POLICE will be able to tell whether drivers who crash were speeding by checking the black boxes being fitted to thousands of cars by Britain’s biggest insurance company.

Norwich Union is installing satellite tracking devices to customers’ cars so that it can charge them an insurance fee for each mile they travel.

Rather than paying a fixed annual premium, drivers will pay different rates according to when and how far they travel.

Those who drive between midnight and 5am and from 7am to 10am will pay a higher rate because the company has calculated that crashes are more likely to happen during these periods.

The system uses satellite positioning to check a car’s location throughout its journey. The information will show how fast a vehicle was travelling before a crash and whether it was accelerating or decelerating. Police will also be able to check a motorist’s behaviour during an entire journey to see whether there is any evidence of speeding or aggressive driving.

The company, which expects to fit the boxes to 100,000 cars over the next year, said that the information would be given only to police investigating a crash and would not be used to issue speeding penalties.

Motorists will be able to request information about their driving to defend themselves against false accusations of speeding of dangerous driving.

Police have already requested information from Norwich Union about two crashes involving drivers taking part in trials of the pay-as-you-drive technology. In one case, police are understood to be checking claims that the driver, who was killed, was travelling too fast. Both cases are still under investigation.

A Norwich Union spokesman said: “We have to provide the data if the police request it but only if it is part of an investigation. It is only right for people who have been breaking the law and driving dangerously to be prosecuted. But we do not use the system ourselves to check speed and it is the insurer’s job to pay out on claims regardless of liability.”

The AA Motoring Trust said that some drivers would be suspicious. Andrew Howard, head of road safety, said: “Norwich Union must make it clear to drivers that information about their speed could be handed to police if they have a crash. Some won’t like the idea but it could improve safety because people are more likely to drive carefully if they know the system is looking over their shoulder.”

Rob Gifford, director of the Parliamentary Advisory Council for Transport Safety, said that the system would make it easier to prosecute dangerous drivers. Only one in nine road deaths results in a prosecution for causing death by dangerous driving. He said: “This should provide important corroborative evidence for police. It will be especially useful now that anti-lock brakes mean there are few if any skid marks from which to determine speed.”

Drivers who use the system will receive a bill itemising each journey. Travelling between 7am and 10am will cost 12p a mile on urban roads and 1.5p a mile on motorways. Between 10am and midnight and 5am and 7am the rates will be 7p and 1p. Norwich Union set the rates after studying 5,000 drivers in a two-year trial. It found that drivers in the morning weekday rush hour were 50 per cent more likely to be in a crash than those driving at weekends or in the evening.