Traffic signal preemption is a type of system that allows the normal operation of traffic lights to be preempte, often to assist emergency vehicles. The most common use of these systems is to allow emergency vehicles priority by changing traffic signals in the path of the vehicle to green (or in some cases, flashing green) and stopping conflicting traffic. 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 are able to remain on schedule and improving commute times.