Flashing headlights on Autobahn

It usually means get your slow ass out of the way.

The main Autobahns have three lanes:

• Outer, for trucks. Nominal speed 100km/h.
• Middle, for normal driving. Nominal speed 160-180km/h.
• Inner lane, passing. Nominal speed: faster than you.

If you are driving in the inner lane and someone flashes their headlights at you, it means move over to the middle because they want to get past. If this happens regularly it's a hint to not drive in that lane, or speed up.

It is an official signal and has a name: it is called Lichthupe in German.

The German road-traffic regulations Straßenverkehrsordnung (StVO in short) mention this in two paragraphs:

• StVO § 5 (5) Outside of towns or villages a driver may warn oncoming traffic with the horn or the Lichthupe if he is going to overtake.
• StVO § 16 (1) A driver may warn others with acoustic or light signals
  • in case of StVO § 5 (5) and
  • in case of danger to warn others

A typical danger could be you see that a driver loses freight. You may not warn other drivers because of a traffic control checkpoint. ADAC (an automobile club in Germany) notes that the fee will be 10 EUR.

It is not allowed to reduce the safety distance between two cars. Reducing the distance and flashing lights will be interpreted as pushing and leads to severe punishment.

NB: It is allowed outside of towns to flash lights from a safe distance to "ask for permission" to overtake.

It can mean different things, depending on context:

• If you are both on the leftmost lane, especially if the other car is very close, and/or coming up quickly, they likely want you to change to he right lane so that they can pass
• If yo are on an entry lane, the car behind you on the lane left of you may flash it's headlights to indicate that you can change lanes
• It can also mean "check your lights"
• Last, but not least, it can be used to mean "attention" in a general sense.

As you can see, there are lots of different meanings, and it's not always clear what the other driver meant.

In many cases it simply means that the guy behind you is crazy thinks he doesn't have to care about speed limits and safety distance. If there are cars on the line right to yours and you are driving faster than these cars, just ignore the guy behind you.

As @Jonas Stein already mentioned its usually just a reminder that somebody is coming on from behind with a much higher speed then you and wants to overtake. They usually signal from a longer distance. This is for safety reasons so that you know somebody is going to pass although you are already are in the correct lane.

It is to prevent that a driver in the middle lane is going to overtake by going to the left while a car is coming from behind with much higher speed which is pretty dangerous! You don't really see that happen that much in Germany as drivers are used to high speed overtakes but in other countries - like the Netherlands - people just go left without proper mirroring.

You should first let cars pass and then move a lane if you want to overtake. It is very rude and very dangerous to just slam the car to the left as the driver from behind will need to break hard or even insanely hard if he is traveling at 250km/h which is not very uncommon on some highways.

Keep mirroring when you are driving reasonably fast at 180km/h and always move a lane to the right when possible as there is always a faster car.

Although the official rule is to move to the right most lane you usually see that card keep their lane in Germany unless they see a faster car approaching and then move a lane to the right when possible. If its not possible you usually see the car from behing overtake and keep his lane until someone approaching him from behind and them moves to the right very early to not hinder the car from behind.

Also, as people mentioned is that German drivers more often signal when they think something is wrong with your car like signals, lights, tyre pressure as all these can cause dangerous situations due to speed differences.