As a Canadian, I can vouch that that happens here, too. As an added bonus, these people seem to be the same ones who think that signal lights are optional 90% of the time, everytime.
I dunno.... I think that if someone feels they have to flash another driver there’s an issue usually. If a car suddenly appears behind you that wasn’t there before, or is getting closer and closer very fast, it’s obvious they are going faster. If you let them go around you an both just go on with your lives as if you had never met. If they get “stuck” behind you though, or you intentionally keep them there or figure “they can go around if they want”- they might get increasingly upset and speed+anger is more dangerous than speed. It doesn’t matter if you’re going 30 and they 40, or you’re going 160(mph) and they 220. It’s up to the police to slow them down, trying to do so yourself can put yourself and those around you in as much or more danger as just letting them go. Slow traffic to the right, fast traffic to the left, slow and fast are relative to the cars around you, and if you aren’t sure what lane to be in and are alone stay to the middle.
Like anthracite says, there rarely is a speed limit on German highways. You are legally allowed to overtake police cars if you do it safely even if you're going 250. And it is common courtesy to clear the left lane if you see people driving a lot faster than you or at least that's what gets taught in driving class.
It’s simple. Don’t get in other people’s way, and get out of the way as quickly as possible when you are. If something you do on your own requires someone else to have to react, you’re driving wrong. If you can pull out in front without them having to slow down or move, and there’s enough space for safety if you both suddenly need to stop, do it. If they would need to slow down, wait for them to pass since they’ll pass you any way.
Sadly, common courtesy, much like common sense, isn't common at all.