I think the solution to this was to move from the inductive loops to cameras which detect motion (or a change of just the asphalt, or *something* -- not sure what). I can't say I've tested this, but I do think they're a lot more reliable than the loops (which I noticed would fail to pick up even a car properly sometimes). Around here (PA/NJ) it seems like they've been aggressively switching to the cameras over the last few years.