Taken from here: http://bicycleuniverse.info/transpo/...g-signals.html

"I read your article on triggering traffic signals with a bicycle and would like to tell you a few facts about vehicle detectors as I am a designer of these units. Firstly, magnets have no effect at all on loop detectors (unless of course the magnet is so big that it acts more like a big mass of metal.) If you want to trigger the traffic lights with 100% certainty, simply get off the bike and lay the front wheel of the cycle flat over the loop towards one corner for a second or two. The rim of a cycle wheel acts like a big short circuited turn of wire and used in this way will produce a bigger signal than most automobiles do. The reason for this is that you can place your wheel flat on the ground so that it is about 10 x closer to the loop than most automobiles can reach."

Also see this article, and the links he gives at the end: http://www.bikexprt.com/bicycle/actuator.htm