The 2.4m widening of the A23 between Handcross and Warninglid in West Sussex is the first of the 14 major road projects Government pledged to start before 2015.
It sets the ball rolling as expected this month on the £1.4bn road improvements spared in last year’s spending cuts.
Although main civil engineering works on the improvement scheme will not begin until next spring.
The project to widen dual carriageway to three lanes is expected to cost between £87m to £103m although efficiency savings are now being developed.
The first phase of work will be site preparation before main civil engineering work begins next spring.
Roads Minister Mike Penning said: “This start of work shows the Government delivering on its promise to invest in transport schemes that reduce congestion, improve safety, and support economic growth.
He added: “This major improvement project will provide much-needed extra capacity at a notorious traffic bottleneck, deliver important safety benefits for drivers and provide more than a six-fold return on investment for the economy.”
The road layout will be upgraded, improving visibility for road users and removing sharp bends, and access to the A23 will be made safer for the homes and businesses where drivers currently have to turn directly onto the busy dual carriageway.
Before main civil engineering work can start, trees and other vegetation in the central reservation and verges along the A23 need to be removed.
Highways Agency project manager Chris Bacon said: “By using the space in the central reservation we are minimising the amount of land we will need on either side of the road, and we will also carry out further replanting later in the project.”
The work will be carried out using a series of overnight restrictions on the A23 between Pease Pottage and Bolney over the next seven weeks.
Before the end of the year, work is also scheduled to start on improvements to the M62 in West Yorkshire, the M4/M5 around Bristol