They damage cars and give drivers a nasty jolt, but now speed bumps have been found guilty of an even worse crime — they are helping to destroy the planet.

The traffic-calming measures double the carbon dioxide emissions and fuel consumption by forcing drivers to brake and accelerate repeatedly, according to a study commissioned by the AA. A car that achieves 58.15 miles per gallon travelling at a steady 30mph will deliver only 30.85mpg when going over humps.

The AA employed an independent engineer who used a fuel flow meter to test the consumption of a small and a medium-sized car at Millbrook Proving Ground in Bedfordshire.

The results, calculated by averaging the performances of the two cars, also showed that reducing the speed limit from 30mph to 20mph resulted in 10 per cent higher emissions. This is because car engines are designed to be most efficient at speeds above 30mph.

A motorist who observed the speed limit on one mile of 20mph road during a daily journey would produce an extra tonne of CO2 in a year compared with driving at 30mph on the same stretch.

In an unusual move for a motoring organisation, the AA called for the introduction of cameras that detect average speeds to replace humps.

Edmund King, the AA’s president, said: “Humps are a crude, uncomfortable and noisy way of slowing people down and this research has shown they are also environmentally damaging. We accept that traffic speed needs to be controlled in residential areas where there is a problem with accidents and children are playing. We think motorists are more likely to accept average speed cameras than humps.”

But he added that drivers would not support a proposal in London by the Mayor, Ken Livingstone, to make 20mph the default speed limit on all residential roads. “The AA accepts that targeted 20mph speed limits in residential areas are popular and improve safety. However, a 30mph limit on local distributor roads may be more environmentally friendly.”

Previous research by the Transport Research Laboratory found that air pollution rose significantly on roads with humps. Carbon monoxide emissions increased by 82 per cent and nitrogen oxide by 37 per cent.

The London Ambulance Service has claimed that the 30,000 humps on the capital’s roads cause up to 500 deaths a year because its crews suffer delays in reaching victims of cardiac arrest.

Mr King said: “Humps tend to breed more humps. If one street has humps installed, the adjacent street calls for humps and eventually you find no clear roads for movement of emergency service vehicles.”

Transport for London has been helping to test average-speed cameras on residential roads in Camden, North London. No tickets are being issued yet, but the mere presence of the cameras has resulted in the proportion of drivers complying with the limit increasing by a third.

The new cameras are not linked but have synchronised clocks and each separately transmits information to a processing centre. This allows several cameras to work together without the need to dig up the road between them to lay cables. In urban areas this can halve the cost of installing the system.

Putting in 50 standard humps on three or four connecting residential streets costs about £150,000. A set of eight average-speed cameras covering the same area would cost £250,000.

The Home Office has been monitoring trials of average-speed cameras for almost three years but has yet to approve them. The camera suppliers believe that the delay is due to a lack of staff to complete the approval process.

Rob Gifford, director of the Parliamentary Advisory Council for Transport Safety, said: “If we remove road humps, the clear alternative method for enforcing lower speeds is through average speed cameras. These will smooth out traffic flow and be fairer to car drivers.”

Mr Gifford said that research had shown that 10 per cent of pedestrians would die when hit by a car at 20mph compared with 50 per cent at 30mph.