I was to Germany several times in 1986 on business. There are parts of the Autobahn that do have speed limits. Urban areas (as in near downtowns) was about 100 km/h. Construction areas were about 80 km/h and those DID get enforced.
One of the construction zones was quite impressive. The pavement is about a meter thick. Rather substantial. One trip I had a bit of a treat. There was an exercise going on, where some RAF Jaguars were using part of the Autobahn as a dispersal airstrip and doing takeoffs and landings. This was about 150km from the Inner German Border.
It used to be assumed that the Autobahn was built in a manner which would allow it to be used as landing strips for the military. That wasn’t true but the American interstate highway system was built with that in mind.