They damage cars and give drivers a nasty jolt, but now speed bumps have been found guilty of an even worse crime — they are bumping up C02 emissions as well.

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.”

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.”