What Are The Lights On Top Of Traffic Signals?


Emergency vehicles require expedient passage through intersections to reach those in need as quickly as possible. Traffic signal preemption systems are designed to detect approaching emergency vehicles and temporarily override normal traffic light operations to grant them the right of way.

Once the emergency vehicle has cleared the intersection, the traffic signals resume their normal pattern. Traffic signal preemption systems have the potential to shave critical seconds off emergency response times and reduce collisions at intersections.

What Are Traffic Signal Preemption Systems?

Traffic signal preemption systems are specialized equipment installed on emergency vehicles and at intersections that allow emergency responders to control traffic signals. An emitter on the emergency vehicle communicates with a receiver at the traffic signal, triggering the light to change and give the emergency vehicle the right of way.

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Three main types of detection systems are used for traffic signal preemption: point detection, zone detection, and area detection. A point detection system detects a vehicle at a single location. A zone detection system detects a vehicle within a certain zone near the intersection. An area detection system tracks a vehicle’s location within a broader area.

How Do Signal Preemption Systems Work?

Signal preemption systems allow emergency vehicles to override normal traffic light operations. They work by detecting an approaching emergency vehicle and transferring a request to change the traffic light in the vehicle’s favor. Three main types of detection systems are point detection, zone detection, and area detection.

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Point detection systems detect a vehicle at a single location. Zone detection systems detect a vehicle within a certain range of the intersection. Area detection systems track a vehicle’s location within a broad area. The systems use equipment on the emergency vehicle to communicate with equipment installed at the intersection.

How Are Requests Confirmed?

Many preemption systems have indicator lights to confirm that the request has been received. A solid white light usually means the traffic signal has been preempted to provide a green light for emergency vehicles. A flashing white light typically indicates the signal has been preempted for emergency vehicles traveling in a different direction.

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Traffic signal preemption gives agencies more control over vehicle movement, allowing for safer, faster, and more efficient operations. The basic components of a priority control system include on-board vehicle equipment, intersection equipment, communication networks, and control center hardware and software.

Most Common Points of Detection

The most common types are pointing detection, using infrared or radio frequency communication between the vehicle and receiver, and GPS-based detection, which uses the vehicle’s location data to predict its path to the intersection.

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Traffic signal preemption aims to reduce emergency vehicle response times, increase safety, and prevent traffic accidents. By controlling traffic lights to give emergency vehicles a green light, they can navigate intersections quickly and avoid being impeded by traffic or red lights.

Key Components of Traffic Signal Preemption Systems

Emergency vehicles are equipped with emitters, computers, automatic vehicle location (AVL) systems, and preemption equipment to communicate with traffic signals. Satellite-based radio navigation systems like GPS are commonly used to pinpoint the vehicle’s location and intended path of travel.

Source: Wikimedia/Kamigata0

Traffic signals contain receivers, signal controllers, and preemption equipment to detect approaching emergency vehicles and override normal signal operations. Detection systems identify vehicles at a single point, within a zone near the intersection, or track the vehicle’s location within an area.

Control Centers

Advanced traffic management systems (ATMS) and IT infrastructure within traffic control centers monitor connected intersections and emergency vehicle activity. Personnel can see the location of emergency vehicles, their destination, and the status of traffic signals along the intended travel route.

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Effective traffic signal preemption relies on coordinating these components to detect and prioritize the movement of emergency vehicles through intersections. Municipalities that invest in these systems benefit from improved emergency response times, increased traffic safety, and optimized traffic flow.

Major Cities That Have Implemented Preemption Systems

Many cities and counties in the U.S. have implemented traffic signal preemption systems to aid emergency responders. Maricopa County, Arizona, installed some of the first systems in the early 1980s. The systems have now become widely used, with many areas expanding to include transit priority for buses, light rail, and emergency vehicles.

Source: Maricopa County

Traffic signal preemption provides emergency responders a faster, safer way to navigate roadways when every second counts. By giving emergency vehicles control over traffic signals, these systems save lives through reduced response times for ambulances, fire trucks, and police.

The Difference Between Signal Preemption and Signal Priority

Signal preemption overrides normal traffic light operations by changing all lights to red except for the direction in which the emergency vehicle travels. This clears a path through the intersection, allowing emergency vehicles to proceed swiftly without stopping.


Signal priority provides transit vehicles such as buses, light rail trains, and streetcars with extended green light time at intersections to help them stay on schedule. By avoiding red lights, signal priority also improves safety by assisting these vehicles in clearing the intersection during the green phase.

Implementation Considerations for Cities

There are several factors to consider for cities looking to implement traffic signal preemption systems. The core components-onboard vehicle equipment, intersection infrastructure, communication networks, and control center software and hardware-must be evaluated based on a city’s current technology and future needs.

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Choosing the right vehicle detection system is essential. Point detection locates vehicles at a single location, zone detection senses vehicles within an area near an intersection, and area detection tracks vehicle location across a broad space. The selection depends on a city’s specific requirements.

A Worthwhile Investment for Cities

Emergency response times are impacted as cities continue to grow in population and traffic congestion increases. Traffic signal preemption systems offer a technological solution to improve emergency vehicle response times and save lives.

Source: Unsplash/Anders Jilden

While the costs to implement these systems can be significant, the benefits to public safety make them an investment worth considering for any city struggling with traffic congestion and concerned with improving emergency response times.

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Charlotte Clad

Written by Charlotte Clad

Charlotte Clad is a brilliant writer who possesses the remarkable ability to craft content that goes viral and leaves an indelible mark on readers. With an innate passion for storytelling and an unwavering commitment to her craft, Charlotte has consistently pushed the boundaries of creativity to captivate audiences worldwide.

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