & Signals Manual). This was the case in the Ohio case example in chapter seven. Traditional Immediate and Smart Preemption There are two terms (not in current use) that will be used throughout this synthesis in order to clarify the type of railway output being used by the traffic signal system in some highway agencies. âTraditional immediate preemptionâ requests that immediate action be taken by the traffic signal upon receipt of the railway output. Historically, this could be an advance request or a simultaneous request, not both. âSmartâ preemption involves receiving information before the railway warning devices become active, but not implementing an immediate termination of current activities at the traffic signal. Instead, the controller implements the TCGI no later than necessary, while also maintaining current opera- tions if there is enough time. Smart preemption requires more than one railway output. This type of preemption is only possible in more state-of-the-art systems and requires collaboration between the highway and railway agencies to achieve a mutually acceptable solution. Smart preemption is a form of traffic signal operation that is similar to priority control used in transit applications, which is discussed in NCHRP Report 812.
