Lower speed limits unlikely to improve air quality

Just weeks after the Welsh government implemented 20mph speed limits on most residential roads, a leading scientist has told Autocar that the move is unlikely to improve air quality in Wales – one of the scheme’s primary objectives. 

Frank Kelly, a professor at Imperial College London and director of the Environmental Research Group at King’s College London, said: “With many urban roads already experiencing congestion during rush hours, speed is often already at 20mph or less, so I expect to see little further benefit to air quality in our towns and cities as a consequence of this new rule.” 

The Labour-led Senedd implemented the 20mph limit on 17 September, applying it to most roads that had 30mph limits.

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The move, which will cost £32 million, is intended to make roads safer and, by encouraging more people to walk or cycle, reduce the number of cars on them. The government has also said the new limit will reduce individual vehicle emissions, because less fuel will be used. 

“Accelerating to 30mph can take twice as much energy as speeding up to 20mph,” reasoned a spokesman. Putting to one side Kelly’s observation that vehicles on many urban roads are already restricted by congestion to 20mph, the government’s claim is supported by research carried out in 2022 by the Transport Research Laboratory (TRL) into the effects of urban traffic, speed limits and driving style on fuel efficiency and emissions. 

This found that the energy required to accelerate vehicles to the speed limit was the largest contributor to fuel consumption and emissions in urban traffic conditions. 

“Vehicles are often more efficient at higher speeds, but this doesn’t account for the energy required to reach these speeds,” claimed the report.