As long as you have to intentionally modify the sign to cause confusion, as opposed to doing random vandalism, I don't think this is a problem at all. It's like a person spray painting extra loops to change a 30 into an 80 or sticking a home-made sign over a real one so it's indistinguishable to most drivers. People aren't going to do that any more than they already do and it'll still be just as illegal.