A reduction in speed from 30mph to 20mph increases CO2 emissions

According to the Highways Agency’s figures, at 30mph average CO2 emissions for vehicles stands at 188g/km, whereas at 20mph this rises to 221g/km. This is usually due to motorists driving in a lower gear than they would at 30mph.  This is demonstrated by the chart below.

Speed and CO2 Emissions


Source: Highways Agency, 2003

This graph represents emissions with free-flowing traffic. The emissions represented are an average for all vehicles. This is the latest study representing CO2 emissions for free-flowing roads.

The graph is only available for kilometres per hour.  To assist your use, see the conversions between kilometers per hour and miles per hour below.

Converting mph into kph

Converting kph into mph

5mph = 8.0467kph
10mph = 16.093kph
15mph = 24.140kph
20mph = 32.187kph
25mph = 40.234kph
30mph = 48.280kph
35mph = 56.327kph
40mph = 64.374kph


5kph = 3.1069mph
10kph = 6.2137mph
15kph = 9.3206mph
20kph = 12.427mph
25kph = 15.543mph
30kph = 18.641mph
35kph = 21.748mph
40kph = 24.855mph
45kph = 27.962mph
50kph = 31.069mph
55kph = 34.175mph

The AA also advise that cutting the speed limit from 30mph to 20mph 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.

They state that transport and highways planners have little or no official guidance on the environmental impact of 20mph speed limits and 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. 

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