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The government is . That's good news for the eight people who still die on our roads each day, but will it reduce CO₂ emissions, as ?

Sure, driving at lower speeds can burn less fuel and cut your car's carbon emissions on motorways, helping to tackle climate change. (, estimates that at 70mph you use roughly 10% more fuel than at 60mph, and about 15% more fuel than at 50mph.)

Why, then, are speed limits considered by some a green no-no? Firstly, some cars are more fuel-efficient at speeds of over 30mph, which is why reducing the speed limit to 20mph can actually boost emissions ().

And secondly, the speed bumps commonly used to enforce speed limits can increase CO₂ emissions by forcing drivers to brake and accelerate repeatedly - by as much as 50% on 30mph roads, reports the Times (, 2008). 'A car that achieves 58.15 miles per gallon travelling at a steady 30mph', transport correspondent Ben Webster writes, 'will deliver only 30.85mpg when going over humps'.

As Dudley Moore said, 'the best car safety device is a rear-view mirror with a cop in it'.

Follow up: ³ÉÈË¿ìÊÖ Bloom: Fuel-efficient driving

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