It's been in the local press that they are installing average speed cameras on that stretch so that's what they''ll be - to the best of my knowledge they aren't up and running yet though.

Thanks both. What is the average percentage of over speed error allowed. ? Some seem to do over the stated speeds at various controlled areas.

It depends on the Police Force. Bear in mind it's the average between one camera post and the next one, so as long as you're average is below the threshold, you can go above the limt for part of the time, as long as you slow down otherwise. The problem is judging that, if you're going to. You'd have to know the distance between the camera posts and calculate on the fly how long it SHOULD take at the threshold (for a ticket) speed to go between them.

Try doing that if there are more than one set of cameras and/or they are not right next to the distance markers (little white posts on the verge). I'd rather be concentrating on the road.

Note sure if some of these cameras also have a spot speed function as well (like a traditional Gatso camera), but I'd never risk it. Note also that quite a few police forces are thinking of enforcing the EXACT speed limit, literally 0.1mph above and you're nicked. Lucky our speedos are designed to always read over by up to 10%. I'd also be wary of relying on a satnav speed nowadays to give an 'accurate' indication and to religiously go at the limit as the satnav says you're doing.

If police forces are to seek prosecutions for going over an exact speed then everyone will need to have a much more accurate and calibrated speedometer than they currently have.

A speedometer is calibrated to under-read by 10%, or whatever the figure happens to be, when it's brand new out of the box. I wouldn't like to even guess what the calibration was like after it had been installed in a vehicle for 10 years or 100,000miles.

Speedometers are never tested at an MOT test, only to test that they are fitted and they work. That would probably need to change, too.