We spoke to some of the guys that operate them, when they were at a car show, and he said that they can get your speed from a fair way away, but can't book you till you're very close (I can't remember the metres exactly, but it was close). His words were, you could be doing 180kms but as long as you slam on the brakes before you get close you'll be fine.
He basically said you have to be a dickhead to get done by one (ie. not paying any attention at all) cos most people keep an eye out and will spot them way before they can book you.
These are the vans I'm talking about. This was also a couple of years ago... so things may have changed.