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Common Problems with Preemption Systems & How to Solve Them

Preemption System

Preemption systems can dramatically decrease response times for emergency vehicles. Nationwide studies have demonstrated a 15 to 50 percent reduction in response times and a 70 percent decrease in crashes involving emergency vehicles. These improvements have a big impact on the outcomes of medical emergencies and greatly improved traffic safety.

Let’s take a look at some common issues with preemption systems, as well as some possible solutions to help further improve response times.
Preemption systems dramatically reduce emergency response times, but there are some common issues with legacy systems. Click To Tweet

Line of Sight

Many preemption systems use optical, infrared or radio technologies, which require line of sight. They do not pass through buildings, over hills or around corners, which means that emergency vehicles may be required to make a complete stop and wait for traffic to clear before moving forward. And common GPS technologies also require a clear view of the sky to function.

For example, a fire engine may receive a call within a firehouse or an ambulance may be in a parking garage. Since there’s no clear line of sight and the GPS can’t resolve without a clear view of the sky, they are often stuck at the first traffic light. This is widely known in the industry as the “first light” problem, and it has a significant impact on response times.

Cellular technologies are a great way to solve the line of sight issues associated with optical and radio technologies. At the same time, dead reckoning GPS systems use accelerometers and gyroscopes to calculate a vehicle’s exact location and heading based on their last-known GPS position, even without a clear view of the sky. Cellular communications allow emergency response vehicles to talk to intersections ahead of time and even around corners and over the hills.

Traffic Jams

Preemption systems often can’t clear traffic in advance, since they’re only activated when an emergency vehicle is close enough to the intersection. But a green signal isn’t very helpful when there’s a traffic jam ahead, which is a common occurrence in busy cities during rush hour and other busy traffic hours (e.g. holidays or events).

Download our emergency response audit worksheet to assess your emergency response times and identify ways to improve over time.

GPS and cellular systems can mitigate these problems by detecting traffic ahead of the emergency vehicle and causing the traffic signal to go green early to flush out the traffic ahead. This not only flushes out the traffic at the first intersection but also the next intersections upstream. Then all of the traffic lights along their route can be changed to clear traffic in advance, rather than waiting for the emergency vehicle to approach. This could save valuable time for emergency responders.


Emergency vehicles approaching an intersection with a green light might not have any way of knowing if the signal has actually been preempted. If they need to take a left turn, the vehicle may need to stop and ensure that all of the oncoming traffic has stopped before proceeding through the intersection, in order to avoid a collision.

A common solution to this problem is to program the traffic signal controllers to always bring all other traffic to a complete stop and provide the emergency vehicle with a green light in all directions. This means that the straight and left turns are active. This also ensures that the driver of the vehicle can see all other approaches are stopped and they have control of the intersection.

Stop Times

The Federal Highway Administration points out that preempted signals that stop motorists for too long may “encourage disrespect for the red signal”. While there’s no specific evidence with preemption systems, disobedience is common among signalized crosswalks and similar scenarios where vehicles are forced to a temporary stop.

After an emergency vehicle has passed through, it’s important to ensure that the signal goes back to normal operation as quick as possible. The solution to this is to put the signal back to normal operation once the emergency vehicle is just 30 feet past the signal. Many older systems have a 15 second timeout and this frustrates drivers dramatically.


Studies have shown that, once a signal was preempted, coordinated systems can take anywhere from 30 seconds to seven minutes to recover to normal operation, depending on the location. During peak traffic hours, these interruptions can lead to significant delays for all vehicles at the intersection and nearby roads.

Don’t forget to download our emergency response audit worksheet to assess the efficiency of your emergency response vehicles and identify ways to improve.

City engineers frequently point out that they often receive an immediate phone call about a malfunctioning signal but receive very few calls attributed to the impact of a preemption event. While drivers are very understanding, there’s still a productivity, environmental, and financial cost to congestion.

Modern traffic signal controllers have advance functions that make it possible to get traffic in coordination quicker. By utilizing these new functions, agencies can make sure traffic signals get into coordination much quicker.


Most preemption systems don’t include any kind of reporting. While emergency vehicles may record their response times, it’s difficult to consolidate information and analyze it to find ways to improve or measure improvements over time. These are critical activities to ensure an effective response.

Next-generation preemption systems interface with central systems to automatically record response times and show trends over time. This data can be helpful when assessing different routes and strategies that first responders use during an emergency situation.

Smart City Solutions

Smart city solutions can make preemption systems safer and more efficient by introducing next-generation technologies, such as GPS and cellular modems, along with integrating the technology into a smart central system.

Applied Information’s Glance Preemption and Priority Systems seamlessly combine cellular, radio transmission, and GPS technology into one, easy to use tool. Enhanced GPS with dead reckoning mitigates any low signal areas, while a rules-based engine makes it easy to configure and adjust the systems.


The preemption systems also include built-in cabinet monitoring to ensure that they’re functioning normally, including monitoring of incoming AC power, BBS battery, conflict monitor, fan status, remote power cycling, and more. If something isn’t working, the traffic engineers are automatically alerted to fix the issue.

On a higher level, emergency response and city planning personnel can monitor the location and status of the vehicles at all times from headquarters. These personnel can also see in-depth reporting data, including total miles, average speed, types of emergencies and average response times, making it easier to to identify problems and improve the system.

The Bottom Line

Preemption systems can dramatically reduce emergency response times and improve traffic safety. Next-generation technologies further improve the safety and efficacy of these systems by solving some common issues that arose with early versions, such as line of sight requirements and traffic congestion.

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