The 12-mile bypass is part of a £1.5bn project to upgrade 21 miles of the A14 in Cambridgeshire.
It had been due to open in December 2020, but fast progress by main contractor Balfour Beatty/Costain/Skanska means that the road will now be ready for drivers to use this December.
The joint venture contractors began work on the scheme between Cambridge and Huntingdon in November 2016.
Work on the upgrade has progressed at pace, with around 10 million cubic metres of earth having been moved – more than 4 times the great pyramids of Giza.
Up to 2,700 staff are working across the whole project to build the new roads and bridges that are needed.
Highways England project director David Bray said: “I’m delighted that we will be able to deliver the benefits of this key section of the project so much earlier than originally planned.
“It will mean that we are able to start work sooner on the improvements we are making to local roads in and around Huntingdon as we start to dismantle the old viaduct over the train station, which will be a real boost for local communities.”
As well as the bypass, the full project includes widening the A1 between Brampton and Alconbury, widening the existing A14 between Swavesey and Milton and improving the junctions at Bar Hill, Swavesey, Girton, Histon and Milton.
The full project remains on track to open to traffic by the end of 2020.