New research from the AA claims introducing speed humps on some roads can actually damage the environment.
The motoring organisation also says slashing speed limits from 30 to 20 miles per hour (mph) can also increase CO2 emissions by 10%.
The research found that the average car achieves 58.15 miles per gallon (mpg) at a steady speed of 30mph.
But if you introduce humps, the fuel consumption drops to 30.85mpg.
Simply cutting a speed limit from 30 to 20mph would also increase an average car’s fuel consumption by 5.8mpg.
The AA argues there needs to be further research into the environmental impact of 20mph zones before they become more widespread.
AA President Edmund King said: ‘Transport and highways planners have little or no official guidance on the environmental impact of 20mph speed limits.
‘It would be a bitter and unpalatable irony if local authorities, which have targeted owners of larger vehicles with environmental charges, are found guilty of pumping up CO2 emissions through indiscriminate use of 20 mph restrictions.’
He added: ‘We need independent research to ascertain both the safety and environmental implications of 20mph zones so that authorities don’t make a huge and widespread environmental mistake.’
Which? motoring editor Richard Headland said: ‘We agree there’s a need for some further research in this area, as although 20 mph areas appear to save lives – especially in urban accident blackspots – it’s true that most cars will use more fuel at 20 mph instead of 30 mph.
‘That said, this research suggests that 20mph zones could be far less environmentally damaging than more widespread use of speed humps – which are also accused of increasing wear on a cars brakes and suspension.’