A coworker got a warning this morning for doing 16 MPH over a 60 MPH speed limit. He was driving on the Dallas North Tollway at around 6:50 AM when this happened.
If the North Texas Tollway Authority bothered to follow commonly accepted speed zoning procedures, or even follow state law, then the speed limit would undoubtedly be the state legal maximum, 10 MPH higher than currently posted.
However, the state cannot create laws of science. Had the state not enacted an arbitrary speed limit cap, informal observations strongly suggest that the correct speed limit could easily be 75 MPH, perhaps higher. In that case my hapless coworker would have been going a screaming 1 MPH over the limit.