Traffic signal preemption

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A 3M Opticom signal preemption system emitter. This unit, mounted next to the lightbar on a fire engine, connects to the control unit mounted in the cab, pictured below.

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 allow emergency vehicles priority by changing traffic signals in the path of the vehicle to green (or in some cases, flashing green[1]) 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.

The key advantages of signal preemption include their ability to reduce response time for emergency services and to increase safety on the road.

These devices generally operate by use of invisible infrared signals or by visible strobe lights. Each emergency vehicle is equipped with an emitter, a device which emits visible flashes of light or invisible infrared pulses at a specified frequency. Receiver devices placed on or near intersection traffic control devices recognize the signal and preempt the normal cycle of traffic lights. Once the emergency vehicle passes through the intersection and the receiving device no longer senses the remote triggering device, normal operation resumes. Some systems can be implemented with varying frequencies assigned to specific types of uses, which would then allow an intersection's preemption equipment to differentiate between a fire engine and a bus sending a signal simultaneously, and then grant priority access first to the fire engine.

There are limitations inherent in the standard signal preemption systems, which rely on line-of-sight communication between the emergency vehicle and the receiver. This leads to some intersections unnecessarily being impacted, and conversely other intersections not receiving the signal in time to clear the intersection for the approaching emergency vehicle. Short-term solutions to these issues have been addressed by the relatively remote placement of receivers (on a pole above the traffic signal, or around a corner that approaches an intersection, for example), and also with physical additions to receiver designs that are less likely to pick up signals unless they are originating from a location certain to be in the path approaching the intersection covered. Research into dynamic route clearance has been made to solve these problems and improve traffic preemption with centrally-based route command and control.

A common implementation of signal preemption systems is a method of communicating to the emergency vehicle operator as well as civilian drivers that a traffic signal is under control of a preemption device. Known as a notifier, this device is almost always an additional light located near the traffic signals. It may be a single light bulb visible to all, which flashes or stays on, or it may be a more sophisticated system such as that available with the OptiCom system from 3M where a spotlight is aimed in each direction. The spotlight will either flash or stay on, which communicates to all drivers which direction a preempting signal is being received from. This informs regular drivers which direction may need to be cleared, and informs emergency vehicle drivers if they have control of the light (especially important when more than one emergency vehicle approaches the same intersection).

[edit] Cheating

Other problems include the potential abuse of the system. Anyone with a strobe light operating at the appropriate frequency can activate some of the older systems which utilize visible light pulses, and infrared emitters have sometimes found their way into the wrong hands as well. Jason Niccum, of Longmont, Colorado made news in March 2006 when he was caught using such a device he bought on eBay for $100. He had been using it to speed his commute for two years. Various systems are available to prevent such abuse, including encoding the strobe frequency and installing encrypted radio emitters to authenticate signals. Moreover, the SAFETEA-LU law signed in August 2005 made the sale or illegal use of a traffic preemption device in the United States a serious federal crime, with a minimum sentence of six months in prison.

An old urban legend that anyone can cheat and trigger a preemption device by flashing their vehicle's high beams is false. No human can operate their high beam switch at any of the exact frequencies required to activate the system, nor is it possible for any type of headlight to flash on and off at that rate even if it was mechanically or electronically controlled to the correct flash rate.

The most common signal pre-emption systems are the Opticom by 3M and the STROBECOM by Tomar Electronics.