By forcing drivers to brake and accelerate repeatedly, the traffic-calming measures can make a car that achieves 58.15mpg while travelling at a steady 30mph deliver only 30.85mpg.

The study also found lower speed limits can be bad for the environment - a 10 per cent rise in emissions results from a fall in speed from 30mph to 20mph. A motorist observing a 20mph speed limit during a daily journey on a one mile stretch of road would produce an extra tonne of CO2 in a year than a driver going 30mph on the same trip. AA president Edmund King spoke out against Ken Livingstone's proposal to make 20mph the default speed limit on all residential roads in London. As a result of its findings, the AA has called for speed cameras to take the place of road humps as traffic regulators.