I’ve noticed that when I’m stopped at a traffic light and an ambulance is approaching, it seems like all the traffic lights stay on red while that vehicle maneuvers by. Two questions:
1. Is that correct, or is it a coincidence with the lights?
2. If it’s correct, how are the traffic lights controlled in these situations?
Comments
Emergency vehicles have control units of some kind (might vary I’m not sure) that alert intersections when they are approaching and switches all the lights.
Yes, emergency vehicles have a device that they can use to change the traffic light so they can pass through intersections unimpeded.
The lights around here also have a little white light on the transponder that you can see turn on when it’s been triggered.
They have a device called a preemption unit which is plugged into the signal controller that controls the lights. When an emergency vehicle comes through and a signal reaches the traffic signal controller inside of the cabinet near the signal, it knows that it needs to end the current phase and give priority to the approach the emergency vehicle is coming from. After that happens, it finishes the cycle from the point it was forced into and then resumes normal operation.
ELI5 answer: They basically have a remote control that tells the signal to take the proper steps to change the light to green