Thousands of commuters from Brighton and Hove face two years of delays from this morning (Monday 2 July).
The main phase of work is due to start on widening a two and a half mile stretch of the A23 after a year of preparations and with tree felling having started last autumn.
The work will turn a two-lane section of the road into three lanes between Handcross and Warninglid. The road is already three lanes wide at each end of the section that is being widened.
A 40mph temporary speed limit will be enforced by cameras, the Highways Agency said, but journey times should improve as a result of the work.
The agency also expects the wider and straighter road to have a better safety record and less congestion for the 65,000 drivers who use it each day.
It said that the “mid-range cost estimate” for the scheme was £76.9 million but added that it would bring economic benefits to the area worth billions of pounds.
The project is scheduled to be completed in autumn 2014.