I bought 4 neodymium magnets from this place.
http://www.kjmagnetics.com/We have a gate at work that to exit, you need to trip a buried loop. Or I can call and bug the guys in the office to open the gate for me.
The magnets did nothing. I never put them on the bike, I just carried them to the loop in the asphalt and ran them back & forth, to no effect.
They live on the side of the fridge, now.
They are extremely powerful, and while playing around with them, I caught a bit of my finger between two of them.

Not good. Blood blister, I think it is called.
I think the one's I bought were 3" x 1/2" x 1/4" thick
http://www.kjmagnetics.com/proddetail.asp?prod=BZ084Now I just exit the building outside the parking lot, set my stuff down, open the gate with my card, and go inside and push the bike out (it's right next to the gate) and pack up and put my helmet on out there.
It works.
Traffic lights suck, though. I've not gone through too many, and I've never been caught.