One type of traffic signal operation is the use of preemptive signaling to speed transit vehicle travel.
Preemption is the transfer from normal operations of signals to a special control mode.
Light rail traffic preemption often occurs when trains approach highway-rail grade crossings. Preemption of traffic signals is typically done when vehicles stopped for a red traffic signal are queuing across the adjacent set of tracks. The objective of a successful preemption is to take control of the intersection traffic signal displays and provide for the passage of a train, no matter where in the normal traffic signal operation the preemption occurs. Traffic signal systems at the crossings can be equipped to handle more detailed data about train position, speed, and estimated time to crossing. The traffic signal controller can then accommodate train movements without the abrupt preemption process, improving safety.