Can't imagine there would be that many vans around, but the ones i see regularly (especially Barrhead) is stickered up. (On the side where you won't see it 'til you're at it).
What was the back of the van like? Darkened windows? sliding window open?
I am pretty sure it was a scamera van. White transit van facing oncoming traffic, no markings on front or on the side that was nearest the road. I didn't get a chance to see the back of it as it was up against trees.Probably about a 55 plate or newer, so it seemed very out of place at the side of the road at 7:00pm. There was some sort of equipment pointing towards oncoming traffic, either behind the drivers or passenger seat - or possibly in the passenger seat. Looked like a camera anyway.
I was looking at info on scamera vans and the markings on them vary widely from clear to hardly any - http://www.speedcam.co.uk/van2.htm