In the UK at least, you can often see a rubber strip in a square where the sensor has been added to the road (they cut a slot then re-seal it). I try to stop with my wheel rims right on strip and it often triggers the lights. In fact, I wonder sometimes if it actually senses stronger because the metal wheel rim is so much closer as I've had the light turn immediately if I get it just so. (the specific junction I use is only detector triggered, it doesn't time back and forth at all)