Was it a poilce speed camera van?

If the van actually said "police" on it, then it was probably radar of some sorts. If it was a DOT (Department of Transportation) van then it might have been a DOT checkpoint for trucks and other commercial vehicles. If it said nothing at all then it was probably a surveyor's truck. I really wouldn't worry too much about it. If it was a speed issue you probably would have been pulled over by a chase vehicle.