Warning: This post contains flashing lights

You recognise a friend. It is safe for the car that is overtaking you to return to their lane. The oncoming car has forgotten to switch their main beam off. Another car hasn't realised they are driving in the dark without lights. You want the car ahead of you to speed up or change lanes. You want to tell the driver of the car ahead that you are about to overtake. You are telling another car that you will give way to them. You are telling the pedestrian they can cross the road. You are warning other drivers of a hazard. You are telling abother driver that there is something wrong with their car. You are celebrating. You are warning other road users of your presence. You are warning other drivers of a police speed trap. Have you ever flashed your headlights on any of these occasions? When other drivers have flashed their lights at you did you always know what they meant?

A couple of years ago, my pupil (in the blue car) was approaching the situation in this diagram. Parked cars on both sides of the road meant that there was not quite enough room for us and the oncoming red car to pass safely. My pupil slowed and watched to see what the red car would do. He slowed to walking speed and flashed his headlights. We took this to mean that he was giving way to us. Unfortunately, the drivers of both the yellow car and the green car thought the same thing and we all moved forward. Who were the flashing lights for? Good question - only the driver of the red car knew for sure. I use this example a lot with my pupils to demonstrate the confusion that can be caused by flashing headlights. In this example, there was nothing wrong with the red car giving way to us (we had equal priority), but by simply slowing down he would have made his intentions clear. Of course, it is possible that the yellow and green cars may still have thought the red car was giving way to them, but I hope they would have been a bit more cautious about emerging at the crossroads.
As far as The Highway Code is concerned, it states:
"110 - Flashing headlights. Only flash your headlights to let other road users know that you are there. Do not flash your headlights to convey any other message or intimidate other road users."
So, flashing your headlights should only be to warn others of your presence. In fact, in many other countries (such as France), a driver flashing his/her headlights at you is warning you that they have priority and you should give way.
The Highway Code also states:
"111 - Never assume that flashing headlights is a signal inviting you to proceed. Use your own judgement and proceed carefully."
I usually find that, when my pupil is waiting at a T-junction or crossroads and an oncoming car flashes them (presumably telling them to go) it puts pressure on them to go, causing them to rush and (especially in a petrol car) stall. I know and my pupils know that the other driver is trying to be nice, but we would much rather you didn't (unless traffic has stopped or is barely moving). Giving way when you have priority may be courteous to the other driver, but it is not courteous to the traffic behind you which is now having to slow down or stop unnecessarily.

You may have seen (or heard) about Mr Michael Thompson, who has been convicted of 'perverting the course of justice' (see this link) and fined £175 (plus £265 costs) after he was caught flashing his headlights to warn oncoming drivers about a police speed trap ahead. His defence was that, by warning other motorists, he was encouraging them to drive safely by warning them to slow down. On TV and radio there have been 'experts' defending Mr Thompson, repeating his claims that he was acting in the interest of safety. But I am not convinced. Does Mr Thompson flash his lights to warn other motorists about a sharp bend he has just passed? Does he flash his lights to warn oncoming motorists to slow down because they are approaching a pedestrian crossing? And how about schools? cyclists? junctions? If Mr Thompson flashes his lights everytime he thinks oncoming motorists should slow down (in the interest of safety, of course) his car would look like a strobe light and his left hand would have RSI.
And ask yourself the following question: If someone flashed their lights to warn you of a police speed trap (assuming you correctly guessed that that is why you were being flashed), would you
a - Slow down because you realised that driving above the speed limit is potentially dangerous?
or
b - Slow down because you didn't want to get caught, fined and penalised, but speed up again once you were past the police?

Maybe car manufacturers should install an extra light that, when flashed in response, means "I can see you are flashing your lights, but are you flashing them at me or someone else? And what the hell are you telling me by flashing your lights?. Perhaps you should just concentrate on your own driving and not worry about mine. Thanks. Have a nice day."