Traffic Preemption: It’s More Than a Green Light

As new technologies continue to expand apparatus safety features, Fire Apparatus Manufacturers’ Association (FAMA) member companies must stay informed and constantly adapt. Part of FAMA’s work is to ensure that new technologies are shared and explored with member companies so they can understand the opportunities, considerations, and potential value of new options as they emerge.

This month’s subject is an interview on traffic preemption between Cory Hohs of HAAS Alert and Mike Lim. Lim is cofounder of Xtelligent, a United States Department of Energy funded venture for connected vehicle and signal control with MIT-/USC-/Berkeley-affiliated researchers. Xtelligent is developing the signal network of tomorrow by reimagining how the ground rules for intelligent, sustainable movement in our cities are adaptively coordinated and communicated; focusing on first responders is a core piece of their mission. Lim also helped lead Los Angeles, California, through automated vehicle, connected infrastructure, and transportation strategies.


Hohs: So Mike, what exactly is traffic preemption, and why is it important to the fire service?

Lim: Preemption allows emergency vehicles to preempt a traffic signal, i.e., the light changes green for the travel direction and other signals turn red, allowing the responder vehicles to pass safely. Preemption studies have shown a 15% to50% reduction in response times, and even 70% reduction in collisions at intersections. This makes a difference, given that approximately 40% of firefighter fatalities occur while traveling to the incident.

Hohs: Let’s start with older traffic preemption—how did it work?

Lim: Older solutions require direct line-of-sight, which can lead to problems if there are large vehicles, bad weather, tree branches, or other blockers. Most systems don’t consider surrounding signals, so if a traffic signal is preempted, it causes congestion at other intersections. Ninety-eight percent of U.S. signals rely on antiquated timer-based approaches that do not adjust signal timing based on real-time traffic nor adjust to first responders. Today’s systems don’t coordinate with each other. This is exacerbated by industry players building a “walled garden” where only their technologies work with each other.

Hohs: What’s next for traffic preemption?

Lim: Significant advancements in traffic signal control, analytics, and connectivity are reshaping the industry and opening up opportunities for first responders to benefit from sophisticated traffic signal timing optimization, priority, and preemption capabilities. This gets a little technical, but important!

Future systems are much more interoperable so that intersections, regardless of the legacy vendor, can optimize with each other. First responders will not have to worry about not being able to benefit from preemption when they cross jurisdictions.

One of the most notable approaches mimics the decentralized command and control paradigm from the Internet developed by the U.S. federal government—the Advanced Research Projects Agency Network—to enable decentralized command and control of the nuclear arsenal in a safe, resilient, and efficient manner. These IP-inspired control algorithms are not only effective at optimizing traffic signal timing but also highly proficient at introducing greater efficiency, adaptability, and resiliency to the system.


Let’s touch on connectivity, recent advancements in Cellular, C-V2X, GPS, 5G, DSRC, and fiber optics are opening new opportunities for coordination. Sophisticated preemption and priority systems fuse the connectivity and signal control technology to provide a coordinated approach to benefit cities and first responders.

Hohs: What about systems to control new signals? If the older systems rely on manual timers, can we now control signals from a central location?

Lim: Absolutely, and beyond. Ubiquitous connectivity has multiple benefits, including better situational awareness on a real-time basis and an opportunity to revisit historical patterns to identify problems and improvement opportunities. It is not only about the fire chief, but the city’s central traffic control, lots of evolution in what “situational awareness” means when it comes to emergency response.

Hohs: If we have the situational awareness, and we have connected apparatus, and intersections, everything will be able to communicate to drop my response time to 30 seconds then, right?

Lim: Well, not sure about 30 seconds! But now that signals can communicate on a real-time basis with vehicles to ensure co-optimization and safety, it’s a real opportunity to see firefighters get to the scene safely, with a reduced chance of collision, all the while not snarling up traffic across the city. The signals intelligently prepare intersections in preparation for the arrival of fire apparatus even before they are within the line of sight, flushing out traffic queues before the vehicles arrive. The system also communicates highly accurate signal timing details to the vehicles to support vehicle control and navigation. After the preemption event, the system would also resiliently recover, allocating signal phase and timing to minimize reverberating congestion.

Hohs: Very cool. As we wrap up, I want to leave the readers with actionable questions to ask when folks are looking to bring traffic preemption into their city. Can you touch on a few key areas?

Absolutely. First, ask if the system can enable optimization in addition to preemption. You want to be able to prepare the intersections prior to arrival and recover after. Additionally, make sure the system can scale rather than only work at a single intersection or corridor. Consider scaling from a technical and cost perspective. If the system cannot deploy widely due to computation requirements, communication limitations, or even high costs, then the benefit of preemption is limited to only a small area. Emergencies can occur anywhere in the city, and the system should be scalable across the city both technically and financially.

Interoperability is key; many systems take a proprietary approach which limits the ability for emergency vehicles to trigger preemption across jurisdictions. Consider whether the system can integrate with other technologies such as digital alerting, CAD, dispatch, etc.

One of FAMA’s responsibilities is to bring subject matter experts to educate its members and the industry. Staying on top of growing technologies is part of FAMA’s commitment to the manufacture and sale of safe, efficient emergency response vehicles and equipment. FAMA urges fire departments to evaluate the full range of safety features offered by member companies.

FAMA is committed to the manufacture and sale of safe, efficient emergency response vehicles and equipment. FAMA urges fire departments to evaluate the full range of safety features offered by its member companies.