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What are the "real" speed limits which are prosecuted by the police?
I live in a forty mile an hour zone but despite a speed camera we frequently see drivers who seem to be travelling much faster. Is the "real" speed limit (that at which police will prosecute) higher than that displayed on the signs?
Speeding is an absolute offence which means a driver travelling at 41mph outside your house is breaking the law.
But the reality is that any speed measuring equipment, whether a speed camera or a police-operated device, has a 'margin of error', and challenges made in court by drivers travelling close to the declared limit which call into question the absolute accuracy of the equipment reading would obviously have a higher chance of success.
Association of Chief Police Officer (ACPO) guidelines suggest prosecution at 10% plus 2mph of the speed limit, thus the "real" limits you allude to are:
| Speed Limit | Prosecuted speed |
| 20 mph | 24 mph |
| 30 mph | 35 mph |
| 40 mph | 46 mph |
| 50 mph | 57 mph |
| 60 mph | 68 mph |
| 70 mph | 79 mph |
Drivers relying on their speedometer to drive up to the "real" speed limit are extremely unwise: speedometers in "normal" vehicles are notoriously inaccurate whereas devices used by the police are (or should be) tested and calibrated on a regular basis.
Each police force in reality has its own (secret) policy, and in some areas prosecutions may not be brought even if the above ACPO-suggested limits are exceeded.
However the pressure within ACPO is to prosecute at lower speeds as increasingly accurate equipment comes on stream. The former Head of Road Policing within ACPO and current Chief Constable of North Wales, Richard Brunstrom, has called for the ACPO suggested figure in future to be the speed limit plus a straight 10%.
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