This article bakes in some false assumptions about compliance.

You can't just set a 40mph speedway on a limited access highway because "tHaT's WhAt'S sAfE gUyZ". Drivers will ignore it. Except that one guy with a dead hooker or brick of cocaine in the trunk or some other specific reason to be very cautious about complying with the law. He'll do 40. And safety will have been reduced because you have increased the variance in speed of traffic on the road.

There's a reason that current civil engineering doctrine is to set speed limits based on traffic speed and not the other way around.