Except most speed limits are complete bullshit, at least in the US.
There are highways in NYC that have a 40mph minimum and a 45 mph maximum. Think that has anything to do with the ability to fine pretty much anyone at any time?
Roads have pretty natural speed limits regardless of the imposed limit, and it would be trivial to figure them out by simply taking an average over the course of a few days. I would venture a guess for the majority of the highways it would be substantially higher than the posted limit.
Remember - speed doesn't kill... a line of courteous drivers, observing correct leading distance and allowing free merges can probably go 100+ on a modern road in modern cars. Recklessness, carelessness, and needless maneuvers is what kills, not to mention distraction and intoxication. Observe Germany's autobahns for an example,