I think Texas got some special dispensation for a west Texas two lane US highway.
Trying to raise the speed limit on the West Texas interstates to 80 mph from 70 mph. The two-lane state and federal highways in the area are already 75 mph so you can go faster on the two lane roads. I travel them all the time -- set speed control to 80 and try to listen to talk radio. Two lane roads in other parts of Texas are 70 mph.