Most traffic lights these days have sensors that detect the presence of cars waiting for the light to turn green. These sensors are buried in the lanes, one at the head of each lane, and usually one or more some ways further back. As I understand it, they detect cars by induction. The sensors are basically a magnetic loop and the presence of a large amount of conductor near by (the metal in a car) induces changes in that magnetic field.
You can tell where the sensors in the road are by looking for a small black diamond shape (the older ones) or a large black circle with a black line running to the side of the road.
However, I’ve noticed that at most (if not all) of the newest intersections, I can’t see where they put the sensors. There’s no black lines on the pavement.
This is not just an academic exercise. I bicycle frequently and sometimes I need to trigger traffic lights. I can usually do this by laying my bike down on the sensor and it will detect it. But on these new intersections I can’t see a sensor. On the assumption that there must be a sensor there even if I can’t see it, I tried laying down my bike at the head of the lane, but it didn’t work.
So what’s going on? I don’t think they are using optical detection of cars as I would expect it to detect me on my bike. Plus I don’t see any special cameras for that. Is there some other technology at work here?