Originally Posted by
cyccommute
If the cars stop beyond the stop line, they will have driven over the sensor and tripped it. If they stop before the sensor, they won't trip it. A car is a large enough conductive object that they can often trip them by just putting the bumper over...or even near...the sensor line.
Somehow, the sensors here know if you've gone past them. Numerous times I've stopped on the sensor and then started rolling a second or two before my light is green and it will stay red and immediately give the other road a green because it knows I'm no longer on the sensor.