There used to be a ten percent allowance plus one for speeding...ie..30mph zone you would not be do doing 33-34mph. Is this still in effect? Or was that not generating enough revenue?
Used to be 3 mph + 10% of the speed limit figure eg 30 + 3 + (10% of 30) 3 = 36 so you would need to be doing 37 to be nicked. In a 70 zone it would work out at 70 + 3 + (10% of 70) 7 = 80 so 81 would get you nicked. This info is taken from the 'Drivers Survival Handbook'. However, I suspect that cameras are being now being set either just over or on the limit to gererate more revenue from them. Get a TalexLite GPS camera detector - perfecltly legal and very discreet unit - best �70 I ever spent !!
According to the Association of Chief Police Officers, their guidelines are still 10% + 2. So in a 30 limit, a fixed penalty is isued at 35 (summons at 50). In a 70 limit, fixed penalty at 79 and summons at 96.
even if thats true you should be aware that youre speedo may still be slightly out and you could easily be doing 2/3mph more than it reads so you shouldnt really take the risk by driving at 35 in a 30 thinking you are safe from the speed cameras.
We have a GPS and it generally reads about 10% less than the speedometer on the car (the GPS is accurate), so this could account for some of the "allowance"
It also depends on the police officer who catches you and what kind of a day he's had!
Speedos are not allowed by law to tell you that you're going slower than you are (ie, it says 70 but you're actually doing 75), but they are allowed to be wrong the other way. Most speedos will therefore over-estimate your speed.
The 10% plus 2 is just guidance, not law, so you could be unlucky and get done doing 71 in a 70 limit if you're unlucky.