Emergency Vehicle Driving and Traffic Preemption

Traffic signal preemption and its contribution to safe passage at intersections.

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.

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.

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