All cameras are required to detect and record the speed of passing vehicles within specified degrees of accuracy. Until July 2003 the standard was a positive error of no more than 3mph up to 100mph, and 3 per cent. above 100mph. The negative error was not greater than 5mph or 10 per cent. above 50mph. From July 2003, in line with European standards, new speed cameras will be required to be accurate to ±2mph up to 66mph and ±3 per cent. above 66mph. This has not yet been implemented for existing cameras, but we consider that all cameras are in fact already accurate to this level.
The enforcement of speed limits on the basis of information provided by the cameras is an operational matter for chief officers of police. The Association of Chief Police Officers has published guidelines on speed enforcement thresholds to support a transparent and consistent approach, while recognising that not all speeding offences are the same. The police retain discretion to take account of the particular circumstances of any particular speeding incident.