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.
What about the cycle lanes?