If it is of any help , I fell foul of one of those vans on the same road ( between Stirling and Cumbernauld ) some years ago . In my case , I came over a hill crest to see a van on an overhead bridge straight in front of me , the distance was certainly much less than a mile . At the time these vans were still 'new technology' and I did not immediately recognise it for what it was ; when I got close enough to read the writing on the side and see the camera pointing out of the open window , I slowed down . Unfortunately , for me , I got a ticket for doing 82 mph .
At the time , I received the enquiry form asking who was driving etc. Being a disorganised sort of person , I managed to misplace the form , but I knew it originated from Central Police HQ in Stirling so I phoned them up and asked to speak to someone in the relevant department ; knowing that I was going to be in Stirling a couple of days later , I asked if I could call by and complete the paperwork ; I was told this would be fine . I duly turned up and was greeted at the door by a young lady who asked if I was there to view a video . On explaining why I was there , I was taken upstairs to an office where they had numerous computers and vcr's playing back traffic recordings on several monitors : I could see the operators shuttling the tapes back and forth and that recordings would show cars typically two or three hundred yards away with number plates clearly visible , time and speed readings also being shown on screen . There were recordings running from a number of different locations , but all were showing vehicles at a similar range . The recordings were clear enough , incidentally , that drivers could be recognised .
I also know from my own experience as a professional photographer and cameraman that , even when using very good tripods costing many thousands of pounds , it is very difficult to keep a camera steady enough to use the amount of magnification that would be neccessary to pick up a vehicle a mile away ( and even more difficult to read a number plate at that distance ) . At these distances even the vibration present in a bridge , or buffeting of the camera by wind would be significant - this would be with the tripod out of the vehicle and standing on the bridge - a tripod mounted in a vehicle subject to suspension movement and possibly with engine vibration if the operator was trying to keep warm would be even worse . It is unlikely that readings will be taken at more than a few hundred metres therefore .
I got the fixed penalty and the three points , now thankfully removed from my licence .