Red Means Stop, Green Means Go, White Means...?

An small poll of my fellow Twin Cities drivers indicates that a lot of motorists don't know what those little white lights mounted on traffic signals are. The most common thought is that they are something to do with catching people who drive through a red light.

They are actually part of a system called OptiCom, developed by St. Paul technology company 3M, which changes the signal priorities for emergency vehicles. Fire trucks and other emergency vehicles carry a transmitter that notifies the traffic signals of their approach, and the signals change to give the emergency vehicle the right of way. The white lights come on or flash to warn drivers that the signals are about to change, and that they may need to move out of the way.

A white light means that the emergency vehicle is on the same road as you. A flashing light means that the emergency vehicle is on the road crossing yours. The signal will soon change to give the emergency vehicle the right of way. If you will be in the way of the emergency vehicle, pull safely to the side of the road, without blocking the intersection, and let the emergency vehicle pass.

Here's a more detailed explanation of how the system works, and examples of what to do if you see a white light, or a flashing white light at traffic signals.