Germans have been flashing their headlights at cars traveling too slowly in the left lane for decades. It means move over to the right lane. No one seems to get road rage from it. At least, they didnt when I drove there. And as I recall there were no speed limits in the left lane. That must have changed.
The problem here in the US is when you are in the left lane, passing a line of cars in the middle lane, and then some jerk traveling 35 miles over the speed limit comes flying up in back and starts flashing his/her lights while riding your bumper. When I am done passing the cars that I am passing I will get over, but I have no obligation to rapidly squeeze into the traffic in the middle lane that I am passing.
I was TDY in Germany for a couple of months just after the Wall came down. A serious road hazard on the Autobahn was "easties" in their two-stroke Trabants, which were just barely capable of overtaking the big Turkish-registered long-haul trucks, which were then limited (I think) to 60 kilometers.
We'd be cruising at 150 in an Audi, and one of these things would suddenly swerve into the left lane. . . . needless to say it infuriated my driver, who would flash mercilessly while waiting until the last split second to brake. . . . . .