Work on what will be the longest stretch of "smart motorway" in England has begun, with disruption expected until 2020.
The 32-mile section of the M4 between London and Theale in Berkshire is being upgraded by Highways England.
It said an average of 130,000 vehicles a day use this part of the motorway, which is "prone to congestion".
Highways England said the upgrade would provide "much needed capacity" as well as aiding economic growth in the area.
The work includes creating an additional lane for traffic by converting the hard shoulder permanently into a traffic lane.
Full carriageway closures on the motorway will be in place overnight throughout the project, which will cost up to £862.4m.
"The key aim remains to keep three lanes open during the peak travel times," Highways England said.
Safety concerns have been raised previously by the RAC over the lack of a hard shoulder, leaving motorists in a vulnerable position if there was a breakdown.
Highway England said "places of relative safety will be every 1.12 miles on average and no more than 1.6 miles apart".
The project will also see the introduction of more "technology on the road to smooth flows and manage incidents", it added.