Magnetic induction loops require metal in close proximity to disturb them. They are placed behind the white line because that's where the bulk of the metal is... in a car! I have found that if I place my bike back from the white line by about a metre then it is in the loop (if I can't see the cuts in the road where it is placed).
I haven't had too many probs with a metal bike but for the Carbon guys... the theory goes that a strong magnet will also disturb the magnetic induction loop.
I know it's a few grams

but gluing, or taping a couple of rare earth magnets to the underside of the BB should be enough to set the loop off.
Let me know how it works if you try it.
FoM