Emergency vehicle signal preemption: how responders get a safe green light
How signals detect responders, what the evidence shows and how drivers should react
Safety Behind the Wheel Foundation built this guide from Federal Highway Administration (FHWA) guidance and traffic control standards, Insurance Institute for Highway Safety (IIHS) data, the Washington State Department of Licensing driver guide and published research on emergency vehicle safety. Every source is listed at the end.
What emergency vehicle preemption does
Emergency vehicle preemption, often shortened to EVP, lets a traffic signal clear a path for an ambulance, fire truck or police car. The Federal Highway Administration (FHWA) says EVP changes the signal to green for an approaching responder and quickly shows red to traffic that could cross its path [1].
The federal rulebook for signs and signals is the Manual on Uniform Traffic Control Devices, or MUTCD. It defines preemption as moving a signal from its normal operation to a special control mode [2]. In plain words, the signal stops following its everyday timing plan and follows a special one.
The manual also defines priority control as a way to obtain or change the right of way [2]. Related ideas include priority for buses and preemption for trains [3, 4]. Our guide to railroad crossing safety covers what drivers should do at tracks.
Why intersections are the pressure point
A 2018 review of 180 publications found that more emergency vehicle crashes have happened at intersections than at other places [5].
National records point the same way. One study of fatal ambulance crashes from 1987 to 1997 counted 339 collisions and 405 deaths [6]. In 53 percent of those wrecks, the ambulance was going through an intersection [6]. More of the harm fell on people outside the ambulance [6].
A newer analysis used police reported crashes from January 2016 to February 2020. In nonfatal injury crashes, side on collisions (called angle collisions) were more likely at urban intersections during emergency responses [7]. Other drivers add risk. The Insurance Institute for Highway Safety linked 1,119 deaths in 2024 to collisions with red light running as a factor [8].
Speed still matters after a collision. A study of 2,268 U.S. counties from 2013 to 2015 compared ambulance response times to accidents. Counties with median response times of 12 minutes or more had a crash death rate 46 percent higher than counties under 7 minutes, after adjusting for other factors [9]. The median is the middle value. That study did not test preemption. It does show why minutes count.
So the goal has two sides. Responders lose fewer minutes at red lights, and fewer vehicles meet in the middle of the intersection.
How the signal learns a vehicle is coming
Every system needs a way to receive the request. FHWA describes two broad families, and GPS adds a third option [1].
Comparison compiled by Safety Behind the Wheel Foundation
Three ways a signal can learn a responder is coming
| Approach | How it works | A good question |
|---|---|---|
| Vehicle based | A transmitter in the responder vehicle talks to a receiver at each signalized intersection. The local signal controller (the box that runs the lights) then changes its timing [1]. | Which vehicles and intersections carry equipment? |
| GPS geofence | Software draws a virtual boundary around an intersection. When a responder crosses it, the system asks the signal to turn or stay green [1]. | How is the boundary set for each intersection? |
| System based | Vehicle tracking, dispatch software and a central signal system work together, with no special field hardware at each corner [1]. | Who runs the central system, and who covers it after hours? |
Automated vehicle location (AVL) tracks where each vehicle is. Computer aided dispatch (CAD) is the software dispatchers use to send units to calls. An advanced traffic management system (ATMS) lets an agency control many signals from one place [1].
FHWA says system based setups let responders pick any route to the scene instead of only equipped corridors. It calls both approaches proven. It says the system based one is stronger on cost, reliability, growth to a whole network and day to day management [1]. A 2018 example from San Jose, California, put the system based setup at about $8 million less than the hardware based alternative [1]. The factsheet’s wording suggests the two options did not cover the same number of intersections, so compare costs with care [1]. Research papers also describe designs that use roadside wireless sensors or radio tags to spot responders [10].
What happens at the intersection, step by step
Details vary by agency and by signal controller. One published research design, tested at an intersection in Tainan, Taiwan, shows the usual order of events [4].
- The signal learns that a responder is coming and which way it is headed.
- The controller checks the request. In the Taiwan design, it confirmed the vehicle’s direction and distance before acting [4].
- Cross traffic is cleared. The signal still honors a minimum green time for the direction that has the light, which protects people crossing on foot. Then it shows yellow and a short all red phase, when every direction has a red light [4].
