Traffic signal preemption

Traffic signal preemption (also called traffic signal prioritization) is a type of system that allows the normal operation of traffic lights to be preempted. The most common use of these systems is to manipulate traffic signals in the path of an emergency vehicle, halting conflicting traffic and allowing the emergency vehicle right-of-way, to help reduce response times and enhance traffic safety. Signal preemption can also be used by light-rail and bus rapid transit systems to allow public transportation priority access through intersections, or by railroad systems at crossings to prevent collisions.

Traffic preemption devices are implemented in a variety of ways. They can be installed on road vehicles, integrated with train transportation network management systems, or operated by remote control from a fixed location, such as a fire station, or by a 9-1-1 dispatcher at an emergency call center. Traffic lights must be equipped to receive an activation signal to be controlled by any system intended for use in that area. A traffic signal not equipped to receive a traffic preemption signal will not recognize an activation, and will continue to operate in its normal cycle.

Vehicular devices can be switched on or off as needed, though in the case of emergency vehicles, they are frequently integrated with the vehicle's emergency warning lights. When activated, the traffic preemption device will cause properly equipped traffic lights in the path of the vehicle to cycle immediately, to grant right-of-way in the desired direction, after allowing for normal programmed time delays for signal changes and pedestrian crosswalks to clear.

Traffic signal preemption systems integrated with train transportation networks typically extend their control of traffic from the typical crossarms and warning lights to one or more nearby traffic intersections, to prevent excessive road traffic from approaching the crossing, while also obtaining the right-of-way for road traffic that may be in the way to quickly clear the crossing. This also allows buses and hazmat vehicles in the USA to proceed through the intersection without stopping at the railroad tracks.

Fixed-location systems can vary widely, but a typical implementation is for a single traffic signal in front of or near a fire station to stop traffic and allow emergency vehicles to exit the station unimpeded. Alternatively, an entire corridor of traffic signals along a street may be operated from a fixed location, such as to allow fire apparatus to quickly respond through a crowded downtown area, or to allow an ambulance faster access when transporting a critical patient to a hospital in an area with dense traffic.

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