Editing Traffic light (section)

=== Preemption and priority ===
{{Main|Traffic signal preemption|Bus priority}}

Some regions have signals that are interruptible, giving priority to special traffic, usually [[emergency vehicle]]s such as [[firefighting apparatus]], [[ambulance]]s, and [[police car]]s.<ref>{{cite web |title=Emergency vehicle traffic signal preemption system |work=United States Patent and Trademark Office |url=http://patft.uspto.gov/netacgi/nph-Parser?patentnumber=6326903 |access-date=7 October 2005 |archive-date=21 December 2016 |archive-url=https://web.archive.org/web/20161221033705/http://patft.uspto.gov/netacgi/nph-Parser?patentnumber=6326903 |url-status=dead }}</ref><ref>{{cite web|url=https://mutcd.fhwa.dot.gov/HTM/2003r1/part4/part4f.htm|title=FHWA – MUTCD – 2003 Edition Revision 1 Chapter 4F|work=dot.gov}}</ref> Most of the systems operate with small transmitters that send radio waves, [[infrared]] signals, or [[strobe light]] signals that are received by a sensor on or near the traffic lights. Some systems use audio detection, where a certain type of siren must be used and detected by a receiver on the traffic light structure.

Upon activation, the normal traffic light cycle is suspended and replaced by the "preemption sequence": the traffic lights to all approaches to the intersection are switched to "red" with the exception of the light for the vehicle that has triggered the preemption sequence. Sometimes, an additional signal light is placed nearby to indicate to the preempting vehicle that the preempting sequence has been activated and to warn other motorists of the approach of an emergency vehicle. The normal traffic light cycle resumes after the sensor has been passed by the vehicle that triggered the preemption.

In lieu of preemptive mechanisms, in most jurisdictions, emergency vehicles are not required to respect traffic lights. Emergency vehicles must slow down, proceed cautiously and activate their emergency lights to alert oncoming drivers to the preemption when crossing an intersection against the light.<ref>{{cite web|title=Emergency Vehicles at Red Signal or Stop Sign|url=http://codes.ohio.gov/orc/4511.03|website=Ohio Laws and Rules|access-date=22 July 2014}}</ref><ref>{{cite web|title=Emergency Service and Vehicles|url=https://leg1.state.va.us/cgi-bin/legp504.exe?000+cod+46.2-920|website=Virginia General Assembly|access-date=22 July 2014}}</ref>

Unlike preemption, which immediately interrupts a signal's normal operation to serve the preempting vehicle and is usually reserved for emergency use, "[[Bus priority|priority]]" is a set of strategies intended to reduce delay for specific vehicles, especially mass transit vehicles such as buses. A variety of strategies exist to give priority to transit but they all generally work by detecting approaching transit vehicles and making small adjustments to the signal timing. These adjustments are designed to either decrease the likelihood that the transit vehicle will arrive during a red interval or decrease the length of the red interval for those vehicles that are stopped. Priority does not guarantee that transit vehicles always get a green light the instant they arrive as preemption does.