The findings come after the Government launched a Net Zero Transport Board last week to help steer a green recovery from Coronavirus that takes into account the imperative to decarbonise transport.

Data analysed by the University of York shows that, at the height of lockdown, roadside nitrogen dioxide levels were 56% lower than normal across the UK. But by 1 July, despite many restrictions having lifted and vehicle miles travelled approaching normal levels, a 30% reduction in the pollutant was still being seen.

“Things are not back to normal according to the air quality data,” said Dr David Carslaw of the University’s Department of Chemistry. “It seems that while traffic levels look like they have mostly returned to normal, concentrations of some air pollutants are still quite a lot lower than expected.

“We think the reason is that congestion has not fully returned, and this has quite a large effect on emissions and hence concentrations.

“The analysis of data gathered during lockdown and coming out of lockdown might give researchers an opportunity to better understand more about linkages between air pollution and congestion,” he added.

Government decision making and action to ensure a green recovery from Covid-19 when it comes to transport will be informed by a new Net Zero Transport Board, which met for the first time last week.

The board brings together industry leaders and specialists from a range of fields –including behavioural change and policy – and will also help shape a transport decarbonisation plan set to launch later this year.

Transport Minister Rachel Maclean said: “The Coronavirus pandemic has led to people making huge changes to the way they live, work and travel, and we want to capitalise and build on the new habits people have developed, as well as champion the power of new, cleaner technologies on our road to a green recovery.”

However Government has also faced fresh criticism this week over the climate impact of its second Road Investment Strategy, which will see £27Bn spent on motorways and major A roads over the next five years.

Research by think tank Transport for Quality of Life claims that the programme will add 20 million tonnes of carbon dioxide to UK emissions between now and 2032. Even with the Government’s most optimistic estimate for electric vehicle adoption, emissions from the strategic road network are not on track to meet ‘net zero’ by 2050, it adds.

“If we are to meet the legally binding carbon budgets that have been set by the Committee on Climate Change, we need to make big cuts in carbon emissions over the next decade,” said the report’s lead author Lynn Sloman. “At a time when we need to cut emissions from the strategic road network, the Road Investment Strategy will increase them.”

Also this week, The Welsh Parliament’s Economy, Infrastructure and Skills Committee has said Wales “cannot afford” to miss the opportunity to drive decarbonisation in the country’s Covid-19 recovery.

It urges the Welsh Government to align any recovery funding with its longer term climate change targets. Committee chair Russell George said: “We believe the Government’s post-Covid recovery plans present an opportunity to drive growth and a culture shift towards greener, more sustainable transport.”