I have attached a magnet to the bottom of my older bike, and it does make a difference. The trick to it is that when you get to the intersection, you have to roll the bike back and forth so the magetic coil underneath the pavement picks up some motion. By doing that, I've been able to go through some intersections without the help of a car or truck.

We used to generate drawings of the standard coil at the engineering firm I used to work at that goes underneath the pavement. I asked one time, and was told that they are not sensitive enough to pick up anything smaller than the typical economy car.