Section 2: Queue Prevention Strategies
Queue management is critical to grade crossing safety. A lack of queue management can become a large contributor when vehicular queues routinely extend onto and/or past the grade crossing. Pre-signals and queue cutters are examples of queue prevention treatments that can provide an alternative to the traditional queue clearance. They are designed to keep the minimum track clearance distance (MTCD) clear at all times. Designers should be aware of various queue management strategies discussed in this section and understand the applicability of each to determine the most effective treatment for a given grade crossing.
Figure 6–1. Geometric Data at a Grade Crossing
Pre-Signals
A pre-signal has traffic control signal faces that are located upstream from a signalized intersection and are operated in conjunction with the traffic control signal faces at the downstream signalized intersection in a manner that is designed to keep the area between the stop line for the upstream traffic control signal faces and the stop line for the downstream signalized intersection clear of queued vehicles. When used in conjunction with a grade crossing, the pre-signal is operated for the purpose of preventing vehicles from queuing within the Minimum Track Clearance Distance. Supplemental near-side traffic control signal faces for the downstream signalized intersection are not considered to be pre-signals.
Pre-signals may be installed when the downstream traffic signal is located relatively close to the grade crossing (less than 200 feet). Figure 6-2 shows the typical layout of a pre-signal installation downstream of the grade crossing.
Figure 6-2. Elements of a Pre-Signal
Pre-Signal Operations
A pre-signal’s phase sequencing shall be timed with an offset from the downstream traffic signal such that the pre-signal’s green indication terminates prior to the downstream traffic signal’s green indication in order to reduce the possibility of motorists queuing on the tracks. The distance between the crossing and the downstream intersection can affect motorist behavior. For example, if the clear storage distance (CSD) is long enough, vehicles may be able to safely queue within the CSD without intruding into either the intersection or the grade crossing. In this case, the offset between the pre-signal and the downstream signal may be timed to allow for queuing in the CSD. The pre-signal offset shall always be timed to clear the MTCD after every cycle. The pre-signal offset should be determined based on the time it takes a design vehicle traveling at the posted speed limit to traverse and clear the MTCD and CSD, if needed. The all-red clearance interval for the downstream signal should be determined based on the intersection width but should exclude the MTCD and CSD measurements as the pre-signal’s all-red clearance interval will be long enough to allow vehicles to safely clear the additional distance due to the pre-signal offset.
Pre-signals operate in conjunction with the downstream traffic signal. Pre-signal faces shall display a steady circular red signal indication during railroad preemption to prohibit highway vehicles from entering the grade crossing area. During the Preemption Clearance Interval, the downstream traffic signal shall display the green signal indications for any motorists between the grade crossing and the downstream intersection. Pre-signal indications shall remain red while the grade crossing flashing-lights are activated. Figure 6-3 depicts an example of a pre-signal phasing diagram, with Ø3 and Ø8 as the pre-signal phases.
Figure 6–3. Phasing Diagram for a Pre-Signal
Pre-Signal Design Considerations
As shown in Figure 6-2, several design elements should be accounted for when considering a pre-signal installation.
Pre-signals are primary signal faces and shall comply with Sections 4D.07 and 4D.08 of
. Pre-signal mast arms should be placed downstream of the grade crossing. A pre-signal may be installed upstream of the grade crossing as necessary to provide the most effective display to approaching motorists as determined by the Diagnostic Team. An example of where upstream placement may be necessary is when the downstream intersection is located extremely close to the grade crossing. If there is not be enough space for both the intersection’s traffic signal equipment and the pre-signal, the pre-signal may be placed upstream so the proper clearances are met.
Limited visibility signal heads shall be installed at the downstream intersection such that when motorists approach the grade crossing, the pre-signal indications are the only indications visible to encourage motorists to stop upstream of the grade crossing. Motorists who have passed the railroad flashers should be able to fully view the limited visibility signal heads and be encouraged to travel through the grade crossing rather than queue on the railroad tracks. With proper design and good motorist compliance, no motorists will be queued between the grade crossing and the signalized intersection.
