Emergency Vehicle Driving and Traffic Preemption

Emergency Vehicle Driving and Traffic Preemption

Article May 31, 2004

Wail, yelp. Wail, yelp. A little air horn here, a little air horn there, and yet the emergency services are still often confronted with the inability to safely and adequately navigate through traffic. This all-too-familiar theme is played out thousands of times a day across North America. Even with the many risk-control technologies that exist to aid the emergency responder with safe passage, there is no guarantee of it.

Although human factors are perhaps the greatest issue surrounding many actions taken by driv­ers of emergency vehicles, engineered preemption of traffic signals has demonstrated efficiency and effectiveness in aiding safety objectives. This article provides a snapshot view of traffic signal preemption and its contribution to safe passage at intersections.

Traffic Signal Preemption Systems

According to Webster’s Dictionary, to preempt is to seize upon, to the exclusion of others. Thus preemption is defined as taking possession before others. Preemption in the context of this topic refers to a traffic management system in which an intelligent transportation technology is utilized to control traffic flow by preempting the green lights.

A variety of preemption systems exist, ranging from radio-based systems to the strobe-based systems in use today. These are communications systems that allow vehicles equipped with preemption transmitters to change traffic flow in their favor by controlling signals equipped with receivers at important intersections.

In a 1996 research project, Chief Wayne Martin of the Oviedo (FL) Fire-Rescue Department, deduced that by reducing the number of emergency vehicle-related crashes at intersections, the use of signal preemption systems (with reduced light and siren response) would save lives. Martin stated that of the services surveyed, 67% agreed that if the journey to an emergency scene could be made safer, while improving or maintaining response time, they would be willing to shut off their emergency warning devices.1 Although this article does not address the use of lights and sirens, the results of the study demonstrate that such an initiative could improve emergency vehicle safety.


Let’s review a typical scenario using traffic signal preemption. Note: Many preemption products with various options exist. This scenario is not intended to be representative of any particular product.

An ambulance equipped with multiple emergency warning lights and a siren is dispatched to an emergency. The ambulance is also equipped with a preemption transmitter that operates using a high-intensity forward-facing strobe light flashing at a rapid rate—much faster than normal attention-getting lights on an emergency vehicle.

When the ambulance approaches within 1,800 feet (line-of-sight) of an intersection with a preemption-equipped traffic signal, the receiver (normally mounted on the cross-arm that suspends the traffic light) locks onto the ambulance’s flashing strobe. The control mechanism then initiates a unique “preemption sequence” in the traffic signal, depending on the direction of the lights.

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If the ambulance already has a green light, the light will remain green. Any other direction that also has a green light (usually the opposite direction) will first get a yellow light, then a red. When all other directions are red, and the ambulance’s direction is the only one with a green light, the left-turn arrow (if one exists) will illuminate and a brilliant white flood lamp mounted near the traffic signal will begin to flash. This flood lamp tells the driv­er of the ambulance that he or she has control of the intersection and the right-of-way (exercising due regard).

If the approaching ambulance has a red light, every other direction with a green light will transition to yellow, then red. When all directions—including the ambulance’s—are red, the traffic signal facing the ambulance will then turn green, along with the left-turn arrow (if one exists), and the brilliant white flood lamp will flash.

Once the ambulance has passed through the intersection, optical communication with the preemption detector on the traffic signal is lost. The traffic signal then defaults back to normal operation. Conversely, until the ambulance passes through the intersection, it will have a green light, regardless of how long it remains there.

If several intersections are within the 1,800-ft. range of the ambulance’s preemption transmitter, they will all respond accordingly to the above operational description. It should be noted that when two or more emergency vehicles reach the intersection at one time, preemption is only given to one vehicle. This is by design and based on a first-come, first-served basis.

Drivers should be trained to recognize that if they do not have a green light and the white floodlight is not flashing, they do not have preemption. In fact, if they cannot account for all lanes of traffic—vehicular and pedestrian—then they must come to a complete stop! It is possible there is another emergency vehicle coming from another direction, which has preemptive status.

As an integral part of traffic management, preemption systems are capable of recording information such as time of day, duration of preemption, direction of travel and effective range control. Additionally, preemption is no longer dependent on a direct line of sight for activation; products exist that are capable of preempting around curves and corners and in most weather conditions. Preemption systems can also recognize whether the activating vehicle is an ambulance, fire engine or police cruiser. There is little if any reason why all emergency response agencies cannot exercise the use of traffic preemption systems.

Getting Started

First, conduct a risk assessment to find out if there is a need to preempt in your location. Next, determine which traffic signals require preemption and in which directions. Preemption devices can address one or multiple directions. Your department may need any variation of such controls depending on circumstances. Consider this: Do your response vehicles always, usually, rarely or never travel in any particular direction at any given intersection? Obviously, this can be different for every intersection. You will also have to decide which of your vehicles, if not all, need to be equipped with a preemption activator. Finally, the agency will need to educate the public and train their personnel regarding the purpose and best-practice use of preemption.

Implementation costs vary, but the overall cost has been demonstrated to pay for itself. After all, consider the internal and external impact of just one emergency vehicle collision versus all the possible collisions that will never occur. Need money? Check out the Federal Transportation Equity Act, or TEA-21.


Preemption is another tool for promoting safe passage through intersections. It does not give the emergency services total right-of-way. Emergency vehicle operators must exercise due regard for the safety of everyone. Still, the use of traffic management systems, not speed, is the key component in response time when it comes to saving lives.


  • Intelligent Transportation Systems: www.its.com.
  • 3M Opticom: www.Opticom.com.
  • MTP Priority One: http://mtp-gps.com.
  • TOMAR Electronics: www.tomar.com.


1. Martin WJ. Unpublished research paper. A safer journey for emergency response vehicles: Strategic management of change, p.15; November 1996.

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