How do traffic light sensors work?

Factual Questions
21

[QUOTE=engineer_comp_geek]
There used to be a hill in Pittsburgh which had a radar speed detector attached to the controllers. Late at night, if you went down the road under the speed limit, you’d get green lights all the way down. If you went over the speed limit, you’d hit red lights all the way down. I don’t know if it still operates that way or not.
[/QUOTE]

Are you sure this was radar-based? Seems a lot easier just to time the lights. (Where I used to live there were several one-way streets which did this. Waves of green propagated down the street at ~30 mph. It you went 32 mph, you were going to outrun the green and have to stop at each light. If you went 28 mph, it was smooth sailing.)

22

On the news reports at the time they made a big deal about the fact that they were measuring your speed. It wasn’t just timing. Of course, you know how things can get garbled by reporters. They were trying to reduce the average speed on this particular road so the city was going out of its way to heavily advertise this new system.

23

Local ones all appear to be induction loops, and are marked by 3 yellow dots to indicate where a motorcycle or bicycle should be placed to trigger the cycle.