Most traffic sensors (at least in the US) are induction-loop types that work the same as the metal detector devices people use at the beach to find coins and jewelry. They use a high frequency electric current in loops just under the pavement. Any nearby metal absorbs some of the energy of the resulting field and this is what triggers the light. No need for any of the metal to be iron or steel, so a bicycle with aluminum rims should be detectable. *BUT* this depends on how high the sensitivity is set on the detector. Many cities just set them high enough to detect cars and trucks which causes problems for cyclists (and some motorcyclists as well).

First make sure you're positioning your bike directly over the sensor location. If the pavement cuts are visible then place your wheel directly over the middle cut (or along an edge is it's a single loop pattern). Hidden loops after a repaving make it more difficult, but the loop is usually directly behind the stop line and in the middle of the lane. If it still doesn't respond then treat it as a broken signal (i.e. proceed when safe after being at a full stop) and contact your local transportation department with a request to adjust the sensor for higher sensitivity. In our area they will come out quite promptly to readjust sensors