At least in PA, 80% of speed traps are on rural roads in the middle of nowhere at points where the speed limit suddenly, and for no fathomable reason, drops to somewhere between 25-35.  Most of these are run by the local Barney Fife PD's.  You will often see 5-6 cars pulled over all at once.  Of course they label these efforts with names like "Smooth Operator" and claim they have nothing to do with generating revenue but are all about public safety.   It is like anything else in politics, just follow the money trail.