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Section 3: Traffic Signal Preemption

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Traffic signal preemption involves a special timing sequence upon the arrival of a train when an at-grade crossing exists nearby on one or more legs of the signalized intersection. A traffic signal controller is preempted by warning from the railroad company. There are two forms of preemption:

  • Simultaneous preemption: occurs when the traffic signal controller is preempted at the same time the active warning devices begin to flash.
  • Advance preemption: occurs when the traffic signal controller is preempted prior to the active warning devices beginning to flash.

The primary function of preemption timing is to ensure that a vehicle that may have stopped on the railroad tracks queued at a red light is given sufficient time to clear the railroad tracks prior to the arrival of the train. It is also used to restrict movements toward the tracks from the traffic signal when a train is approaching or within the crossing.

NOTE: Refer to the Traffic Operations Division website for further updates on policy regarding preemption.

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Need for Preemption

Preemption circuitry should be installed any time an at-grade crossing is within 200 feet of a signalized intersection (see Section 8C.09 of the TMUTCD). However, preemption should also be considered any time traffic may back up over the tracks. If traffic backs up over the tracks before an intersection that is not signalized, a traffic signal with preemption may be installed under Warrant 9 in Section 4C.10 of the TMUTCD.

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Preemption Sequence

Most railroad preemption sequences include the following steps:

  1. Right-of-Way Transfer. This phase clears out any existing vehicular or pedestrian movements when the traffic signal controller first receives a preemption call. The traffic signal controller will ensure that a programmed minimum green, pedestrian walk, and pedestrian clearance time is met prior to terminating the phase. It is also known as a selective phase.
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  3. Track Clearance. This phase clears out the traffic going over the at-grade crossing towards the traffic signal, including a vehicle which may be stopped on the tracks at a red light.

    NOTE: The higher the amount of advance preemption time requested from the railroad company, the higher the required track clearance green time will be under most circumstances in the absence of a gate-down circuit.

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  5. Dwell or Limited Cycle Phases. After track clearance, the traffic signal controller serves any phases that do not move over the tracks. If only one phase is served, the traffic signal is said to be in dwell. If multiple phases are served, the traffic signal is said to be in limited cycle.
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  7. Exit Phase. After the preempt call is dropped from the railroad company equipment, the signal transitions into an exit phase. This phase is typically the phase with the heaviest traffic movement towards the tracks and may be the same phase as the track clearance phase.
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Total Preemption Time

The total time requested from the railroad company for traffic signal preemption includes:

  • buffer time
  • minimum time
  • clearance time
  • advance preemption time
  • equipment response time.
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Buffer Time

Buffer time is additional time provided to account for trains that may accelerate on the approach to the crossing. Although typically at five seconds, buffer time is determined by the railroad company.

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Minimum Time

The minimum time required for active warning devices to flash is 20 seconds prior to the arrival of the train as required by FRA. This includes three seconds of initial flash time prior to the gates descending, and five seconds while the gates are down prior to the arrival of the train.

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Clearance Time

Any additional required time from the railroad company for the active warning devices to begin flashing prior to the arrival of the train is clearance time. Clearance time is typically required at skewed crossings or where multiple tracks exist.

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Advance Preemption Time

Advance preemption time is the additional time requested to preempt a traffic signal controller from the railroad company equipment beyond what is provided to the railroad active warning devices.

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Equipment Response Time

Equipment response time is provided to determine the speed of an oncoming train. Although typically at five seconds, equipment response time is determined by the railroad company.

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Traffic Signal Design

The following are recommendations when designing traffic signals with railroad preemption:

  1. At the traffic signal, if the track clearance green phase includes left-turning vehicles and there is through traffic on the opposite side of the intersection, include a left turn arrow for a protected left turn during preemption. The track clearance green phases should include both the through movement and protected left turn. NOTE: The protected left turn movement is not always used during normal operation (when traffic signal is not preempted).
  2. Include battery backup at the traffic signal controller.
  3. Use a minimum of 8 conductor traffic signal cable between the railroad cabin and traffic signal controller.
  4. Include a label in the traffic signal controller to alert technicians of the presence of railroad preemption circuits.
  5. Traffic signal poles should not block the view of railroad gates, mast flashers, and cantilevers.
  6. Care should be taken to restrict any vehicular movements towards the tracks when the traffic signal is in dwell or limited cycle phases. Restricted left turns or blank-out signs (i.e. No Left Turn) may assist.
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Preempt Priorities

There are many forms of preemption that may occur at a traffic signal controller. Some examples include:

  • emergency, fire, or law enforcement vehicle
  • bus rapid transit.

The TMUTCD states that railroad preemption should be the highest priority within the traffic signal controller when multiple preempts exists. In addition, the designer should confirm:

  • the traffic signal controller has enough preempt plans to support all preempt modes needed
  • the traffic signal cabinet has enough relays to support all preempts.

When using multiple preemption circuits for railroad preemption (see below for example circuits), the circuits must be prioritized properly in the traffic signal controller to ensure the preemption functions as designed.

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Other Preemption Design Scenarios

Other scenarios for railroad preemption occur from time to time and require a deviation from generally accepted practice. Some of these scenarios include:

  • at-grade crossings which are downstream of a traffic signal on one-way streets where the direction of travel is from the traffic signal to the tracks
  • at-grade crossings with an adjacent traffic signal on both sides of the tracks
  • signalized intersections where the tracks cross on two legs of the intersection
  • pre-signals: may be used when the storage distance between the tracks and near edge of the pavement at the intersection is less than the length of the design vehicle such that the design vehicle would be stopped on the tracks when at a red light
  • queue cutter signals may be used when queues at a traffic signal extend over an at-grade crossing at a significant distance beyond the intersection
  • an at-grade crossing in the middle of signalized intersection.

See Chapter 14 for other design manuals for assistance.

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Types of Circuits

There are several different types of circuits used with railroad preemption. Some common circuits include:

  • advance preemption circuit
  • gate down circuit
  • supervised circuit.
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Advance Preemption Circuit

The advance preemption circuit sends a call to the traffic signal controller to go into preemption to begin right-of-way transfer.

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Gate-Down Circuit

The gate-down circuit ensures that the track clearance phase terminates when the gates go down. The gate-down circuit provides two separate benefits. First, it eliminates the preempt trap by ensuring track clearance green does not terminate prior to the gates descending. Otherwise, a red indication could occur at the traffic signal and another vehicle could queue back over the tracks. Second, it ensures that the track clearance green is not unnecessarily long when maximum right-of-way transfer time is needed.

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Supervised Circuit

A supervised circuit monitors the cable connection between the railroad company and the traffic signal and has the ability to send the traffic signal into flash if the connection is broken after right-of-way transfer and track clearance phases.

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Constant Warning Circuitry

Constant warning circuitry from the railroad company is recommended. Constant warning circuitry along the rail can detect the speed of an oncoming train and know when to preempt the traffic signal as well as knowing when to drop a preempt call if a train stops on the approach. Railroad circuitry is installed based on the fastest speed allowed over the crossing, but not all trains will travel this speed over the crossing.

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Adjacent Crossings

As circuitry along the tracks may extend beyond adjacent grade crossings, the need for preemption at adjacent crossings should be considered. Otherwise, preempting adjacent crossings at different times can be significantly more costly.

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