An agreement between Transport Scotland and contractors means advance work can start without delay on the M8 between Glasgow and Edinburgh and the connected M73 and M74.
Hundreds of jobs will be created through the project, which has seen costs driven down by about £105 million, he said.
The work continues to break new ground, by being the first UK roads project to use bond finance since the global financial crash and the largest non-profit distribution project to reach this stage, he said.
"In order to deliver the project at the earliest opportunity, I am also pleased to announce that Transport Scotland has entered into a pre-start works agreement with the contractor," Mr Brown said.
"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 - and we expect this project will deliver similar benefits to the local economy.
"The project will also provide sustainable long-term employment opportunities throughout the 30-year operation and maintenance on these key routes."
The project will take 18 minutes off journeys at peak times for vehicles using busiest sections of the M8 each day, Transport Scotland says.
The cost savings follow "robust" talks with preferred contractor Scot Roads Partnership.
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.
"We are committed to providing significant long-term financial support for the scheme."