I know someone who works the camera vans and the threshold is surprisingly high. Over this 10% +2 quote. Different areas will maybe vary though.
the camera vans that sit on dual carriageways don’t pick up trucks sitting on the limiter
I’m not confident enough that the perth-dunblane cameras are the same tho so I sit at 50 on the GPS
There HAS to be a tolerance to allow for speedo inaccuracy, also to keep themselves right, who is to say the distance set between the cameras is measured accurate to the millimetre, and isn’t longer or shorter if you are in the left or right lane depending it the road curves left or right. I’ve been going through them at 1mph over on GPS and I doubt I’m going to hear anything of it.
In the Times this weekend : Drivers warned to stick to 70mph as motorway speed cameras arrive by stealth. Within the last 10 weeks 520 motorists have received tickets between junc 5/6 M25 for exceeding the 70mph limit. Kent Police refused to say the minimum speed at which they will take action. The margin of 10% + 2mph is a guideline and they use proportional discretion on whether or not to prosecute. Apparently on sections of the new “Smart” motorways new Hadtecs3 cameras have been installed and these are being used to rigidly enforce the limits. Now bearing in mind that it seems open season for any drivers as far as the police are concerned why should the cameras on the A9 be any different ?
norfolk:
why should the cameras on the A9 be any different ?
but if they cant read your numberplate they cant do you, i,m sure the local boys were going faster cos the cameras couldnt see them 
got to say the A9 has been a bit rough this week
chaversdad:
got to say the A9 has been a bit rough this week
A bit? I wanted to stop for a ■■■ n a cuppa last night n most of the laybys north of Pitlochry were full of 5ft high walls of snow !
Chris1207:
chaversdad:
got to say the A9 has been a bit rough this weekA bit?
Th A82 was just as bad trying to get up the hill yo the black mount car park north of Bridgr of orchy. Watched a police land rover pull an empty artic out of a ditch to get the road clear
Chris1207:
chaversdad:
got to say the A9 has been a bit rough this weekA bit?
I was up at the beginning of last week, coming back down, from 'Ness to just north of Perth, there wasn’t one layby I was going to risk going into… one just north of Inveralmond roundabout is the one I went into… Few minutes under my 6 hours… I hope 
this was the dual carriegway at Dalwhinnie on Thursday dinnertime, not much using the outside lane 
Here is a short video of the A9 dual carriageway just after Dalwhinnie heading to Perth at 18:20 on the 14/1/15