- The responder’s direction gets green and keeps it until the vehicle passes.
- The system detects that the vehicle has cleared. The design used a timer so the signal cannot stay stuck in emergency mode if radio messages stop [4].
- Normal timing returns, and the controller makes up lost time for the cross traffic that waited [4].
A 2003 Virginia Tech Transportation Institute study, cited by FHWA, found that an average preemption lasted 25 seconds. It also found that effects on side streets usually cleared within one cycle, which is one full run of the signal’s normal sequence [1]. Some designs add a special flashing display to warn everyone nearby. The MUTCD allows distinctive indications for emergency vehicle preemption as an exception to its ban on flashing displays next to signal indications [3].
What the evidence shows, and where it is thin
FHWA’s 2024 factsheet gathers results from local programs, many of them decades old [1].
- St. Paul, Minnesota: a 1977 study reported 71 percent fewer emergency vehicle crashes after EVP was installed [1].
- Denver, Colorado: a 1978 study found response times fell 14 to 23 percent, about 70 seconds on average [1].
- Plano, Texas: seven intersection crashes at signals in the three years from 1981 to 1983, then four in the 20 plus years after EVP [1].
A health study in Kanazawa, Japan, followed 1,161 cardiac arrests from 2003 to 2011. Ambulances with a preemption system had a median response time of 327 seconds, against 381 seconds for those without one. One year survival was 7.0 percent with the system and 2.8 percent without [11]. The study was observational and came from one city, and its statistical range was wide. It suggests a benefit but does not prove one.
Treat all of this as promising, not settled. FHWA’s list of proven safety countermeasures includes signal items. Two examples are yellow change intervals (the timing of the yellow light) and backplates with retroreflective borders (framed panels behind a signal that make it easier to see). The list does not include emergency vehicle preemption [12]. Preemption also costs other drivers time. A 2025 research paper notes that it interrupts normal signal timing and can add delay for other traffic [10].
Who chooses, buys and installs it
Public traffic signals belong to a road agency, such as a city, county or state transportation department. Fire and EMS agencies supply the vehicles and the dispatch tools. FHWA says an EVP program works best when the signal agency and the public safety agency plan together [1]. Safety Behind the Wheel Foundation gives every community the same advice.
Public traffic control devices must be selected, designed and installed by the responsible agency. The MUTCD says the decisions it calls for must be made by an engineer, or by someone supervised by an engineer with the right experience [2]. Residents should never try to install or adjust preemption equipment.
The MUTCD also reaches beyond public streets. It applies to roads open to public travel whether they are publicly or privately owned, and to site roads open to public travel without full time access limits [2]. Federal rules make it the national standard for traffic control devices on any street open to public travel [13]. So a campus, a business park or a private road owner who runs signals on such a road should follow it.
Typical buyers are road agencies, municipalities, campuses and private road owners. They usually buy through a traffic engineering firm, an infrastructure supplier or an approved installation contractor, using the agency’s own purchasing process. For system based options, the dispatch center and the signal agency must link their systems, so both have to work together.
How to choose and check a system
FHWA says equipment alone is not enough. Agencies also need a policy for when preemption is used, and responders need training. Tracking how often the system activates and what benefits it brings helps an agency show the program’s value [1]. A corridor review can start with these questions.
The railroad question matters near tracks. The MUTCD’s warrant for a signal at an intersection near a grade crossing says preemption control must be provided under its rules [3].
Limits and common mistakes
A 2018 review advises emergency vehicle drivers to confirm that nearby vehicles have stopped before they enter an intersection [5]. It also notes that when several vehicles answer the same incident, intersection collisions have followed [5]. A 2025 paper adds that most preemption models are built for a single emergency vehicle, although two can arrive in the same lane [10].
Common mistakes include:
- Equipping only some corridors. Vehicle based systems work only where equipment exists, while system based setups let crews choose any route [1].
- Skipping policy and training, which FHWA says agencies need [1].
- Ignoring the cost to cross traffic and to coordinated signal timing [10].
- Forgetting people on foot. Minimum green and crossing time need to stay in the plan [4].
- Never reviewing the data. Without activation records, nobody can tell whether the system is helping [1].
