No there are other in car systems, the WHITE SQUARES you see painted on the road are for following checks of suspected speeders.
Remember that the speed check must FOLLOW a SUSPICION of speeding cases have been thown out where this was the prime evidence and where no suspicion could have been formed as to speed due to raod etc.
You could also use the PACE clause in that if you receive a NIP you plead guilty then point out that PACE has been breached in that you were not cautioned before being asked for a reply, look up issues in MCN as they carry a lot of features about this and by the god of men MR LOOPHOLE.
if you have not been stopped at the time it is highly unlikely you will herar anything as they are too far away to read reg sat up a slip road, as stated it is done as a car passes the markers, which have to have a set distance, hence the square boxes, so if you see these be on your guard.
The speed recording equiptment on camera vans has alos been called into question over it's accuracy lately, and cases have been thrown out.
tabetha