The £1.5bn A14 scheme to improve journeys between the East of England and the Midlands was originally planned to open to traffic by the end of 2020.
It will now open ahead of schedule this spring.
Main contractors on the job are Balfour Beatty/Costain/Skanska.
Highways England chief executive Jim O’Sullivan said: “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.”
A new 12-mile bypass south of Huntingdon, which forms part of the overall project, opened a year ahead of schedule in December.
Work on the A14 improvement project began in November 2016 and it has employed around 13,000 people in total, with up to 2,500 working on site during the its peak.
Alongside the Huntingdon southern 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.
Approximately 24 miles of new routes for cyclists, walkers and horse riders are also included in the scheme.
The spring opening date will mark the end of permanent roadworks and reduced speed limits on the new A14 and the A1307, but the project team will still need to carry out a number of completion activities such as landscaping, installing some of the new technology, and work in the verges.
Glennan Blackmore, highways sector director, Skanska said: “The delivery team’s collaborative approach is industry changing. With all partners truly integrating, it demonstrates the success that can be achieved.”
Richard Stuart, Costain sector director, highways said: “This has been an exemplar project and should set the model for other major infrastructure projects to follow and build on.”
Julian Lamb, A14 Deputy Project Director for Balfour Beatty and the joint venture delivery team, said: “With a focus on delivering a safe and sustainable road network, our expertise and capabilities has allowed us to deliver this project safely and efficiently ahead of schedule.”