Emergency vehicle preemption

Emergency vehicle preemption (EVP) interrupts normal traffic signal timing to provide a green light to approaching emergency vehicles so that they can pass through intersections to get to emergencies safely and quickly.

Key characteristics




WSDOT regions

Other names

  • signal preemption

Strategy description

Emergency vehicle preemption (EVP) provides a green light for emergency vehicles who are en-route to a scene. By providing a red light to other drivers that may cross the path of the emergency vehicle, EVP can help reduce collisions. EVP aims to improve other drivers’ awareness of emergency vehicles as well as remind them to yield the right-of-way, increasing overall safety.

It also reduces the travel time for the emergency vehicle which improves the chance to save the lives of anyone who was seriously injured.

For example, an FHWA evaluation of EVP performance in St. Paul Minnesota showed collisions involving emergency vehicles decreased by 70% and emergency response times improved between 14% and 50% (1).

EVP approaches and technologies

Basic systems employ zone detection. Equipment mounted on the emergency vehicle emits a special infrared signal that is detectable at the intersection once the vehicle enters a given range. When the traffic signal receives this message, it attempts to modify its signal timing to provide a green light as soon as possible. 

However, technological limitations of the infrared emitter means that intersection EVP equipment can only register whether a signal from the vehicle was detected or not. It cannot determine how far away an approaching vehicle is or how quickly it is approaching.This  can result in the green light being provided too early or too late.

More advanced EVP systems incorporate GPS vehicle-location equipment and wireless radios along with EVP requests. These send  vehicle headings and estimated time to arrival information to the signal. This provides more reliability and  advanced notice.

The most advanced EVP systems include cellular communications between emergency vehicles and the central traffic signal system. EVP requests are intelligently routed to any centrally-connected traffic signal along the emergency vehicle’s path.

When to use this strategy

EVP makes sense for any signalized intersection where other traffic may delay or cross the path of the emergency vehicle en-route to a scene. It is especially valuable along key routes used by emergency responders.

Strategy benefits:

  • Improves safety by reducing emergency vehicle-related collisions at intersections
  • Supports incident response and management by improving emergency vehicle travel time to the incident scene

EVP’s impact on other traffic:

Because EVP systems interrupt a traffic signal’s standard signal cycle, there are potential negative impacts to traffic flow and coordinated signal timings. Each time an intersection is preempted, it may take several cycles for the signal to get back to its normal setting. This can have far-reaching impacts on congestion and flow if it is coordinated with other signals in the network.

Preempted signals that stop vehicles for too long or are activated too frequently may also encourage drivers to disobey red lights. 

What you need in order to implement

Planning needs:

  • Cooperative involvement of participating agencies--including law enforcement, fire, emergency medical services, and traffic signal agencies--to develop a joint EVP program
  • Coordinated policies on the operating rules for EVP, what equipment is required, and how it will be implemented to ensure that the system is compatible across jurisdictions and used in common ways
  • Updates to the traffic signal agency’s traffic signal policies to include the use of signal preemption systems

Equipment needs:

  • Onboard technology to locate the vehicle, assess its current heading, and communicate the preemption request to the traffic signal or signal system
  • Roadside or central technology to receive the preemption request and determine whether to grant the request, as well as signal software to process the request and store the data

Monitoring and performance reporting needs:

  • Reporting capabilities and ongoing monitoring of preemption activations to ensure that emergency response and traffic performance measures are being achieved and that preemption activations are justified

Learn more about this strategy

Federal Highway Administration (FHWA) Office of Operations Traffic Signal Timing Manual, Advanced Signal Timing Topics

Works cited:

(1) Signalized Intersection Safety Strategies (PDF), Employ Emergency Vehicle Preemption, USDOT

About key characteristics

Location notes:

EVP works well for intersections along key routes for emergency responders and for any locations that would benefit from improved emergency response times.  

Cost notes:

Costs to install and operate an EVP system are relatively low and vary based upon the number of signalized intersections and emergency vehicles outfitted with EVP equipment and the sophistication of the EVP solution implemented. Typically, the traffic signal operating agency pays for the equipment at the intersection and the emergency response agency pays for the equipment on-board the emergency vehicles.

Technology notes:

Technology needs vary based on the design of the system but typically include onboard vehicle technology to locate the vehicle and generate a preemption request, wireless communications technology to relay requests between the vehicle and the intersection, and intersection technology to receive preemption requests and modify traffic signal timing.

Collaboration notes:

EVP requires collaboration between the emergency response agencies and the agencies operating the traffic signals. These agencies must agree on overall system goals, what equipment will be installed and when the installation will occur, and who will be responsible for maintenance and repairs.