The whole idea of measuring the speed of moving object in any given moment is not an optimal solution. It is much better (and easier) in many cases to measure the time in which it took a car to travel some distance. You just set up ALPR cameras every few kilometers and record travel time. If the limit is 60km/h and you managed to drive 20km in 15 minutes, you were clearly over the limit at some point, and therefore you should be fined.

I think France uses this system. Also there was a huge uproar on the news of potential introduction of it in Poland. Mostly because it's so effective.

EDIT:

http://en.wikipedia.org/wiki/SPECS_%28speed_camera%29