I thought that was VASCAR,it works as follows, you pass a set point and they start a stopwatch, they pass the same point and start the distance measure, you pass the second point and they stop a stopwatch, they pass the second point and stop the distance counter (all this in the VASCAR unit - alternatively they can pre-input a distance and just start and stop the clock over the distance ) they now have time over a distance and VASCAR crunches out the speed. 
  
I just wondered how this thing was operated.  They see you pass a bridge or start???? see you pass and stop????? 
  
Obviously parallax type errors and reaction times would make an impact.  Just my suspicious mind in operation perhaps.  If I was 2 mph over a ban limit and it hadn't been done with something very reliable I would be kicking and screaming all the way to court.....