Absolutely nothing! I'll slow down, make eye contact and maybe even nod (I do this on the bike and no-one sits there wondering what I am doing) but flashing the headlights will be seen by everyone and because so many people don't know how to drive/ride safely, many see it as a signal to them that means 'go on it's safe'! As such, the flashing of the headlights isn't the problem. The problem is the way they are recieved and then acted upon. If EVERYONE used these signal, including hazard lights, in the way they are supposed to be used we would have a lot less accidents. The problems occur when people decide to change the rules and meaning of signals to suit themselves. As I said, even the police flash the headlights, and they should know better, so the safest way is go back to basics and use them correctly.