Originally Posted by
RubeRad
What does that mean? I thought traffic light sensors were all triggered by weight. Chatting with another cyclist here in San Diego, he told me over the last few years, many sensors have been tuned to detect cyclists, not just cars.
The loops in the pavement are inductors driven by an alternating current. The resulting magnetic field causes current to flow in nearby conductive (usually metal) objects which in turn changes the loop's inductance and resonant frequency which is picked up by the traffic light controller. The circuit can be adjusted to trigger based on how big the change is, where the losses are higher when there's more conductive material nearby and a bicycle needs a much lower setting than a 4000 pound SUV.
The substance only needs to be conductive - it needn't be magnetic too. Aluminum rims trigger the circuit.
I'm somewhat curious about how things work for people with carbon rims on carbon bikes (although the epoxy is an insulator the carbon fiber it surrounds is a conductor).