In the eyes of the law, even 0.1mph above the signed speed is not permissible. I find cameras are usually set to flash at somewhere around 10% above the limit. For instances where youre caught in person (camera van, police officer looking at you) they have to form a prior opinion you were speeding then gun you. The gun corroborates their opinion (much like the 2 photos from the camera corroborate the radar sensor's opinion of your speed) and makes you guilty. It's likely that operators in this context will only reliably be able to form an opinion if you're doing a good chunk above the limit. If I received a ticket from a camera van saying I was doing 31 in a 30 I would definitely be challenging it on grounds of prior opinion. If you're followed and the copper's opinion is of excess speed as corroborated by his calibrated speedo, then again, 10%+2 would probably see you with a *******ing or finger wagging. You could fail the attitude test at this point and get a ticket though - "You can't ever talk your way out of a ticket but you can certainly talk your way into one"
You're either guilty of speeding or not, so there isnt a requirement to say how much faster than the speed limit you were going. You can be done for excess speed simply by 2 coppers saying "we were walking the beat and saw car X, who I believed was exceeding the limit and this opinion was seconded by my beat partner"
For the guy who asked how the cameras know, it varies. Gatso send out radio waves that are reflected back at a different frequency depending on your car's speed. Outside a permitted frequency range and the camera flashes. It has been known that loose or moving surfaces (a motorcyclist's backpack flapping int he wind for example or air con fans on nearby buildings) can trip the camera accidentally. Gatso are capable of flashing people in either direction of travel but are only approved for rear facing shots. Stories you hear of the guy on the other side of the road getting done because he was speeding at the same time as a same-side driver speeding, are bogus.There are lines on both sides of the road to stop drivers avoiding prosecution by swerving over and doing their speeding in the other lane
Some cameras take only a single picture (truvelo) and can be forard facing. In this case the picture shows the car's front wheel either ahead of, within or behind 3 lines on the road, indicating a speed >10%, within 10% or <10% of the posted limit.The initial speed opinion is done by pressure sensors spaced accurately in the road
