Almost every trap names "Abstand" (which means distance to the car in front of you) in Germany is wrong. I never saw a stationary distance trap on any german motorway at all.

What is really marked are devices which are used for the german toll system for trucks. You can see such a device in the following picture:

http://zelos.zeit.de/bilder/online_aktuell/2005/15/maut_320x280.jpg

Sorry, i rechecked some of the Marks, probably they mark bridges from where the police COULD install a mobile distance trap.
Sorry for my mistake