A new "Circle Freeway" will be built under a Labor government, with the aim to link Perth's outer suburbs, reduce traffic congestion and improve road safety, Opposition Leader Mark McGowan has said.

Sunday's announcement is the latest of a string of election promises that Mr McGowan says will be delivered alongside labor's rail infrastructure system Metronet.

The new freeway would be completed by 2018 to allow uninterrupted travel between Perth's northern, eastern and southern suburbs, said Mr McGowan.

He said the new freeway would link five of Perth's most important roads and reduce the build-up of traffic at traffic lights.

It would also improve safety at five of the ten worst intersections for crashes in Perth.