M4 upgrade work
M4 'smart motorway' upgrade work under way
Work on 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.
An average 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.