The £25.8million project will see 2km of single-carriageway widened to accommodate an extra lane in both directions.
Starting in the middle of last year, the scheme will see improvements between the new Fitzalan link road in Littlehampton and the Body Shop roundabout, and between the Station Road junction in Angmering and the A280 roundabout.
A spokesman for West Sussex County Council said work is ‘progressing well’.
“We would like to thank the public for their patience while the works are ongoing,” said the spokesman.
“Once completed, the improvements will reduce traffic congestion by increasing road capacity and so cut both journey times and pollution.
“Works have been progressing well since mid-2020 when site clearance and archaeological investigations recommenced.
“Despite some challenging weather over the winter months, progress has been made on the initial earthworks, which are required for new utility diversions and the storm drainage network.
“Once the utility diversions are complete – currently scheduled for November, 2021 – the old services can be removed and the new carriageway construction can begin in full.
“With one half of the new dual-carriageway complete, road users and pedestrians will be moved over onto the new section of carriageway while the contractors will take full possession of the opposite side of the road and start to complete some minor, outstanding utility and drainage works.
“Then the old carriageway will be removed to make way for the new carriageway construction on that side.”
The mammoth task, which involved controversially clearing large amounts of vegetation from the roadside, is scheduled for completion by the autumn of 2022.
It has been jointly funded by the Coast to Capital Local Enterprise Partnership, West Sussex County Council and private developer contributions.
Road improvements were suggested to combat an influx of new housing developments in the area, including the 1,200-home Hampton Park in Littlehampton.
Work on a Lyminster bypass agreed as part of the planning permission for Hampton Park is also well underway.