Police in Thames Valley are making so much money from speeding drivers that they cannot spend it all.
Roads with high numbers of injury accidents are targets for cameras
Up to £3m in speeding fines may have to be returned to the government this year because a local road safety group has already covered its costs.
But campaigners have defended the scheme, saying the money would stop rolling in if motorists stopped breaking the law.
Chris Scoxton, from Thames Valley Police Safer Roads Partnership said: "There is a probability that Thames Valley Police will return to the government about £3m this year.
"The partnership is allowed to bid for money for the operation of the safety cameras scheme but we have no idea what the fines will be.
"If our fines are in excess that goes back to the Treasury and we have no say in what's done with it."
A national newspaper has labelled one of the team's speed cameras "Britain's most profitable" after IT reportedly earned £25,000 a week.
The Sun reported the Gatso camera, which is hidden in the back of a van, had a success rate of catching one motorist every 95 seconds.
It is operated in two roads in the centre of Reading, Berkshire - Vastern Road and King's Road.
The Sun said the camera caught 152 drivers breaking the 30mph limit in just three hours 40 minutes.
Kate Smith from Thames Valley Police said the figures were recorded during a few hours.
Fall in deaths and injury
"The camera is not watching every day of every week so it does not have an income of £25,000 a week," she said.
"The camera is there as often as it needs to be. It wouldn't be very profitable if everyone stuck to the speed limit."
She said over the past three years, 25 people have been injured on Vastern Road, one of them seriously.
Most accidents were speed related.
In areas where speed cameras are used the number of deaths or serious injuries on the roads has fallen by one person a week, according to Mr Scoxton.
"We don't need to be on this road from a finance point of view because there comes a point when every single penny goes to the government," he said.
"We are trying to change driver behaviour in general not just on this quarter mile of road but by doing something there we hope it will affect their behaviour across the whole region."