I've found those just cause people to brake hard before them and speed up after. If they become well known, they make people speed even more before/after, as it's not expected to have another speed control measure so close to a trap.
They also do this thing where there are two sets of cameras, and if your average speed is above the limit between two points your in trouble. At the moment they target heavy vehicles, but my guess its only a matter of time before they target all vehicles.