Several Melbourne arterial roads in the west will be duplicated and widened as part of a $1.8 billion public-private partnership to improve traffic flow in some of the city's most congested outer suburbs.

The project, which will be completed in 2022, will also improve some 700 kilometres of arterials throughout the wider western suburban road network.

The 20-year construction and maintenance package will be funded through an availability payment model that Premier Daniel Andrews said was an Australian first for arterial and suburban road works.

Eight "high-priority" road upgrades on the suburban fringe, including several well-known "choke points", have been identified for duplication and widening works to be completed over the next five years.