[Penguin]

I don't know whether this happens elsewhere, too, but in the UK people communicate with their lights while driving.

People often flash cars going the other way if they've just passed a speed trap and the cars going the other way are speeding.

If someone doesn't have their lights on and it's starting to get dark, other drivers will flash their lights to remind them to put them on.

People flash to show that they're giving way, although during the daytime most people will just wave their hand.