The system is called Emergency Vehicle Pre-emption or EVP. These devices are controlled by a white strobe light on each fire department emergency vehicle, which emits a signal to a receiver on the traffic light when the fire department vehicle approaches. The traffic light will change to green in the direction the fire department vehicle is traveling and turn red in the opposite direction.
 

In this picture, you can see the flashing white light on the engine

In this picture, you can see the solid white confirmation light on the traffic signal.

A flashing white confirmation light appears above the traffic signal when the system is activated to acknowledge the signal will be changing. A solid white confirmation light means an emergency vehicle is approaching from the opposite direction.

If you see emergency lights or hear emergency sirens, slow down and pull to the right to allow the emergency vehicle to pass in either direction. Once the vehicle has passed carefully resume your normal speed. Remember "Pull Right For Sirens and Lights."

For more information please call the Mount Prospect Fire Prevention Bureau at (847) 818-5253.