You have certainly got some good - and bad- information on this one. Since no one else has addressed your question based on you picture, I'll give it a shot. Tripping the light is more a matter of location of the bike than any other factor. You want to position your bike over the most sensitive part of the loop (motorcyclist make this mistake, too.) Once you know where to find that place, most lights become easy to trip.

To understand the physics of the induction loop, you have to think of what's going on. The wire in the ground is only the physical manifestation of the induction loop. Unseen to us, since we don't see in magnetic fields, is another loop of magnetic lines extending out from the wire for a few inches to a couple of feet. The magnetic lines have strengths that are dependent upon their distance from the wire. The closer to the wire you are, the stronger the field strength. (All wires actually do this. You can get current detectors that clamp around the wire and allow you to measure the strength of the field as well as various other properties of the power within the wire.) This goes all around the wire from the signal box, out to the street and back to the signal box.

Now think of you and your bike coming along and going through this magnetic field. (Sorry but I can't draw worth beans so we'll have to make do with words.) Looking at your picture, if you cross over the magnetic field in the middle of the circles, you only break the field in 4 places and for pretty short duration. Most of the current sensors for these loops are set pretty low since they only have to detect cars directly over them. Cars have great masses of conductive materials in them so they detectors don't have to be set too sensitive.

But look at your picture again. There is a wire going from the detector box to the loop in the pavement. But what you forget is that this has to be a curciut, so there has to be a wire going back to the box. Usually, but not always, the wire going out and the wire coming back are in the same groove. (You only have to cut one slot that way.) Because of the magic of electric fields, the magnetic field of this 2 wire system is going to be twice as strong as those in the circles from your picture. If you ride with your wheel directly over this line (to the right of the circles), you will trip the sensor! Presto! Chango!


The loop that you are trying to trip is one of several different designs. Some are better for bikes than others. The 'figure 8' loop is probably the best since the most sensitive part of the loop is the easiest to find. The double circle one is probably easy to trip but it will put you in a bad lane position. I've seen them on the 'net but I have yet to run across one in the wild.