Does the 10% rule still apply in relation to speeding?
ACPO Guidelines on speeding, and the UK law regarding prosecution: Vehicle construction and use regulations require a vehicle speedometer accuracy to be in the range of -0->+10%. The implications are that it must never under-read – for obvious reasons – but may over-read. As the cost of manufacturing a speedometer with -0% error would be very costly they all over-read by a few percent without exception. Even if speed is measured correctly the display may not be accurate, so a speedometer error is allowed. Because of this, the Association of Chief Police Officers (ACPO) have an official formula for calculating a speeding offence. It allows a leeway of 10% plus 2mph. In reality, most speed traps are triggered at higher speeds than this because if they were set bang-on those guidelines, the sheer amount of paperwork generated would overrun the police speeding departments.
