Fuel For Thought
20 mph roads pump up CO2 emissions by 10%
25 January 2008
Cutting the speed limit from 30 mph to 20 mph on the wrong roads can pump up CO2 emissions by more than 10 per cent1, AA research has discovered. The UK's leading motoring 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 (1.3 miles/litre). Over a year this will significantly increase CO2 emissions.
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 47 per cent when installed on 30 mph roads. Compared to a 20 mph road, speed humps along a 30 mph road increase fuel consumption by 41 per cent.
The AA accepts that targeted 20 mph speed limits in residential areas are popular and improve safety. Along shorter roads with junctions and roundabouts, limiting acceleration to up to 20 mph reduces fuel consumption. However, a 30 mph 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 by 10 per cent through blanket and badly-placed 20 mph restrictions.
"Transport and highways planners have little or no official guidance on the environmental impact of 20 mph 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 20 mph restrictions," says Edmund King, the AA's president.
"The Green Party has been advocating 20 mph limits across the whole of London, perhaps without realising that this policy would backfire in terms of environmental emissions. We need independent research to ascertain both the safety and environmental implications of 20 mph zones so that authorities don't make a huge and widespread environmental mistake. Researched guidance on 30 mph versus 20 mph limits versus speed humps will help road engineers to make informed decisions on where best to site lower speed restrictions on urban roads."
AA Fuel for Thought research results:
1 30mph/20mph/speed humps petrol car consumption figures
- Steady 30mph (4th gear):
60.7 mpg (small petrol car), 55.6 mpg (medium petrol car)
= 58.15 mpg (average) - Steady 20mph (3rd gear):
55.5 mpg (small petrol car), 49.1 mpg (medium petrol car)
= 52.3 mpg (average) - Speed humps (6 over 1/2 mile at 20-30 mph):
33.8 mpg (small petrol car), 27.9 mpg (medium petrol car)
= 30.85 mpg (average)
30mph/20mph/speed humps petrol car consumption differences
- Change 30 mph zone to 20 mph:
increases fuel consumption by 5.85 miles per gallon, or 10.1 per cent. - Add speed humps to a 30 mph zone:
increases fuel consumption by 27.3 miles per gallon, or 46.9 per cent.
Notes to editors
In 2000, the then Department of Environment, Transport and the Regions argued against reducing the 30 mph limit for fear of increasing emissions.
The AA's fuel consumption tests were carried out at Millbrook proving ground by an independent engineer and car tester, using a fuel flow meter.
Burning 1 litre of unleaded petrol produces 2.36kg of CO2
25 January 2008