It's 400 miles from Dallas to Amarillo. much of it on roads that are empty. Besides, the German Autobahen used to have no speed limit but there were not an extraordinary number of deaths. You have a point aboiut the trucks. But the big problem is their speeding in urban areas.
I saw a TV show not too long ago which pointed out that there are still no-speed-limit zones on the Autobahn. The Autobahn isn't one big no-speed-limit zone anymore, however.