High Quality Content by WIKIPEDIA articles! Traffic signal preemption is a type of system that allows the normal operation of traffic lights to be preempted, often to assist emergency vehicles. The most common use of these systems is to manipulate traffic signals in the path of an emergency vehicle, stopping conflicting traffic and allowing the emergency vehicle right-of-way, to help reduce response times and enhance traffic safety. Sometimes, signal preemption is also used at railroad grade crossings to prevent collisions, and by light-rail and bus rapid transit systems to allow public transportation priority access through intersections to ensure they remain on schedule and improve commute times.