Nobody who drives a car ever has to think about the various technologies used to trigger traffic signals. You just drive up to the intersection, and within at most 120 seconds (but probably less) you’ll have a green light.
Anybody who rides a motorcycle or bicycle on the road has encountered “defective” traffic lights — those signals that don’t detect the presence of your bike.
Some of learn the deep mysteries of traffic signal technology, others rely on myth and magic, while the rest of us just shrug and either run the light or hobble to the sidewalk to push the pedestrian beg button. I think it’s especially pitiful when bike advocates cheer the placement of beg buttons that are nominally placed for cyclists, as if somebody is doing us a big favor. But I digress.
Probably the most common vehicle detector used at traffic actuated traffic lights are inductive loops. These are coils of wires embedded into the pavement. You can often tell the location of these loops but rectangular cuts in the pavement at intersections.
These loops generate a magnetic field, and a chunk of metal traveling through this field induces eddies in the magnetic field, kind of like pitching a rock into a smoothly flowing stream induces eddies in the current. Big rocks, like big cars and trucks with big chunks of metal, induce big eddies. Small pebbles — like bicycles — induce barely detectable ripples.
Bikes are detected because the wheel (not the frame, as is commonly believed) generates these eddies. You can maximize these eddies by turning your bike horizontal. I demonstrate this at an intersection near my home. This works as well for my fancy carbon fiber road bike as well as the steel Schwinn I ride in this video. I should have placed the camera closer to me so you may need to go full screen to see what’s going on.
A word about magnets
That’s the “myth and magic” I alluded to before. Three or four years ago, I bought a set of fancy neodymium rare-earth magnets exactly like these bar magnets. I placed four of these magnets at various places directly over the induction detection loop. Nothing happened. I expected this because of how induction detectors work, but so many people swear by the effectiveness of their magnets I had to test this empirically.
The fact that I expected the test to fail, however, means my test was likely poorly designed. If you suggestions for improving this test, we should do a Mythbusters style video showing the effectiveness of magnets.