Welsh government is also proposing clean air zones that charge drivers of older vehicles.
Welsh motorists will see temporary 50mph limits imposed on stretches of the M4 after the Welsh government announced a series of proposals to tackle air pollution.
The Welsh Minister for Environment, Hannah Blythyn, said the new speed limits would come into force within the next two months as the government attempts to tackle air pollution concerns.
The limits will be introduced on sections of the M4 near Newport and Port Talbot, as well as on the A494 in Deeside, the A483 near Wrexham and the A470 near Pontypridd.
According to the government, the measure is expected to reduce harmful nitrogen dioxide emissions in those areas by up to 18 percent.
The news comes as Blythyn announced a £20 million air quality fund for Wales and the launch of a consultation on proposed clean air zones, which would see older and more polluting vehicles charged for entering areas with poor air quality.
Blythyn made no mention of which cities could get clean air zones, although she confirmed that she would be encouraging local authorities to introduce the schemes, which would be similar to the T-Charge in London and the proposed clean air zone in Bath.
‘I am delighted to announce a package of measures to improve air quality in Wales, including a £20 million fund to help local authorities put arrangements in place to reduce air pollution in their areas,’ said Blythyn.
‘We will encourage local authorities to introduce Clean Air Zones, where evidence suggests they are needed to reduce harmful emissions, as well as launching a new website which allows people to check the air quality in their area.
‘I am confident the measures I am announcing today will help support the changes we need to make in Wales to deliver cleaner air.’
The RAC, however, has reacted with scepticism, claiming the case for reducing the speed limit is ‘questionable’.
RAC head of roads policy Nicholas Lyes said: ‘In principle, we are supportive of clean air zones where they target the most polluting vehicles that do the most miles in the most polluted areas.
‘Air quality clearly needs to be improved, but it’s questionable whether reducing the speed limit to 50mph on two stretches of the M4 that are regularly subject to major jams is likely to make a difference to harmful nitrogen dioxide levels. Slowing down traffic can help to improve flow, but if there are simply too many vehicles for the road space then jams are still likely to occur, so this is as much a question of capacity as anything else.’