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.