How Does a Pre-emptor Work for Traffic Signals?

Traffic signal pre-emption speeds emergency response.

Traffic signal pre-emption systems are designed to interrupt a normal traffic signal cycle to give certain emergency vehicles a green traffic signal as they approach a signalized intersection. Specialized equipment installed on the vehicle and at the intersection's traffic signal controller work together to reduce traffic delay during an emergency response.

Technologies

Common technologies for traffic signal pre-emption are light-based, infrared-based, sound-based and radio-based emitter/detector systems. The use of Global Positioning Systems (GPS) is being investigated for use in traffic signal priority systems. Depending on the technology, a vehicle is equipped with an emitter that can be detected at those intersections that provide traffic signal pre-emption.

Operation

Detectors of a vehicle's emitter are placed at an intersection and are connected to the intersection's traffic signal controller. Each approach of an intersection requires a detector, although typically, only the main street approaches of an intersection are equipped with detectors. The intersection's traffic signal controller is programmed to adjust the timing and sequence of the traffic phases to give priority to the emergency vehicle's approach, either by extending an existing green traffic light until the emergency vehicle clears the intersection, or interrupting the timing and sequence of the traffic signal phases to provide a green traffic signal.

The Federal Highway Administration's "Manual on Uniform Traffic Control Devices" details specific requirements for safely interrupting the normal traffic signal phasing sequence. The traffic signal controller must change the pedestrian phase to "flashing don't walk" to clear pedestrians from the intersection approach. Traffic on the cross street must be given a yellow indication that is of the normal duration for that intersection approach.

Benefits

Emergency services providers say the benefits of traffic signal pre-emption systems include improved response time and reduced chances of an intersection crash with an emergency vehicle.

Challenges

Because different detection technologies are in use, emergency services providers may not get the benefits of traffic signal pre-emption when asked to respond to incidents in neighboring jurisdictions.

Traffic signal pre-emption disrupts the normal coordinated traffic signal cycle, and it may take several minutes for the traffic signal control system to return to normal operation after an intersection signal has been pre-empted.

Jurisdictions restrict the use of traffic signal pre-emption devices to authorized users. It is a violation of U.S. federal law to sell or use a non-authorized traffic signal pre-emption transmitter.

Other Applications

Traffic engineers install traffic signal priority systems at signalized intersections close proximity railroad crossings. In some jurisdictions, transit vehicles also are given priority at signalized intersections.

References

• U.S. Department of Transportation: Traffic Signal Preemption for Emergency Vehicles
• Alabama Department of Transportation: Traffic Signal Design Guide and Timing Manual
• Federal Highway Administration: Manual on Uniform Traffic Control Devices
• Los Angeles Fire Department: Traffic Signal Preemption System for Emergency Vehicles
• U.S. Government Printing Office: United States Code Title 18, Section 39, Traffic signal preemption transmitters