Upkeep matters as much as installation. Receivers, GPS settings and dispatch links need regular testing, and each fix needs an owner. FHWA says newer technology brings cost effective setups with fewer operating problems and lower maintenance costs, which makes EVP worth a new look [1]. A system that fails without anyone noticing can mislead responders who count on it.
What drivers should do
Rules differ by state, so read your own driver guide. Washington’s guide, for example, says an ambulance, fire truck or police car with lights or sirens on always has the right of way. It tells drivers to move to the right edge of the road and stop, then wait for the vehicle to pass before signaling and reentering traffic [14].
At a signal, a few habits help. A green light is not a promise that the way is clear.
- Listen for sirens and glance at your mirrors and cross streets as you approach an intersection.
- If the light changes for no clear reason, or shows an unfamiliar flashing display, slow down and look for a responder.
- Do not enter the intersection on a fresh green until you have checked both sides.
- Follow your state’s rule for stopping and wait until the vehicle has passed. Then look for a second vehicle.
- Reenter traffic only when it is safe, and keep your usual following distance.
Digital alerts may help too. FHWA notes that they can warn drivers that a responder on a call is coming up from behind, so they can yield [15]. Our guide to work zone and move over safety covers what to do when responders are stopped on the roadside.
What residents, schools, campuses and small employers can do
You cannot install preemption, but you can ask good questions. Start by finding who runs the signal. For a public street, that is usually the city, county or state transportation department. For a private road, it is the property owner.
Then talk with the fire or EMS chief about slow or risky intersections on common routes. Put your request in writing. Name the intersection, describe what you have seen, and ask for an engineering review. Our guide to safer road design walks through how to request a road safety review and what to include.
Frequently asked questions
Does every light turn green for an ambulance?
No. Only equipped intersections respond, and only when the system detects a responder. At those intersections, the signal turns or holds green for the responder and shows red to cross traffic [1]. Elsewhere, responders face ordinary lights and must check for cross traffic themselves [5].
Can I buy a preemption device for my own vehicle?
Preemption works only when the vehicle’s equipment, the intersection’s equipment and the agency’s rules all match [1]. This equipment belongs to agencies, not private drivers. Ask your local fire department or road agency if you think a vehicle or intersection needs it.
Why did the light change when I saw no emergency vehicle?
The system can detect a responder before it reaches the corner. GPS based systems, for example, watch for vehicles entering a boundary around the intersection [1]. The change may happen while the vehicle is still out of sight. Treat an unexplained change as a cue to slow down and check every direction.
Does preemption also handle trains?
Railroad preemption is a separate system with its own rules. The MUTCD’s warrant for a signal at an intersection near a grade crossing calls for preemption control under its rules [3]. Near tracks, ask the agency how train and responder requests are timed. See our railroad crossing safety guide for what drivers should do.
Does preemption make roads safer overall?
It can help responders, but proof is limited. FHWA’s own case examples are local and mostly old [1], and the technology is not on its list of proven safety countermeasures [12]. Think of it as one layer in a larger plan, like the one in our Safe System approach guide.
How much does it cost?
It depends on how many vehicles and intersections are involved and which approach is used. FHWA’s San Jose example reported a gap of about $8 million between system types, though the intersection counts differed [1]. Ask for a written cost comparison that covers equipment, upkeep and staff training.
The bottom line
Recommendation from Safety Behind the Wheel Foundation
Safety Behind the Wheel Foundation encourages drivers to treat any signal change as a cue to look and listen, and encourages communities to ask how local preemption is used and tested. Emergency vehicle preemption gives responders a safer, faster path through signals by changing the light for them and holding cross traffic. The best programs combine matched equipment, careful timing, clear policy, staff training and regular testing.
Everyone else can stay alert near intersections, yield as the state requires, and ask the road agency and fire chief how local signals handle emergency calls.
