I've got a fork crown mounted headlight, fenders and a relatively skinny tire on 26" wheels. I have no troubles getting the bike onto a bus rack and mounted securely enough to handle potholed roads. That means the arm ends up correctly jammed right up against the fork crown, between my headlight and the head tube. Since none of the local buses take the interstate, I don't know for sure about at highway speeds.

On a 28 or 29" wheel bike, it *might* be more problematic. But that's not something I'd expect from how much clearance I have when loading my bike.