20mph zones unlikely to improve air quality, leading scientist warns

“For the modelled Ford Focus Ecoboost, a cruising speed of 30mph, accounting for the acceleration phase, is only more efficient than a cruising speed of 20mph if the vehicle can drive for [0.3 miles] without stopping, which is unlikely under urban driving conditions.” 

Nevertheless, according to Nick Molden, founder and CEO of vehicle testing firm Emissions Analytics, who contributed to the report, the TRL considered 25mph to be the optimum cruising speed (assuming that it could be maintained). 

Meanwhile in England, Prime Minister Rishi Sunak is set to curb 20mph zones as he tackles anti motorist policies.

60 mph limits

In an effort to reduce emissions and improve air quality, National Highways is trialling 60mph limits on short sections of its network, including the M1, M4 and M6. 

The focus of its research is on learning what impact reducing the speed limit from 70mph to 60mph has on NO2 levels. The locations it has chosen have levels exceeding the legal annual limit. 

National Highways had intended to publish the results of the trials last spring but said the complexity of the data had caused a delay and that, in the meantime, the 60mph limits would remain in place. It said early indications are that the lower limit reduces emissions by 17%. 

According to a review of the UK government-owned organisation’s methodology undertaken in 2019, especially concerning Euro 6-compliant petrol cars, any lower limit (for instance, 50mph) shows “significant variability in NOx emissions”.

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