Not to disagree but worth noting 4/6 lane divided highways are engineered to sustain an 80 mph rate of travel. 65 mph is arbitrary, based in large part on fuel efficiency, and really too low for most highways. Which is why 70-75 speeds aren't usually punished. In fact, a large safety problem with too low limits is that something like 90% of drivers will travel at the safe rate of speed for any given highway, which is usually higher than the speed limit. Thus those travelling the posted rate are as dangerous to safe travel as the few exceeding the actual safe rate, if they don't stay in the right lane when available so as not to impede traffic. Doubling down on too low limits is a lost cause as you're fighting human nature and the actual design of the roads. 

Physical calming measures work in neighborhoods by impacting the physics. If we want highway speeds to match lower arbitrary limits we need to stop designing 80 mph highways.