Drivers often claim they will not be fined for exceeding the speed limit by 10% or less, but police have clarified the reality. Under UK law, speeding is a strict liability offence, meaning police and the Crown Prosecution Service do not need to prove intent. If a vehicle travels above the legal limit, the offence is complete, and drivers have no legal right to a margin of error. In theory, a driver caught at 31mph in a 30mph zone could be fined.
NPCC guidance on enforcement thresholds
In practice, police forces do not apply the law strictly. London's Metropolitan Police, in a freedom of information response, stated: "The Met observes the current National Police Chiefs’ Council [NPCC] guidelines which states that enforcement action should commence as soon as a speed is at +10% plus 2mph. This is normally the minimum speed at which enforcement is undertaken in all speed limits. NPCC guidance also states that whilst tolerances exist, they do not and cannot replace police officers’ discretion."
North Wales Police, in a separate freedom of information request, said it "applies the NPCC guidelines which outline the enforcement thresholds of not less than 10% plus 2mph". Due to the Welsh Government's widespread 20mph speed limit on residential roads, North Wales Police added: "Following the change in the default speed limit, chief police officers allowed us to increase to 10% plus 4mph in 20mph limits only, meaning we start to prosecute at 26mph in a 20mph limit".
Speed camera calibration and legal defences
Official documents suggest this tolerance is built into infrastructure and local government strategy. A Bedford Borough Council document on speed camera policy said there would need to be "clear evidence that... a significant number of motorists are regularly exceeding the posted speed limit", but it considered the limit to be 10% plus 2mph above the signed speed.
Regarding speed cameras, the Met Police said: "The minimum speed at which a speed camera is operated may be varied at any time but there are no plans to use a threshold that is not contained in the current guidance." All enforcement cameras are calibrated according to manufacturers and Home Office guidelines, and drivers could have a legal defence if they could prove equipment was faulty. Drivers can request the calibration certificate for the specific camera or radar gun used.
