Although in Germany there is no express speed limit on the highways, the lines where you can go 300 or more are quite limited and it is only feasible on certain days, since heavy traffic during the week prevents it anyway. On most highways there is a speed limit and it is usually 120 or 130, and these indications come in a red circle and often with radar control.
But if not...
https://piped.kavin.rocks/watch?v=7BWE2ftcF4Q
The Speedlimit on German Highways is only the one that is marked on traffic signs, if there are no signs, the limit is only what the car, traffic or common sense allows. It is also prohibited to go slower than 60 km/h on highways, if there is any reason for this, as it is also prohibited to go with vehicles that do not reach a minimum of 60 km/h.
This, in view of the high speeds of others, is also logical.
On the other hand, in general German drivers are quite disciplined and the police are very attentive to violators, high fines and a point system (with 15 points you can ride a bicycle), where there is a speed limit it is sure that it is controlled by radar and at rush hour there are even helicopters controlling traffic, also with patrol cars with civilian or police cars, all of them high-end (BMW, AUDI, Porsche).
Those with 300 or more are mostly rich foreigners "Highway Tourists" who want to experience it with their supercars (Japanese, English, and others) because it is the only place where it is allowed.