She says the busy highway in Toronto's west end would be extended 6.6 kilometres to Major Mackenzie Drive in York Region.
The project has been on the Ontario government's radar for some time.
The Liberal government said in its May 2 budget that it would proceed with the planned extension, among other highway projects in the province.
The Highway 427 project includes three new interchanges, nine bridges and protection for a future transitway and transit stations along the west side of the highway.
But the Liberals need the support of the New Democrats to pass the budget and avoid triggering an election.