Bill Roper's Journal

Another theory: It's generally understood that "Speed limit n" really means "Speed limit n+10". The cops don't ticket people driving in this range. The legislatures don't want to change the legal limits to match the de facto limits, because that would just create a new, higher set of de facto limits. So anyone who is driving the de jure limit can be blocking people who want to drive up to the de facto limit.

Yes, it's crazy, but it's consistent craziness.