Calls for 20mph limits to reduce NOx emissions

Campaign groups are urging councils to lower speed limits to 20mph in urban areas in an attempt to reduce the quantity of NOx pollutants being emitted from diesel cars.

A study conducted by Imperial College London has suggests that the average Eoro 4 compliant diesel (between 1.4- and 2.0- litres) discharges an excess of 8.2 per cent of NOx at 30mph compared to at 20mph. This is calculated as 0.81g/km at 30mph as opposed to a less significant 0.74g/km at 20mph.

Figures show that in 2014, 37 per cent of cars on UK roads were diesels. Thus reducing the speed limits in urban areas could lead to a significant depletion in NOx emissions.

The campaign group 20’s Plenty for Us, said that a blanket reduction of speed limits is necessary to improve safety and enhance air quality. Rod King MBE, founder and director of the campaign, said: “A 20mph built up limit simply, immediately and effectively reduces dirty fumes. This government should urgently wake up to the air quality gains from 20mph."

He added: “Lower speeds give so many road safety, active travel and public health benefits.”

The research found that CO2 emissions from diesel cars have the potential to be reduced by 1 per cent if urban speed limits are decreased to 20mph. However, a reduced speed had the opposite effect on petrol cars, increasing CO2 emissions by around 2 per cent, which raises questions of whether the reduced limits would really make a substantial impact on air quality.

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