- Sep 2, 2009
It's not pressure, it's the steel of the car as it passes a magnetic field in an induction loop (usually buried inside the road, but sometimes suspended overhead). As the car enters or exits the magnetic loop, the field strength changes, which induces a voltage spike in the loop that gets detected.
I have one of those near my house. The light stays red almost forever unless I drive right up to it.
Much of what makes cycling here better is what makes it more tedious: duffers all over the &^%$ed place. I just need to remember it is them dithering all about the road and sidewalk that makes drivers look out for me, and I'd rather be delayed by a grandmother riding her bike more slowly than I stroll than hit by a car driving faster than the speed limit (the Canadian model).