Cutting the speed limit from 30mph to 20mph on the wrong roads can pump up CO2 emissions by more than 10pc, AA research has discovered.

The organisation is now warning local authorities that some well-intentioned safety schemes may backfire in environmental terms.
On average, petrol car fuel consumption on longer and relatively free-flowing 20mph urban streets can worsen by 5.8 miles per gallon or 1.3 miles/litre. Over a year, a petrol car commuting along such a road could generate nearly a quarter of a tonne of additional CO2 every quarter of a mile.
In the first piece of research for its new Fuel for Thought campaign, updating widely-used test results from 1999, the AA argues for further detailed research into the environmental impact of 20mph zones - before the more widespread use of the new speed limit.
Even speed humps, although very popular with residents wanting to slow traffic in their street, pump up fuel consumption by 47pc when installed on 30mph roads. Compared to a 20mph road, speed humps along a 30mph road increase fuel consumption by 41pc.
The AA accepts that targeted 20mph speed limits in residential areas are popular and improve safety. Along shorter roads with junctions and roundabouts, limiting acceleration to up to 20mph reduces fuel consumption. However, a 30mph limit on local distributor roads may be more environmentally-friendly.
Some councils that are hitting owners of larger vehicles with considerably higher charges for parking and access may be guilty of hypocrisy if their policies increase CO2 emissions 10pc through blanket and badly-placed 20mph restrictions.
The AA’s research suggests that driving a Band G “gas guzzler” instead of a Band B “fuel miser” along a free-flowing 30-mph route may produce 11 times less extra global-warming gas than driving the same route at 20mph in the small car.
“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, that have targeted owners of larger vehicles with environmental charges, are found guilty of pumping up CO2 emissions through indiscriminate use of 20mph restrictions,” says Edmund King, the AA’s president.