The reason the Netherlands employs so many mobile speed traps is because people quickly learn where the fixed speed traps are and simply ignore the speed limits in between, or try to "make up for time lost by having to brake for a speed trap" (yes, I've heard and seen plenty of people do this).
This is why radar detectors are illegal in the Netherlands.
It always amuses me to hear local radio news about yet another multi-vehicle collision.
In Germany, the country known for it's wonderful Autobahn, you can temporarily loose your license for tailgating.
PS: I'm dutch.
The worst part of that is when overtaking ... a traditional Dutch driver will only look to see if he has the place to go, not if the car coming at the back is much faster than him/her.
So you end up with a lot of braking when driving on the left-most lane. The "funniest" is to see that happening with Dutch drivers outside the Netherlands ... because drivers from other countries are not used to that behavior :) (disclaimer : I live in NL, but I'm not Dutch and did not learn to drive in the Netherlands)
Besides, there are still quite a few fixed speed cameras in NL AND some "trajectcontrol" camera (as mentioned by a previous poster). I actually find the latter one the only one worth it, because then you are sure that people slow down for a longer period.