You're being obtuse just for the sake of arguing. Anyone who knows even 1 traffic cop can tell you that they have, what they call, "honey holes" or "fishing holes". These are areas usually at the bottom of a hill, coming onto or off of a highway, or in places where its unclear if you've transitioned to or from a highway or normal city street.
There is no magical perfect science figuring out speed limits. They are always either 30, 50, 80, 100, or 110km/h in my area. There are many areas of my city where 50 is too slow, but 80 is probably too fast... so the limit defaults to 50. And traffic police will routinely sit at these "fishing holes" and pull over anyone missing that mark by a mere 10km/h (the lowest deviation they can write a ticket for). They also always seem to be set up at the end of the month too. What a coincidence.