The flashing of headlights can have so many different meanings:

1) I'm really angry with you, you just cut me up!
2) It's fine to pull out in front of me
3) Thankyou
4) I'm a lorry driver and so are you!
5) Do you realize that your boot is open?
6) There's a policeman with a radar gun up ahead

What is spooky is that you can nearly always tell what is meant from the context. It's like some weird oriental language with only one word, which means loads of different things depending on the way you say it.