Section 3: Traffic Signal Preemption

Introduction

Traffic signal preemption involves a special timing sequence upon the arrival of a train when an at-grade crossing exists
within 250 feet
 of the signalized intersection. A traffic signal controller is preempted by warning from the railroad
crossing equipment
. 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 a vehicle stopped on the railroad tracks at a red light is given sufficient time to clear the railroad
crossing
 prior to the arrival of the train. It is also used to restrict
traffic
 movements toward the tracks from the traffic signal when a train is approaching or within the crossing.
Review
 the for updates on policies regarding preemption
and training
.

Need for Preemption

Preemption circuitry
must
 be installed anytime an at-grade crossing is within
250 feet
 of a signalized intersection (see Section 8C.09 of the TMUTCD). However, preemption should also be considered anytime traffic may back up over the tracks. If traffic backs up over the tracks before an
unsignalized
 intersection, a traffic signal with preemption may be installed under warrant 9 in Section 4C.10 of the TMUTCD.

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
    (TSC)
     first receives a preemption call. The TSC will ensure a programmed minimum green, pedestrian walk and pedestrian clearance time is met prior to terminating the phase.
  2. Track Clearance.
     This phase clears out the traffic going over the at-grade crossing, including a vehicle which may be stopped on the tracks at a red light.
    The higher the amount of advance preemption time requested from the railroad company, the higher the required track clearance green time under most circumstances in the absence of a gate down circuit.
  3. Dwell or Limited Cycle Phases.
     After track clearance, the
    TSC controls
     any phases that do not move over the tracks. If only one phase is
    active
    , the traffic signal is said to be in dwell. If multiple phases are
    activated
    , the traffic signal is said to be in limited cycle.
  4. Exit Phase.
     After the preempt call
    has ended
     from the railroad company equipment, the
    traffic
     signal transitions into exit phase. This phase has the heaviest traffic movement toward the tracks and may be the same phase as the track clearance.

Total Preemption Time

The total time requested from the railroad company for traffic signal preemption includes:
  • Minimum time.
  • Clearance time.
  • Buffer time.
  • Equipment response time.
  • Advance preemption time.

Minimum Time

The
FRA requires the active warning devices to flash for a minimum of
 20 seconds prior to the arrival of the train
at the crossing
. This includes 3 seconds of initial flash time prior to the gates descending and 5 seconds while the gates are down prior to the arrival of the train.

Clearance Time

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

Buffer Time

Buffer time is additional time provided to account for trains that may accelerate on the approach to the crossing.
This is determined by the railroad company and is normally 5 seconds.

Equipment Response Time

Equipment response time is provided
for the crossing equipment
 to determine the speed of an oncoming train. This is determined by the railroad
and is normally 5 seconds
.

Advance Preemption Time

Advance preemption time is the additional time requested
for the TSC to activate the preempt phase
 from the railroad company equipment beyond what is provided to the railroad active warning devices.

Traffic Signal Design

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
    TSC
    .
  3. Use a minimum
    12 color coded
     conductor traffic signal cable between the railroad cabin and
    the TSC
    .
  4. Include a label in the
    TSC and railroad cabin
     to alert technicians of the presence of railroad preemption circuits.
  5. Traffic signal poles should not block 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.

Preempt Priorities

There are many forms of preemption that may occur at a
TSC, including:
  • Emergency vehicles.
  • Public transportation vehicles.
The TMUTCD states that railroad preemption is the highest priority within the
TSC
 when multiple preempts exists. The designer should
also
 confirm:
  • TSC
     has enough preempt plans to support all preempt modes.
  • Traffic signal cabinet has enough relays to support all preempts.
  • Solid state preemption controllers may be used to eliminate the need for preempt relays.
When using multiple preemption circuits for railroad preemption, the circuits must be prioritized in the
TSC
 to ensure the preemption functions as designed.

Other Preemption Design Scenarios

Other scenarios for railroad preemption occur
on occasion
 and require a deviation from generally accepted practice. Some of these scenarios include:
  • At-grade crossings downstream of a traffic signal on one-way streets
    with
     the direction of travel
    coming
     from the traffic signal
    and going towards
     the tracks.
  • At-grade crossings with an adjacent traffic signal on both sides of the tracks.
  • Signalized intersections where the tracks cross 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.
  • At-grade crossing in the middle of signalized intersection.
See Chapter 14 for other design
resources
.

Types of Circuits

Several types of circuits are used with railroad preemption. Some common circuits include:
  • Advance preemption circuit.
  • Gate down circuit.
  • Supervised circuit.

Advance Preemption Circuit

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

Gate Down Circuit

The gate down circuit ensures that the track clearance phase terminates when the gates
are horizontal
. The gate down circuit provides 2 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.

Supervised Circuit

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

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 and know 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.

Adjacent Crossings

As circuitry along the tracks extends beyond adjacent grade crossings, the need for preemption at adjacent crossings should be considered. Preempting adjacent crossings at different times can be significantly more costly
otherwise. However, this is determined by the railroad company.

Cutover and Annual Testing of Preempted Traffic Signals

Any change to a preempted traffic signal should include a cutover inspection after the work is completed using TxDOT Form 2625 to document the results. This includes any modification to the traffic signal cabinet. An annual joint inspection, also documented on TxDOT Form 2625 should be done with members of the railroad, traffic signal maintainer, state railroad or federal railroad inspector, and district staff.