A stretch of the M4 outside Newport is set to be restricted by a permanent 50mph speed limit by mid-March in a bid to tackle climate change.
Approximately seven miles of motorway between junctions 24 and 28 will be subject to average speed limit checks when the enforcement cameras go live by 15 March.
It forms part of plans to reduce congestion and the risk of accidents - both of which are particularly problematic in that area, according to the Welsh Government.
In 2018, speed limits were introduced as a trial along several stretches busy Welsh roads - including the M4 outside Newport - to cut nitrogen dioxide levels.
Emissions were found to have dropped at the five traffic hotspots in and around Newport, Deeside, Wrexham, Port Talbot and Pontypridd.
Installation of the cameras has already begun, with full night-time closures of the eastbound carriageway starting on 1 March.
Full west bound closures will begin from Monday between 8pm and 6am.
There will be a bedding-in period allowing motorists time to get used to the new speed limit, and for full systems and enforcement checks to take place.
Speeding fines will be issued from the summer.
The change is intended to:
Help manage congestion issues along this stretch of the M4
Improve journey times and provide smoother traffic flows
Reduce the risk of accidents
Improve air quality by reducing emission levels
Economy and transport minister Ken Skates said the work is part of Welsh Government plans to tackle the climate emergency.
He said: "The South East Wales Transport Commission produced a number of recommendations to tackle congestion which we were happy to endorse.
"Transport for Wales’ Development Unit is now tasked with supporting us to take forward Lord Burns’ wider recommendations forward and we will continue to work with partners to improve journeys in the area."
South Wales Police have endorsed the move, following "a number of collisions over the years" along that stretch of the motorway.
Assistant Chief Constable Mark Travis said: "I welcome the introduction of a permanent reduced limit and corresponding Average Speed Camera scheme to ensure compliance on this section of the M4 and to reduce death on Welsh roads.
"The Welsh Government and four Welsh police forces are committed to working in partnership to maximise the amount of safe and efficient journeys our residents and visitors make on our roads.
"Safe roads are vital to the wellbeing of our communities, in particular young people, it is right that we make our communities as safe as they can be."