Posted speed limits in Mexico are ridiculously low. All drivers greatly exceed the posted speed limit on a daily basis which makes all drivers subject to arrest.
It's been my experience that Federal Police don't enforce speed all that much. I hate to see that change - in cities and towns speed enforcement is done for safety while rural (open road) speed enforcement is centered on revenue.
I've seen the same thing happen in Quebec; a culture that accepted fast driving will change with enough threat of penalty.
It's too bad; creeping along a multi lane highway at 36 miles per hour isn't my idea of fun.