An agreement reached between Transport Scotland and contractors means advance work can start immediately on the M8 between Glasgow and Edinburgh and the connected M73 and M74.
The motorway between Glasgow and Edinburgh is the busiest stretch of road in Scotland, carrying more than 90,000 vehicles a day.
The work involves the construction of a new section of motorway between Baillieston and Newhouse in North Lanarkshire to replace the A8, cutting peak time journeys by up to 18 minutes, with an existing stretch of the M8 motorway between Easterhouse and Baillieston being upgraded.
Major improvements will also be made to the Raith interchange at junction five, where the A725 provides links to the M8 and M77, along with the widening of 11 miles of the M73 and M74.
Construction is due to be completed in spring 2017.
Plans to upgrade the motorway network in central Scotland were given the go-ahead in December 2010 and work was scheduled to begin last year, but the plans were hit by cash problems.
It is the first UK roads project to use bond finance since the credit crunch and the largest project under the Scottish Government's non-profit distribution (NPD) model to reach this stage. NPD allows ministers pay back the cost to private financiers over 30 years.
Transport Minister Keith Brown said: "In order to deliver the project at the earliest opportunity, I am pleased to announce Transport Scotland has entered into a pre-start works agreement with the contractor. This will allow advance works to progress without delay, with the contract expected to be signed during February.
"The motorway link between the country's two largest cities is absolutely pivotal to Scotland's sustainable economic growth and the improvements made by this project will drive significant economic benefits for businesses and improve road safety and accessibility for road users the length and breadth of the country.
"Completing the M74 directly supported 900 construction jobs - the large majority of which were filled by workers from the local area. We expect this project will deliver similar benefits to the local economy."
The project also includes upgrades to the connections to the M73 motorway at Baillieston and a new junction at Shawhead, which will connect to the A725 East Kilbride road.
There will also be new junctions at Eurocentral and Chapelhall.
Transport Scotland said construction costs had been driven down by £105 million, from £415m to £310m, following "robust" talks with its preferred contractor, Scot Roads Partnership, a consortium comprising of Ferrovial Agroman and Lagan.
An additional £180m is being spent on other aspects, such as land acquisition, putting the overall cost at around £490m.
Funding support will come from the European Investment Bank and a group of investors managed by Allianz Global Investors.
Jonathan Taylor, bank vice-president, said: "The European Investment Bank is committed to supporting crucial investment in essential infrastructure across Europe and we recognise the importance of the M8 scheme to upgrade Scotland's core motorway links.
"This project will not only provide economic benefits during construction, but will also improve safety and reduce costs for business in the years ahead through improved travel times."