An upgrade to a notoriously busy roundabout in Melbourne's south-east has been brought forward by three years, as the Andrews government pushes ahead with promises to marginal seats following the November state election.

Traffic lights will be installed at the Thompsons Road intersection with Dandenong-Frankston Road in Cranbourne by mid-2020.

Acting Transport Infrastructure Minister Jaala Pulford. Credit:Eddie Jim

Acting Transport Infrastructure Minister Jaala Pulford said this would be three years ahead of schedule.

Queues at the current roundabout intersection can be as long as 1.5 kilometres on Dandenong-Frankston Road and more than 700 metres on Thompsons Road during peak travel times. An average of 60,000 vehicles cross the intersection each day.