The proposal is part of the Welsh Government’s plan to protect public health and tackle roadside nitrogen dioxide (NO2) pollution.
The government had introduced temporary 50mph speed limits in June this year at five sections of the Motorway and Trunk Road Network but is now proposing to make this permanent along with the introduction of variable diversion routes in Newport.
It has placed a Direction on Cardiff and Caerphilly Councils to undertake an assessment by 30th June 2019 to identify the best option to achieve legal limits for NO2 “within the shortest possible time”.
Two local authorities have also received £20 million for projects aimed at achieving compliance for air quality levels.
The government will be publishing a Clean Air Plan for Wales next year, which will identify key pollutants, their effects on public health and the natural environment and the actions that need to be taken to improve air quality.
Environment Minister Hannah Blythyn said: “Everyone in Wales has the right to clean air. We are working actively, alongside the other UK nations, to transform our most polluted towns and cities into clean and healthy urban spaces.
“While the focus of this plan is to reduce concentrations of NO2 around roads where exceedances have been identified, we are developing a range of other measures to improve air quality. We are committed to making improvements to air quality in Wales through planning, infrastructure, regulation and health communication measures.”