Police indicator lights on stoplights...

Where is the light on the vehicle? On the vehicle we have a blinking white light that is used to control the traffic lights as we approach them. It gives use the green light only when it is blinking.

Here in NJ they have the blinking white lights on top of the school buses too!

The white light is an Emergency Vehicle Preemption (EVP) confirmation light located on or near a traffic signal (a.k.a. traffic light, stop light, semaphore). Its main purpose is verification for emergency vehicles that the EVP signal has been received and the traffic signal will remain green until the emergency vehicle has safely crossed the intersection. Obviously, a solid or blinking light in both directions serves that purpose and is used in many locations.

The indicator lights described in this question are advanced EVP confirmation lights that are in use at many traffic signals in Minnesota, Illinois, and Iowa. Advanced confirmation lights are especially helpful to emergency vehicle drivers when more than one emergency vehicle approaches the same intersection from different directions.

If you see a steady white light, the emergency vehicle is approaching your direction from either side and the traffic signal will be red for you.

If the white light is flashing on and off, the traffic signal for you will be green and the emergency vehicle is either coming straight towards you or is approaching from behind you. The emergency vehicle may be a mile or more ahead or behind, so be alert for the flashing lights and siren since you are expected to continue driving until you see or hear those indicators. That keeps the intersections clear and traffic moving until the emergency vehicle approaches.

Note: In British Columbia (Canada), a white light indicates signal light preemption in your direction of travel and a blue light indicates the intersection is being controlled by an emergency vehicle approaching from another direction.

The information for Minnesota can be found on the state transportation website at: http://www.dot.state.mn.us/trafficeng/faq/faq-signals.html#K

I am a Defensive Driving Instructor (U.S. National Safety Council certified).