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.
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.
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.