Emergency vehicle detectors: How do they work?

You might recall that bus accident that happened near the corner of 11th and Broadway in Quincy Wednesday.

One of the emergency vehicles was parked facing 12th and Broadway and didn't turn off it's strobe light.

The detector continued to pick it up, backing up traffic for blocks.

You might have seen these little black sensors on top of the cross bars of a traffic light in Quincy. They're emergency vehicle detectors. But how do these detectors work...?

"The strobe light is detected at the intersection, and then three legs would go to a red light and the remaining leg for safe passage of the vehicle would go to a green light," said City Engineer Jeff Steinkamp.

What if the accident is AT the intersection?

"There is a switch that we can shut that light off. In addition, there's also a mechanism as we set our parking breaks on our vehicles that light goes off when we do that so as we're parked that does not operate anymore," said Quincy Deputy Fire Chief Steve Salrin.

But that didn't happen Wednesday...

"Apparently there was a signal that was still hitting into the 12th Street, 12th and Broadway intersection and because of that we got a call to City Hall and knew the traffic was backing up. We sent out a technician immediately, went to the intersection and disabled the detection system there until the accident scene was clear," said City Engineer Jeff Steinkamp.

City Engineer Jeff Steinkamp says the detector then resets itself after an emergency vehicle passes through the intersection.

The detectors generally only take a few seconds to regenerate.