Even in such a situation, it is almost always the driver's fault if he or she speeds. I routinely drive fast on roads where the posted speed limit is 30mph but could easily allow for faster speeds. I haven't been ticketed, but if I am, it doesn't matter what the design speed of the road is.
The times when a cop does tail me, I just set the cruise control to 30mph and watch as the "parade" backs down behind me.