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."

The dispute began on Tuesday in Anaheim when an un-uniformed officer grabbed a 13-year-old boy walking across his lawn, accusing him of making a threat.
Footage of the incident shows two other youths shove and punch the officer, who pulls a pistol from his waistband.
About 300 people took to the streets of Anaheim on Wednesday night.
Several windows were smashed on homes during the rally, police said.
Some 24 people were arrested, including eight women and six juveniles, according to the Los Angeles Times.
Anaheim police told the newspaper the dispute arose from "ongoing issues" with young people walking across the officer's property.
During the ensuing argument, the LAPD officer detains the 13-year-old, whom he says threatened to shoot him.
The boy insists he only threatened to "sue" him, and accuses the officer of directing an obscene epithet at one of the girls in the group, which the man denies.
"Let me go," the boy says repeatedly.
During the stand-off, a group of young people stand by, telling the man to release him.
One of the young onlookers shoves the officer over a hedge and another throws a punch at him.
The man, still holding on to the 13-year-old, pulls a pistol and a single gunshot rings out, though it is not clear if he fired deliberately.
Police arrested two of the boys, but not the off-duty officer.
A 13-year-old faces charges of battery and making criminal threats.
A 15-year-old was detained on suspicion of assault and battery.
The off-duty officer has been placed on administrative leave, pending an inquiry.
An Anaheim woman, who claims to be the 13-year-old's mother, described the video on Facebook as "devastating" and "disturbing".
A regular congestion hotspot on the M4 is to get a £13.7m upgrade in a bid to ease rush hour traffic problems.
[NEXT_CONCEPT]
Hundreds of protesters have taken to the streets of a Los Angeles suburb after an off-duty policeman fired a gun during a fracas with teenagers.