I've driven thousands of kilometers on the German Autobahn. The safety issue is not so much the speed relative to the ground, but the speed relative to other drivers. If you're going 170kph in the left lane in your BMW, and grandma in her 1990 Volkswagen swerves left to overtake a truck, you've got pretty limited distance to slow down (at high speed). German autobahns are sensibly limited to 130kph in urban areas, for windy stretches of road, or two-lane portions.
French interstates work quite well in that respect. Everyone does about 130kph (that's 81mph), and this speed limit is strictly enforced. Relaxed driving, really.
The other thing to take into account is driver training. The kind of tail-gating I'm seeing here in the states rarely happens in Germany. Yes, you get the angry BMW driver flashing his lights at you when you're in "his" lane (road rage is universal), but that's typically over in a few seconds. Drivers wisely keep their distance.