Speed camera convictions have virtually trebled in three years to nearly three million a year and police are taking a tougher line on motorists who are just over the limit, according to the Automobile Association.
The Exchequer, local authorities and police are making up to £180m a year from motorists who usually choose to pay the fixed penalty of £60.
Andrew Howard, head of road safety at the AA Foundation, said that some police forces were allowing less leeway for those who break the law. While the Association of Chief Police Officers suggested motorists might be allowed to drive 10 per cent over the limit plus two miles an hour, the margin was being lowered in some areas.
The conviction rate could increase further if there is widespread introduction of digital devices that operate all the time. It is estimated that of the 5,000 cameras installed only about 1,000 will contain film.
There is growing concern among motoring organisations that some limits are unrealistic and that cameras are increasingly used in relatively safe areas to raise revenue.
Mr Howard said evidence from a pilot scheme showed that cameras helped to reduce accidents. But at some stage he believed "diminishing returns'' would set in as devices spread from accident black spots. While the number of cameras was increasing, most of the worst areas had been covered.
In Northamptonshire, Lancashire and the Thames Valley police were offering law-breakers the option of "speed awareness'' courses rather than fines.
An AA spokesman said many local authorities had introduced limits which motorists thought were not "realistic, reasonable or necessary".
Research by the AA found that about 80 per cent of the public supported cameras. "But there is a growing minority of drivers who believe the expansion of speed cameras is solely to generate revenue from fines, not to prevent road users being killed and injured."
Steve Hounsham of the campaign group Transport 2000 thought the AA estimate of the number of convictions might be too high. "However, it seems clear that many motorists don't respect speed limits. We need to educate drivers about the dangers of speed. A third of road deaths are attributed to excessive speed.''
A spokesman for the Department for Transport said some of the revenue from fines paid for cameras and the rest went to the Exchequer. He said the Government hoped revenue would decline as motorists realised they could not get away with breaking the speed limit. He denied reports that the department was investigating the feasibility of placing satellite-tracking devices in all cars that could be used to catch drivers who break speed limits.