20mph speed limits could increase pollution at city hotspots – new modelling
Nicholas Thomas, local democracy reporter
The switch to 20mph default speed limits in Wales could end up increasing air pollution levels according to a new report.
Newport council’s Air Quality Action Plan sets out how pollution will be monitored and tackled across Newport over the next five years.
More electric buses, “green walls” of vegetation, and more education in schools are all among likely strategies to cut nitrogen dioxide levels across six of the city’s pollution hotspots.
But at a meeting on today (27 October) councillors also learned that modelling for the new 20mph limits “suggests detrimental impacts from a reduced speed of traffic speed” – although the report said this should be taken in the context of “the overriding road safety benefits” of lower speeds.
Steve Manning, the local authority’s chief scientific officer, acknowledged the council’s research had “thrown up a few notional increases” in air pollution levels when the lower speed limit was taken into account.
But he told the meeting of the overview and scrutiny management committee the “dichotomy” of modelling meant it was difficult to know for sure how 20mph might have an impact on pollution.
The council report also stated that “in reality it could likely lead to traffic calming, a smoother drive profile, and therefore less emissions”.
Committee member Matthew Evans questioned this.
“You’re saying the methodology [is suggesting 20mph] is going to make pollution worse, but you’re saying the reality is it won’t – how does that work?” Cllr Evans asked.
“Do any modelling, and you’ve got to try and reconcile what the modelling has thrown out at you… [with] what the real world situation might be,” Mr Manning said.
He gave the example of George Street, where modelling was done for road speeds at 30mph, but “we probably know, anecdotally, just walking around George Street, it rarely gets above 15mph between the two sets of traffic lights”.
He said the estimated impact of 20mph on pollution levels should be taken with an “open mind” because it’s “never a cut-and-dried scenario that’s been predicted”.
Noting wider research on the lower speed limit’s impact on air quality, Mr Manning said “in truth the jury is out whether [20mph] makes a difference, one way or the other”.
“The air quality aspects associated with 20mph, in my experience, are neutral if not negligible,” he added.
The Welsh Government introduced 20mph default speed limits on September 17 this year, primarily citing road safety reasons.
It said research had found lower speeds would “reduce collisions, save lives and reduce injuries”.
The government noted 20mph limits had been deemed “pollution neutral” in one study, adding that “many things contribute to pollution levels”.
But it claimed behaviour change could also end up improving air quality.
“We believe the lower speed limits will encourage more people to choose active ways to travel and there will be fewer polluting cars on the roads,” the government said when 20mph was introduced.
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