The A1 is now three lanes wide for a seven-mile stretch in Cambridgeshire where it joins the A14.
The improved section of Britain's longest road opened on March 25 and the 70mph speed limit has been restored.
The work is one of six stages of the £1.5 billion project to upgrade the A14, which is set to be completed by December 2020.
The A1 has been widened between Alconbury and Buckden and a redesigned junction where it meets the A14 at Brampton Hut has been put in place.
Highways England's project director for the A14 scheme, David Bray, said: “This is a big step in delivering a new, improved A14, and drivers can see just how much work has been done over the last couple of years. The upgraded A1 will fit seamlessly together with the new A14 and improve journeys on both roads."
A decision is expected soon on making part of the upgraded A1 and A14 motorways. If the plan goes ahead, there would be a motorway link between London and Peterborough for the first time.
Some finishing works on the A1 are still to be done, such as commissioning gantries to share information with drivers and completing the links with the A14. This will require some overnight closures, and information will be shared in advance by Highways England.
Local link roads
Another of the improvements from the scheme, a link road between Girton and Dry Drayton, opened last week.
It's one of a number of local roads that are being put in place as part of the A14 upgrade programme.
A local access road will run alongside the upgraded A1 northbound from the new Ellington junction. The Dry Drayton link road will eventually form part of a five-mile local road running parallel to the A14 between Cambridge and Huntingdon.
The overall A14 works
The A14 project is now more than halfway finished, and will see the A14 widened between Swavesey and Milton. There will also be improved junctions at Bar Hill, Swavesey, Girton, Histon and Milton.
Nine of the 34 bridges and structures involved in the project have opened to traffic. The longest, the River Great Ouse Viaduct, stretches half a mile and was completed in February.
Graham Bull, leader of Huntingdonshire District Council, said: “The new A14, and the improvements to this section of the A1, are key to ensuring that Huntingdonshire can fulfil its potential as a truly great place to live, work and invest, and we are delighted to see new sections of the road beginning to open.
"It is part of an important wider network, including the A428 and the A1 between Buckden and Black Cat, which we are keen to work with partners to continue to improve.”
