A new road bridge is about to be moved into place in Adelaide's Darlington project, in an extraordinary feat of civil engineering.

Weighing 3,000 tonnes and measuring 180 metres long, the bridge has been built over four months on land near the Southern Expressway, and will soon carry Main South Road over the motorway.

So-called "self-propelled modular transporters" — using remote controls — will shift the bridge into place this weekend, at the slow speed of 100m per hour.

"This is a common construction method in other parts of the world, such as Europe and America," Transport Minister Stephen Mullighan said.

"But it's the first time it has been used on a civil construction project of this size in Australia."

The Darlington bridge is about to make its big move into place. ( ABC Radio Adelaide: Spence Denny )

The State Government said if the bridge had been constructed in place, it would have required at least seven closures of the Southern Expressway and Main South Road.

Instead, the roads will be closed only this weekend, and for one more weekend in the future for the bridge's second span.

"Some of the busiest intersections in Adelaide are located on this part of the North-South Corridor so these bridges will improve traffic flow and cut travel times for vehicles travelling in all directions," Mr Mullighan said.

The 180-metre bridge is longer than the Adelaide Oval. ( ABC Radio Adelaide: Spence Denny )

There will be eight new bridges in total for the Darlington project, which runs from the existing end of the Southern Expressway and north to Tonsley.

Taxpayers are forking out $620 million for the upgrade with 80 per cent of the funding coming from the Commonwealth.

A concept image showing the new bridge above the Southern Expressway. ( Supplied: Department for Planning, Transport and Infrastructure )

The project coincides with the Torrens to Torrens upgrade of South Road in the inner-west.

The second bridge is being built the same way and will be installed at Darlington next year.