Mr Brown said the dualling of the Kincraig to Dalraddy section of the A9 had been brought forward to next summer as a result of savings on the construction of the new bridge over the Firth of Forth.
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A £50 million contract has now been advertised for the scheme, the first of 12 in the £3 billion roadworks.
Mr Brown said: "Dualling the A9 between Perth and Inverness is a huge challenge with some 80 miles of road to upgrade, but we are determined to make early wins.
"As the Queensferry Crossing completes we are anticipating more savings being realised next year from the project's contingencies, allowing us to unlock investment for the A9 dualling scheme now.
"With that investment identified and the progress in making statutory orders for the five mile long Kincraig-Dalraddy scheme, I have given the go-ahead to get procurement under way."
Finance Secretary John Swinney said ground investigation work at Kincraig was supporting in excess of 30 jobs and more would be created in the construction phase.
He said: "Communities and businesses here and along the length of the route can look forward to feeling all of the benefits of this investment as we press ahead in developing the whole road to dual carriageway status from Perth to Inverness.
delivering the full programme of road development ."
Responding to the announcement, Scottish Liberal Democrat MP Danny Alexander said: "Highlanders will treat SNP claims that the dualling of the A9 is ahead of schedule with the derision they deserve.
"Regular users of the A9 have had to endure seven wasted years of a SNP government that has focused on central belt priorities while turning a deaf ear to Highland concerns."