If anything, the Autobahn proves that the system works when everyone respects the rules and each other, all cars undergo regular roadworthy inspections, driver training is rigorous, and the road is designed for speed variation. In the US, none of that is the case.
Having rigorous enforcement of minimum radius in curves*, preserving more available cornering/braking traction and wide lanes, providing for better sightlines ahead (for faster traffic) and behind (for slower traffic contemplating pulling out to pass even slower traffic).
* - I did an internship for Mercedes in the early 90s and we had testing access to a section of ex-A8 near Stuttgart that was retired because it didn't meet the modern autobahn requirements and so had been replaced with a re-routed A8. To my American college-student mind that seemed incredibly wasteful, but it sure was convenient for our testing. I can't find it now on Google maps, but it's been 30 years so may have been demolished by now.
The Autobahn is much more challenging to drive on due to the extreme speed variation between trucks in the right lane going 80 kph and race car drivers in the left going 250.
IMO the only reason that it's acceptably safe is strict German driver's licensing requirements and fairly strong enforcement of traffic laws. If you had the same regime but American style speed variation, I think you'd see considerably safer roads in Germany.
