Lined up on the wire
There’s nothing more frustrating than getting stuck at an on-demand signal and having to wait for a car to come up from behind to trigger the light. In some jurisdictions, if you’re unable to trigger the light, it’s legal to proceed after stopping, but that doesn’t help when you’re at a cross street with heavy traffic moving in both directions. The good news is that in many cases it’s possible to trigger a light with your bicycle.
On-demand signals use what they call “induction loop vehicle detectors” to sense when a vehicle is waiting at a light. These detectors are essentially metal detectors embedded into the pavement. They work by sensing changes in an electromagnetic field and have nothing to do with the weight of the vehicle. You can often see evidence of loop detectors as lines cut into the road surface just behind the crosswalk. Wire sensors are embedded in these cut lines, and it’s possible to trigger a light by placing your bicycle wheels precisely on top of one of the wires to disrupt the magnetic field. Some sensors seem to be more sensitive than others; in those cases where the light isn’t initially triggered, I’ve had some success by leaning my bike over toward the inside of the detector loop. In cases where there are two side-by-side loops, lining up over the center where the two loops meet doubles your chances of triggering the light. Once I understood exactly how loop detectors work, my rate of success at triggering lights considerably improved; I’m currently getting somewhere approaching a 90% success rate on the detectors where I live.