Most drivers have seen them, the little white lights that sometimes flash above a traffic light… but how do they really work?
That white light on top of a stoplight is an indicator for the Emergency Vehicle Preemption system. The device helps to control traffic when emergency vehicles are responding to call. The system can prompt a change in the traffic lights within an intersection before an emergency vehicle approaches, allowing them to move through intersections more safely, while alerting other motorists to what is coming down the road.
Solid White Light = emergency vehicle is traveling the same direction as your vehicle.
If the light facing you is solid white, that means an emergency vehicle is traveling in the same direction as you, either in front or behind your vehicle. Be prepared to slow down, pay close attention to traffic in all directions and if the emergency vehicle is approaching behind you, move your vehicle to the side of the road.
Flashing White Light = emergency vehicle is traveling in a different direction than your vehicle.
If the light facing you is flashing white, that means an emergency vehicle is approaching the intersection from a different direction than you are traveling, either from a cross street approaching from your left or right, or traveling toward you. Expect traffic lights in your lane to transition to red. Pay close attention to traffic in all directions and be prepared to stop.
How it Works
Inside each emergency vehicle is an emitter which can send an infrared signal to preemption devices that are directly in front of them. The signal prompts the traffic lights ahead to change so that the emergency vehicle can pass through safely. At the same time, the preemption device stops all cross traffic from entering the intersection, and in most cases, stops traffic coming from the opposite direction.