Conduct rigorous testing prior to deployment of an emergency preemption system to avoid potential problems and negative system impacts.
The experience of multiple agencies with emergency vehicle preemption systems.
Made Public Date


United States


Fairfax County
United States


St. Paul
United States

Traffic Signal Preemption for Emergency Vehicles: A Cross-Cutting Study


Emergency vehicles operating in metropolitan areas with high congestion levels are at an increased risk for involvement in crashes and are subject to unpredictable delays in reaching the scene of a fire or crash. One way to offset the effects of congestion is the installation of emergency vehicle preemption (EVP) equipment at signalized intersections. This ITS technology provides a special green interval to the approach serving the emergency vehicle while providing a special red interval on conflicting approaches. Providing a green light at signalized intersections will reduce conflicts, driver confusion, and emergency response times.

In January 2006, the Federal Highway Administration produced a cross-cutting study to examine emergency preemption at signalized intersections in a number of communities across the United States. To show a wide range of deployment options, three jurisdictions – Fairfax County, Virginia; Plano, Texas; and St. Paul, Minnesota – were identified and officials were interviewed. The purpose of the study was two fold: to increase awareness among stakeholders – police, fire, rescue, and emergency medical services (EMS) – of the benefits and costs of EVP, as well as to reduce the time it takes to move from a plan to realizing improvements in the delivery of emergency services.

Lessons Learned

By providing emergency vehicles (EV) with a green light on their approach to a signalized intersection throughout the length of their run, a jurisdiction can reap substantial benefits. Not only does the emergency vehicle reduce its response time, but the overall safety of the traveling public is also improved when there is less potential for a conflict with the emergency vehicle.

However, each EVP system needs to be tailored to fit the needs of the jurisdiction. Due to differences in traffic signal controllers and traffic signal systems, an EVP system is not simply a "plug and play" type system. During system installation, adjustments will need to be made prior to system-wide deployment. The system will need to be debugged using the parameters for that jurisdiction and, because of the nature of deploying this type of system and exposing the traveling public and the emergency vehicle to potential conflicts, a field test should be performed before system-wide deployment.

Interviews conducted at the three jurisdictions provide the following guidance for an EVP system installation:

  • Bench test the equipment and software in the shop with the same equipment that is found in the field. Bench testing can help prevent potential problems that may occur in the field when the system is deployed. Traffic controllers should be set up in the shop the same way as in the field to replicate any issues that may occur. In Fairfax County, the Virginia Department of Transportation (VDOT) found that their traffic signal controller software required an upgrade to allow dual use of the technology for both EVP and transit signal priority. If provided, transit signal priority should always be a secondary request to an emergency vehicle preemption request. Prior to the software upgrade, VDOT found that the transit priority requests were granted the same level of precedence as an EVP request. If this situation had not been tested before deploying the system in the field, a serious situation may have occurred.
  • Wire the vehicle emitter into the EV parking brake or transmission lever to turn the emitter off when the EV is stopped. When an emergency vehicle stops in the vicinity of an intersection, a continuously running emitter will hold the signal in the preemption phase indefinitely, possibly causing significant traffic problems. Preemption systems usually include factory-installed emitters that include a power interrupt tied to the transmission shift lever that disables the emitter when the vehicle is in "park." In both Fairfax County, Virginia and Plano, Texas the technicians had to develop custom power interrupt solutions for vehicles with locally-installed emitters.
  • Maintain an open line of communication among stakeholders during the acceptance testing period to avoid poor system performance and perhaps avert a dangerous situation. Resolving system performance issues requires cooperation and communication between EV drivers and EVP maintenance technicians. Certain signalized intersections may pose problems in terms of emitter-detector line-of-sight reducing detection ranges. Finding the right solution requires detailed problem descriptions. The EV drivers need to be involved when the system is being installed and should continue to communicate to technicians any problems in order to maintain optimum system performance. In some cases the maintenance technicians may work for the traffic engineering department and in other cases the technicians may belong to the emergency service department. In either case, it is important that good communication is maintained to provide a high level of system performance and to avoid any possible dangerous situations.

Improving emergency response, in support of reduced response times and intersection safety as well as homeland security and disaster preparedness, is a high priority for most regions. The benefits of an EVP system far outweigh the potential for any negative impacts the system might have on traffic delays. When EVP is implemented well, the negative effects on traffic flow are not significant and public acceptance of the system is high. Public awareness grows quickly and complaints about the system decrease. Being sure that the system has been debugged and field tested before it is deployed system-wide will lessen the potential for a dangerous situation to occur and will improve the public's perception of how the system functions. EVP systems provide ITS solutions that help meet local and regional transportation goals to improve safety, mobility and customer satisfaction.

Traffic Signal Preemption for Emergency Vehicles: A Cross-Cutting Study

Traffic Signal Preemption for Emergency Vehicles: A Cross-Cutting Study
Publication Sort Date
William C. Louisell, SAIC
Prepared by the FHWA USDOT

(Our website has many links to other organizations. While we offer these electronic linkages for your convenience in accessing transportation-related information, please be aware that when you exit our website, the privacy and accessibility policies stated on our website may not be the same as that on other websites.)

Focus Areas Taxonomy: