Another signal on the autobahn in Germany is when a car is trying to pass you in the left lane is for them to turn on their left blinker. If you are also in the left lane and high beams flash several kilometers behind you, it's time to find a place to hide, because they are coming fast. Usually it's a Mercedes, with a BMW or two drafting behind. If you don't comply they have permission to take you into the woods and teach you a few things.
In WA state, flashing your high beams is considered aggressive driving, and you can be ticketed.