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.