This can be intentional. If there is a dangerous piece of road where the average speed of traffic is higher than the road will take, then put a trap in to slow the traffic down to the limit. It doesn't have to be over the entire highway system to have an effect on accidents.

For example, in NZ, there is a speed camera half-way down Ngauranga gorge in Wellington - a place where there is a blind turn down a steep slope and where the limit goes from 100kph down to 80kph.

http://en.wikipedia.org/wiki/Ngauranga_Gorge