Most every driver knows that the effective highway speed limit is 10 mph over the posted limit as they won't ticket you up to that point. But it is the variance in speed that creates the greatest danger. When the speed limit is 70 the majority are going faster. The 'speeders' are exceeding 80, the expedient are going 78, the law abiders are right on 70, and the clueless are puttering along at 64. It would be safer if everyone were traveling the same speed, whether that was 70 or 80. And when we get to the point where the speed of all vehicles is remotely controlled, that speed could reasonably be 80. But that's likely decades away.
If I'm not on city streets I'm always using cruise control so I'm quite aware of the speed at which I'm traveling.