Is this a speed camera van or not?

I was driving on the M5 yesterday when I noticed a van with clear police markings parked on top of one of the bridges on the opposite side of the motorway. It appeared as though it could have been checking the speed of cars by their rear number plates. I did not see any tripod or police officers operating any equipment outside the van just noticed the black window open facing towards traffic moving away from it. Is this a speed camera van or not? and can it detect how far you are travelling from the opposite side of the motorway?