Finishing South Australia's largest infrastructure project will cost upwards of $9 billion and take another 10 years with the state government's latest budget revealing the final plans for Adelaide's north-south road corridor.
The development will provide for 78 kilometres of non-stop motorway that will cut freight costs and reduce travel times.
It will allow motorists to bypass 21 traffic lights and remove an estimated 110,000 vehicles from surrounding roads.
In its 2021/21 budget on Tuesday, the government allocated $1.96 billion over the next four years to start the work on the final and most difficult portion of the South Road project.
It said it had also settled on a plan to include two tunnels in the 11km section, one 4.3km long and a second 2km long.
That will cut the number of properties to be bulldozed to about 390, down from about 900 with a conventional roadway.
The cost of the work is currently projected at $8.9 billion but the final cost estimate will not be known until the full business case is finalised in mid-2021.
Treasurer Rob Lucas said SA had already secured significant federal government funding and was 100 per cent confident of further commitments from the Commonwealth.
He said despite the high cost of the project the economic benefits were "unarguable".
Prime Minister Scott Morrison said the road corridor would deliver thousands of jobs to support South Australian families and motorists would benefit for generations to come.
"The north-south corridor is a game-changing piece of infrastructure that will change the way people move across Adelaide," Mr Morrison said.
"This is the most significant and eagerly anticipated road infrastructure project ever delivered in South Australia."
SA Transport Minister Corey Wingard said the tunnelling options would minimise community impact and deliver greater safety benefits.
"The other design options we assessed failed to meet the state's transport needs while getting the balance right between easing congestion across the network and enhancing the liveability of local communities," he said.
"With the hybrid solution, we can protect iconic community and heritage assets including the Thebarton Theatre, Hindmarsh Cemetery, Hoffman Brick Kiln and Queen of Angels Church, to name a few.
"This solution requires far less property acquisition, will have reduced impact to business during construction and saves a number of heritage buildings from demolition."
Premier Steven Marshall said work was already under way with geotechnical studies being carried out at dozens of locations.
Work next year will include ground investigations, detailed design and the compulsory acquisition of land.
Mr Marshall said the project was expected to create 4000 jobs.