Lots of posts here about the old induction coil sensors. I recently spoke with a local traffic engineer about one of the newer traffic light sensors that are being used in newer signals. Thought it might be of general interest.
The lights use a "mass sensor"... a camera-looking thing mounted a couple of feet above the boom. (They can't be mounted on lights suspended by wires... no way to fix the detection zone.) There's typically also an emergency vehicle detector, the thing that looks like it has a floodlight, mounted on the boom. The sensors have a fairly confined detection area, typically set up for an area from 5' in front of the stop bar to 30' behind it, and is pretty narrowly focused on the lane, so activity outside that (e.g. pedestrians) don't trigger it. There is also a sensitivity adjustment, and there is a reset mechanism, so cars that roll up to the light and then turn right on red don't trigger a green. All these settings can be adjusted. The engineers advice to me was that if there were no cars, I should be sure I'm in the detection zone. I find they work the best if I roll up to the bar, and stop near the center of the lane. And if the light doesn't respond, notify the appropriate local DOT that it needs to be adjusted.