if i was gordon brown i would send a fine to the astra driver and then one to you that way i will get double my money.
if i understand it correctly the camera takes a few pictures if there are lines on the road so that they can prove that the astra van drove over 2 of the lines , do a simply maths equation and work out his speed. if it is one of the newer cameras they fire a laser at the van and that tells them the speed.so for you turning it would show less than 30 and for the van it would show 35,36.37 or whatever he was doing hope that makes sense.
only joking about the gordon brown ?