If a car is coming up behind me at the light, I always make room for them and motion them onto the sensor. They usually appreciate the gesture, even though they don't know why I'm doing it.

I took a magnet out of an old hard drive - those suckers are strong! - and stuck it underneath my bottom bracket. If you have a non ferrous frame you could always clamp it to one of the chainstays. When there are no cars around, I line my frame up with the sensor cut-outs in the road if they haven't been paved over yet.

Any rare earth magnet should disrupt the sensor field enough to trip the light.

Both systems work for me just fine.