The use of stop lines at pre-signal locations may vary depending on the location, but it is important to clearly indicate to motorists an appropriate stopping point. When multiple stop lines are placed – for example, one at the grade crossing, and one at the intersection – it can become confusing for motorists to understand where to stop and may lead to instances where they will queue at risky locations. If the CSD can accommodate the design vehicle, stop lines may be placed at both the grade crossing and the intersection; otherwise, the intersection stop line should be shared with the grade crossing stop line. The stop line located at the grade crossing shall be accompanied by R10-6 sign(s) and supplemental traffic signal heads where possible to emphasize the appropriate stopping point. Signs and traffic signal heads located upstream of the crossing shall not obstruct visibility of any railroad warning device.
Crosswalk placement may also lead to confusing motorist perception of the proper stopping point. Crosswalks should not be placed on the same leg of the intersection as the grade crossing. If a crosswalk is installed adjacent to the grade crossing, a crosswalk design (such as a continental configuration) should be chosen that does not resemble a stop line (such as a standard configuration). Refer to TMUTCD Figure 3C-1 for crosswalk marking options.
Queue Cutter Traffic Signals
As the CSD increases, advance preemption times can become very long due to the time required to clear the MTCD and CSD. In cases where the grade crossing is located farther than 400’ from a signalized intersection, an actuated queue cutter traffic signal can provide adequate queue management at the grade crossing. Queue cutter traffic signals are independently-controlled traffic control signals (not operated in conjunction with the traffic control signal faces at a downstream signalized intersection) located at a grade crossing that control traffic in one direction only on the roadway for the purpose of keeping the MTCD clear of vehicles. A queue cutter traffic signal functions by “cutting” the queue upstream of the grade crossing by turning red before vehicles queue onto the railroad tracks. Queue cutter traffic signals should utilize queue detection downstream of the grade crossing to prevent on-track queuing. Generally, any detection type may used to detect the vehicle queue, but vital loop detection should be considered. The Diagnostic Team should discuss detection types and evaluate each grade crossing individually to determine the best solution.
Many design features of a queue cutter traffic signal are similar to a pre-signal as shown in Figure 6-4. The most notable difference is the detection zone which is essential for queue cutter traffic signal operation. Queue cutter traffic signal indications shall comply with Sections 4D.07 and 4D.08 of the TMUTCD. Queue cutter mast arms should be placed downstream of the grade crossing, where possible, as it places the stop line closer to the crossing. A queue cutter may be installed upstream of the grade crossing as necessary to provide the most effective display to approaching motorists as determined by the Diagnostic Team.
Figure 6–4. Elements of a Queue Cutter Traffic Signal
Queue Cutter Traffic Signal Operations
The queue cutter traffic signal queue detection zone should be located far enough away from the crossing to detect a growing queue, transition the queue cutter traffic signal by serving any minimum green and yellow change interval time, and then allow a design vehicle that has already passed the queue cutter traffic signal’s stop line to clear the MTCD before the queue extends to the grade crossing. Once the queue cutter traffic signal displays a red signal indication, the queue cutter traffic signal shall continue to display a red indication as long as the downstream vehicle detection system continues to detect the presence of stopped vehicles.
Supplemental queue detection zones should be considered when accounting for any turning movements between the grade crossing and the downstream intersection. These detectors should detect the formation of any intermediate queues to activate the queue cutter traffic signal prior to any on-track queuing occurring. Supplemental detection zones could be used to account for different platooning and arrival patterns during different times of the day and/or year.
