The speed limit on some sections of motorway is being cut to 60mph in a bid to tackle illegal levels of pollution.
Highways England (HE) has said it will introduce temporary lower limits on at least nine stretches of motorway as part of a wider monitoring programme that takes in 101 roads.
The roads agency has identified 30 areas where average nitrogen dioxide levels exceed legal limits, with one section of the A3 near Guilford twice the permitted limit of 40 µg/m³.
To try to tackle this it has identified nine areas where the limit will be or has already been lowered and is exploring other measures to cut pollution in other locations.
Among the locations trialling lower limits are the M1 between junctions 34 and 33; the M6 from junction six to seven and the M602 from junction one to three. A stretch of the M5 between junctions one and two will also have the limit lowered as part of the trial.
HE says that reducing the speed limit by 10mph lowers emissions by 17 per cent and can accelerate efforts to bring affected areas within legal limits by one to two years. It argues that the trial zones cover a maximum of 4.5 miles, so will have a “negligible” impact on journey times.
Mike Wilson, chief highways engineer at HE, said: “Improving air quality and tacking air pollution are priorities for the government. Poor air quality is the largest environmental risk to public health in the UK, known to have severe effects on vulnerable groups.
“Highways England is committed to improving air quality in and around the strategic road network, and to supporting the aims of the government’s UK nitrogen dioxide plan.”
Three more locations with poor air quality - along the M4, A1 and M32 - already have lower limits in place due to road works, while the M621 at junctions six to seven has a permanent 50mph limit. These will also form part of the HE study.
HE says the measures are not permanent but will remain in place “until the results of the monitoring show that we can remove the restrictions and maintain clean air within the limit value”.
It hopes that a move to lower emissions vehicles will provide a longer-term solution but has not ruled out implementing lower limits in more locations if the trial proves successful.
Other measures being investigated include a 9.3-metre-high “air quality barrier” along the A3, and diverting heavy goods vehicles off the A500 from Nantwich to Stoke-on-Trent at peak times.