The permanent 50mph limit with average speed enforcement on the M4 near Newport will go live this month.
The speed restriction between junctions 24 and 28 forms part of the Welsh Government’s approach to tackling congestion in the area following the work carried out by the South East Wales Traffic Commission under Lord Burns into alternatives to a new route for the motorway.
Work will be undertaken over the next two weeks to finalise the installation of the cameras and road signs, with the new mandatory limit due to go live by Monday 15 March.
The Welsh Government said the move is intended to help manage congestion issues, improve journey times and provide smoother traffic flows, reduce the risk of accidents, and improve air quality.
Ken Skates, minister for economy, transport and North Wales, said: ‘This work shows this Government’s commitment to tackling congestion in the region. We are doing this in the context of the climate emergency and our clear objective of reducing emissions from the transport sector.
‘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.’
The 50mph speed limit will be monitored by Go Safe, the Welsh Road Casualty reduction partnership, during the bedding-in period to allow for full systems and enforcement checks to take place and ‘give motorists time to get used to the new speed limit’.
Notices of prosecution will begin to be issued in the summer, officials said.
The works will see night time full closures of the eastbound carriageway this week, followed by westbound full closures from 8pm to 6am next week.