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Traffic signal preemption (also called traffic signal prioritisation) is a system that allows an operator to override the normal operation of traffic lights. The most common use of these systems manipulates traffic signals in the path of an emergency vehicle, halting conflicting traffic and allowing the emergency vehicle right-of-way, thereby reducing response times and enhancing traffic safety. Signal preemption can also be used on tram, light-rail and bus rapid transit systems, to allow public transportation priority access through intersections, and by railroad systems at crossings to prevent collisions.