20mph roads and CO2 emissions

Lower limits can increase fuel consumption and CO2 emissions

road sign showing start of 20mph zone Cutting the speed limit from 30 mph to 20 mph on the wrong roads can increase CO2 emissions by more than 10% with the result that 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 – burning 1 litre of unleaded petrol produces 2.36kg of CO2.

sign warning of road humps ahead Speed humps - popular with residents wanting to slow traffic in their street - pump up fuel consumption by 47% when installed on 30 mph roads. Compared to a 20 mph road, speed humps along a 30 mph road increase fuel consumption by 41%.

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. But on local distributor roads a 30 mph limit may be more environmentally-friendly.

sign warning of road humps ahead Transport and highways planners have little or no official guidance on the environmental impact of 20 mph speed limits. It would be ironic if local authorities that have targeted owners of larger vehicles with environmental charges, are found guilty of increasing CO2 emissions through indiscriminate use of 20 mph restrictions.

In the past the Green Party advocated 20 mph limits across the whole of London, perhaps without realising that this policy would backfire in terms of environmental emissions.

It is important to ascertain both the safety and environmental implications of 20 mph zones.

Fuel consumption figures

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.

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.

In 2000, the then Department of Environment, Transport and the Regions argued against reducing the 30 mph limit for fear of increasing emissions.


25 January 2008