The opening of the £1.5 billion upgrade to the A14 will take place this spring - more than six months ahead of schedule.
It follows the completion of the 12 mile Huntingdon southern bypass in December, which was ready a whole year early, and Highways England estimates that the new section of road has already been used to make more than five million journeys.
However, work in the Huntingdon area is set to continue until 2022 for the demolition of the viaduct over the town's railway station and the construction of new link roads.
The long-awaited A14 upgrade, Britain's biggest road project, started in November 2016 and was scheduled to open to traffic at the end of this year.
Highways England chief executive Jim O'Sullivan said: "The A14 is a vital route used by 85,000 drivers every day and including more than 21,000 hauliers transporting essential goods around the country.
"Opening this scheme more than six months early and on budget shows what the UK construction industry can achieve working with Highways England on the strategic road network. I would like to thank them for their focus on our joint success and for their one team approach."
Mr O'Sullivan said: "Also, I would like to thank road users, residents and stakeholders for their patience and support during our work. This road is not just a piece of national infrastructure - it brings benefits to the region and local towns and communities too."
Roads Minister Baroness Vere said: "I'm delighted that the A14 upgrade will open ahead of schedule, not only meaning drivers will benefit from quicker and safer journeys sooner, but also ensuring that key access between the region's ports and the West Midlands will be boosted.
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"Investing in key transport links such as this is part of this Government's plan to level up access across the country, ensuring all regions are better connected and improving journeys for all."
Around 13,000 people have worked on the scheme in total, with 2,500 employed on-site during the project's peak.
In addition to the Huntingdon bypass, the project includes an upgrade to the A14 between Swavesey and Milton, and a new local access road, the A1307, which runs parallel to it between Cambridge and Godmanchester. There are also around 24 miles of new routes for cyclists, walkers and horse riders.
The new opening date will mark the end of permanent roadworks and reduced speed limits on the new A14 and the A1307, but there will still be some temporary road closures while final touches are made.
In the St Neots area Highways England expects to seek planning permission later this year for a new dual carriageway A428 between Caxton Gibbet and the Black Cat junction with the A1 in Bedfordshire.
The old A14, now known as the A1307 east of Huntingdon and along the Alconbury spur, together with the A141 west of Huntingdon, will be handed over to Cambridgeshire County Council once the project is completed It will be used as a local access road running parallel to the A14 and serving the surrounding communities.
Information about the A14 Cambridge to Huntingdon improvement scheme, including advance notification of road closures, can be found at www.highways.gov.uk/A14C2H, @A14C2H on Twitter or at www.facebook.com/A14C2H/ on Facebook.