You're talking about posted speeds. There are long stretches of other highways in the Austin area where, if traffic conditions allow, you can safely drive much faster than the posted speed limits. This is and has been true over much of the U.S. for decades.

One time I was driving on the NJ turnpike, passing a car at about 100mph, and far behind me, on my rearview mirror, I saw cars gaining rapidly on me and flashing their highbeams almost desperately. I barely managed to get in the right lane ahead of the car I was passing, and those cars that had been far behind passed me at what must have been 150mph. They were all beemers and such. Now, I wouldn't do that, but apparently that was a common sight at the time -- young drivers driving rich New Yorkers cars between NY and FL when their owners were flying to FL to spend the winter or back to NY for the summer. I've driven on quite a few highways in the U.S. where 80mph wasn't breaking a sweat and where I believe 100mph would have been perfectly safe weather and traffic allowing. I'm quite certain that our highways are simply designed for much higher speeds than posted.