Main roads in Adelaide's north are promised to be less congested following a $51 million dollar announcement by the state government.

The money will be allocated in the June 18 state budget to bust congestion on two of Adelaide's busiest main roads.

The investment will allow for three frequently used intersections to be upgraded.

Transport minister Stephan Knoll said the three intersections were nearing operational capacity.

"They are accommodating between 58,000 and 78,000 vehicles per day," Mr Knoll said on Saturday.

"We're building these intersections so South Australians spend less time stuck in traffic and have a faster and safer journey."

The state and federal governments have each committed 50 per cent of the funding.

Of that total, $19 million will go towards the intersection of Main North Rd and Nottage Tce.

An additional right-hand turn lane on Main North Rd and extension of the existing bus lane have been proposed.

The intersection of Main North, Kings and McIntyre roads will also be upgraded.

Meanwhile, the intersection of Grand Junction, Hampstead and Briens roads will get a $19 million boost.

This intersection is predicted to be at operational capacity by 2021.

Planning will now begin, with early works expected to start in the 2019-2020 financial year.