You are driving at 100 km/h on a road not inside a built-up area. In front of you a truck is travelling at 70 km/h. What is the minimum distance you must be from a crest in the road when commencing an overtaking manoeuvre?

Outside built-up areas, you may only start overtaking if you have a sufficiently long, clear view of the road ahead and can complete the manoeuvre without endangering anyone. Near a crest (the top of a hill), your view of oncoming traffic is limited, so German rules require a much larger “visibility reserve” before you pull out.

At 100 km/h while the truck is doing 70 km/h, your speed advantage is only 30 km/h. That means you need relatively long to pass, because you close the distance slowly. During that whole time you must have clear sight of the oncoming lane and enough space to return safely. The minimum distance from a crest given for this situation is 800 m, because anything shorter could mean an oncoming vehicle appears too late from behind the crest while you are still alongside the truck.

Rule of thumb for similar questions: the smaller the speed difference, the longer the overtaking takes, so the required clear distance/visibility (especially near crests and bends) becomes very large. If you cannot see far enough ahead, do not start overtaking.