A 16-month project will start on Tuesday to reconfigure and "increase capacity" of junction 28 of the M4 near Tredegar Park at Newport.
The adjacent roundabouts at Bassaleg on Forge Road and Pont Ebbw at Tredegar Park's entrance will also be upgraded.
The Welsh Government wants to "reduce delays" in the area.
Permanent traffic lights will be installed at the neighbouring roundabouts which will have "smart signalling" so the they will "be synced to ensure the smoothest and quickest traffic flow".
Two lanes will be built through the middle of junction 28 to allow traffic from the eastbound carriageway of the M4 to access the Southern Distributor Road (SDR) around Newport without going around the roundabout.
Improvements will also be made on the Pont Ebbw roundabout to ease traffic flow on the SDR.
There are fears the project, which is expected to be completed in July 2018, could increase congestion while the construction work is taking place.
But the Welsh Government said: "Existing junction traffic capacity will be maintained during peak hours - prior to 09:30 GMT and after 15:30 during weekdays - throughout the construction phase, albeit the lanes will be narrower in order to provide working room.
"Off-peak lane closures will be utilised between 09:30 and 15:30 to facilitate the works when traffic flows are lighter."
A regular congestion hotspot on the M4 is to get a £13.7m upgrade in a bid to ease rush hour traffic problems.