When the queue cutter traffic signal is displaying a green signal indication and is preempted by the traffic control relay (i.e. advance preemption), the queue cutter traffic signal shall transition to preemption operations by serving any remaining minimum green followed by the yellow change interval and steady red signal indications. If the queue cutter traffic signal is preempted by the crossing active relay (i.e. simultaneous preemption), it shall immediately transition to the yellow change interval and steady red signal indications. The queue cutter shall continue to display red signal indications until the train clears the grade crossing. When there is no active preemption call and no queue is detected by the downstream detection, the queue cutter traffic signal shall finish timing any active minimum red interval, if used, and transition to green signal indications. Queue cutter traffic signal faces shall not display green signal indications when the grade crossing flashing-lights are activated both before the train arrives and after the train leaves the crossing. Figure 6-5 provides an illustration of a queue cutter traffic signal phasing diagram.
Figure 6–5. Phasing Diagram for a Queue Cutter Traffic Signal
Various types of queue detection may be used. However, the Diagnostic Team should consider the critical nature of the detection zone when recommending a detection type for a queue cutter traffic signal. The failure modes of the queue cutter traffic signal system and the downstream vehicle detection system shall be evaluated and accounted for in the design of any such system. Fail-safe design techniques shall be used in the system design, such as health monitoring and self-check operation to validate the proper functioning of the system. If the detector system fails to properly monitor its health status, then the queue cutter traffic signal shall display flashing red indications until the issue is resolved and the system functions normally.
Placement of downstream detection is key to effective operation of a queue cutter traffic signal and can be an iterative process. Placement should be measured relative to the far limit of the MTCD (i.e. 6 feet from far rail) at a distance equal to the time to detect stopped vehicles (T) plus the yellow change interval (Y), based on the 85th percentile speed. The time, T, should be based on field observations, but is typically around 4 seconds. Yellow time shall comply with TxDOT timing standards. The designer should conduct field observations of queue buildup times during the peak periods. If the observed queue buildup times are significantly less than the time required to transition the queue cutter traffic signal to red than a hybrid queue cutter/pre-signal operation should be used.
Figure 6–6. Detection Placement for Queue Cutter Traffic Signals
Hybrid or Non-Actuated Queue Cutter Traffic Signals
At certain grade crossings with variable traffic flow conditions, TMUTCD Section 8D.12 allows a variable-mode (hybrid) or non-actuated queue cutter traffic signal operation to be used in which the queue cutter traffic signal at the grade crossing may act as a pre-signal in conjunction with the downstream traffic signal during certain hours of the day, but operate independently as a queue cutter otherwise. The queue cutter traffic signal and intersection traffic signal should have separate traffic signal controllers with the queue cutter traffic signal interconnected to the railroad signal system.
For the hybrid function, the queue cutter operation is synchronized with the downstream traffic signal to optimize vehicle flow while maintaining queue prevention at the grade crossing at all times. The green signal indication at the queue cutter traffic signal should coordinate with the green indication of the downstream traffic signal to minimize stopping between the two signals while allowing for some storage between the grade crossing and the downstream intersection.
Hybrid Pre-Signals
A hybrid pre-signal may also be used in which the pre-signal signal indications and the downstream traffic signal indications are both controlled by one traffic signal controller, but may have a more dynamic signal operation than a pure pre-signal. The hybrid pre-signal may change its operations based on time of day to improve the efficiency of the intersection operations. Operation modes that could be considered include using queue detection between the grade crossing and the downstream intersection that can operate the pre-signal as a queue cutter or a pre-timed operation where the pre-signal leads the downstream traffic signal indications to manage queues by providing queue prevention at the grade crossing at all times.
Queue Management Selection
ITE provides guidance on the applicability of the aforementioned queue management strategies depending upon the clear storage distance measurement, as shown in the table below. Generally speaking, the closer a crossing is to a signalized intersection, the more likely that a pre-signal installation will be the most appropriate design choice. When the grade crossing is located further away from the intersection, a queue cutter may be deemed more fitting. However, for CSD measurements between 200’ and 400’, designers should rely on engineering judgment, the Diagnostic Team and analyze the local traffic conditions and roadway features to determine which strategy would be more effective.
CSD | Queue Management |
|---|---|
≤ Design Vehicle Length to 200 feet | Pre-Signal |
200 feet to 400 feet |
|
> 400 feet | Actuated Queue Cutter |