Donald Sensing discusses speed limits and, specifically, why the German Autobahn is much safer than the US interstate system (despite the fact Eisenhower modeled it after the Autobahn).
In addition to Don’s comments, with which I am in total agreement, having driven on the Autobahn myself (1989-92), I’d add:
The Autobahnen are designed for long distance driving and have relatively few exits in light of this. In the US, when driving through major cities the interstates are clogged with local traffic with exits every half mile or so. That is just idiotic.
Also, the Germans regulate when military convoys and large trucks can be on the road. Eighteen-wheelers shouldn’t be on the major roads during rush hour. They clog up the roads, block people’s line of sight, serve as moving road blocks because they can’t maintain momentum on hills, and are otherwise unwieldy and aggravating.