Under UK law, speeding is a strict liability offence, which means police and the Crown Prosecution Service do not have to prove that a driver intended to speed or was aware they were doing so. If a vehicle exceeds the legal limit, the offence is committed.
Motorists have no legal entitlement to a margin of error. In principle, being caught at 31mph in a 30mph zone could technically result in a speeding fine. However, enforcement guidance from the National Police Chiefs' Council (NPCC) means action typically starts at 10% plus 2mph above the limit.
Police guidance on enforcement thresholds
London's Metropolitan Police explained in a freedom of information response: "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 also confirmed in a separate freedom of information request that it "applies the NPCC guidelines which outline the enforcement thresholds of not less than 10% plus 2mph".
Speed camera calibration and legal defences
The Met Police confirmed that speed cameras are generally calibrated to the same guidance, stating: "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."
Every enforcement camera is calibrated following manufacturers' and Home Office guidelines. Motorists could have a valid legal defence against a speeding penalty if they could demonstrate the equipment was defective. Drivers may ask for the calibration certificate for the particular camera or radar gun that was used.