Related articles
Sources and further reading
- 1Federal Highway Administration (FHWA), Office of Operations. Next-Generation Traffic Incident Management: Emergency Vehicle Preemption (EVP) for Saving Lives (FHWA-HOP-24-019). Published 2024. Data year: 1977 to 2018 (local case examples). ops.fhwa.dot.gov/…/fhwahop24019.pdf
- 2FHWA. Manual on Uniform Traffic Control Devices for Streets and Highways, 11th Edition, Part 1 (General). Published December 2023. mutcd.fhwa.dot.gov/pdfs/11th_Edition/part1.pdf
- 3FHWA. Manual on Uniform Traffic Control Devices for Streets and Highways, 11th Edition, Part 4 (Highway Traffic Signals). Published December 2023. mutcd.fhwa.dot.gov/pdfs/11th_Edition/part4.pdf
- 4Lee WH, Chiu CY. Design and Implementation of a Smart Traffic Signal Control System for Smart City Applications. Sensors, 2020;20(2):508 (PubMed record). Published January 2020. pubmed.ncbi.nlm.nih.gov/31963229
- 5Hsiao H, Chang J, Simeonov P. Preventing Emergency Vehicle Crashes: Status and Challenges of Human Factors Issues. Human Factors, 2018;60(7):1048-1072 (PubMed record). Published July 2018. pubmed.ncbi.nlm.nih.gov/29965790
- 6Kahn CA, Pirrallo RG, Kuhn EM. Characteristics of fatal ambulance crashes in the United States: an 11-year retrospective analysis. Prehospital Emergency Care, 2001;5(3):261-269 (PubMed record). Published 2001. Data year: 1987 to 1997. pubmed.ncbi.nlm.nih.gov/11446540
- 7Hossain MM, Zhou H, Das S. Data mining approach to explore emergency vehicle crash patterns: A comparative study of crash severity in emergency and non-emergency response modes. Accident Analysis and Prevention, 2023;191:107217 (PubMed record). Published July 2023. Data year: January 2016 to February 2020. pubmed.ncbi.nlm.nih.gov/37453252
- 8Insurance Institute for Highway Safety (IIHS-HLDI). Red light running. Updated July 2026. Data year: 2024. iihs.org/research-areas/red-light-running
- 9Byrne JP, Mann NC, Dai M, et al. Association Between Emergency Medical Service Response Time and Motor Vehicle Crash Mortality in the United States. JAMA Surgery, 2019;154(4):286-293 (PubMed record). Published April 2019. Data year: 2013 to 2015. pubmed.ncbi.nlm.nih.gov/30725080
- 10Dai Y, Hu L, Zhou S, Liu Y, Yang A. A bi-level dynamic emergency route planning system considering signal preemption control using CV technology. PLoS ONE, 2025;20(5):e0323209 (PubMed record). Published May 2025. pubmed.ncbi.nlm.nih.gov/40388501
- 11Tanaka Y, Yamada H, Tamasaku S, Inaba H. The fast emergency vehicle pre-emption system improved the outcomes of out-of-hospital cardiac arrest. American Journal of Emergency Medicine, 2013;31(10):1466-1471 (PubMed record). Published September 2013. Data year: April 2003 to March 2011. pubmed.ncbi.nlm.nih.gov/24035049
- 12FHWA. Proven Safety Countermeasures. Updated date not shown on page; reviewed October 2026. highways.dot.gov/…/proven-safety-countermeasures
- 13U.S. Government, Electronic Code of Federal Regulations. 23 CFR 655.603 Standards. Current as of October 2, 2026; last amended December 19, 2023. ecfr.gov/…/section-655.603
- 14Washington State Department of Licensing. Washington State Driver Guide (text-only version), section 4.9, Sharing with emergency vehicles. Publication date not shown on page; reviewed October 2026. dol.wa.gov/…/washington-state-driver-guide-text-only
- 15FHWA, Office of Operations. Responder-to-Vehicle Alerts Factsheet (FHWA-HOP-22-015). Publication date not shown on the document. Data year: 2018 to 2020 (case examples). ops.fhwa.dot.gov/…/EDC-6_Factsheet_TIM_Responder-to-Vehic…
This article reflects the safety priorities of intersection safety for emergency responders and everyone who shares the road with them. It is general education, not legal, medical, insurance, financial or engineering advice for any specific intersection or vehicle. Rules change, so verify details with official sources. For your own situation, contact your city or county traffic engineering office, your state transportation department, your local fire or EMS agency, or your state driver licensing agency. Reviewed October 2026.
Cite this article: Safety Behind the Wheel Foundation. “Emergency vehicle signal preemption: how responders get a safe green light.” drivewithcare.org, https://drivewithcare.org/emergency-vehicle-signal-preemption/. Reviewed October 2026.
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