Traffic is slowing in Australia's major cities as roads become more congested, with speeds in Adelaide the most sluggish and conditions in Melbourne deteriorating quicker than any other capital city, a new report has shown.

Key points: Sydney is the nation's most congested city when average speeds are compared to free-flow speeds

Sydney is the nation's most congested city when average speeds are compared to free-flow speeds The road to Melbourne's airport is more congested than the route to any other Australian capital city airport

The road to Melbourne's airport is more congested than the route to any other Australian capital city airport The cost of congestion to the national economy is projected to rise to $37.3 billion by 2030 without major policy changes

Sydney remains the nation's most congested city, the Australian Automobile Association report shows.

Its Road Congestion in Australia report is based on five years of mapping and location data from all of the nation's capital cities.

In the five years to June this year, average speeds declined more in Melbourne than in any other city, dropping 8.2 per cent to 59.9 kilometres per hour.

Speeds in the South Australian capital were just 54.3 kilometres per hour, while speeds in Sydney and Brisbane fell by 3.6 and 3.7 per cent respectively.

Different benchmarks of congestion are assessed throughout the report.

"Sydney is Australia's most congested city when average speeds are compared to free-flow speeds," the report said.

Free-flow speeds are the average observed speeds over a number of hours in an overnight period when there is no traffic.

The Tullamarine Freeway to Melbourne Airport recorded the largest increase in congestion of any route between a capital city and its airport, with congestion up almost 13 per cent over the period.

Dave Jones, the manager of mobility advocacy at RACV, said much of the added congestion was due to major projects that had started over the past five years, including the Tullamarine Freeway widening, the Metro Tunnel and level crossing removals.

"The price of having such a backlog and years of doing nothing is that we are going to have road works and public transport works for years into the future,'' Mr Jones said.

Sorry, this video has expired Growth is putting strain on our roads and public transport

Victorian Roads Minister Luke Donnellan said the State Government had begun a series of major projects that were contributing to current congestion.

"An enormous amount of work is going on and sometimes that does have an impact on travel times for the community, and I understand that's frustrating but to sit still is not an option," Mr Donnellan said.

"More's gone on in the last four years than ever before, and we know we've got to do a lot more."

Australian Automobile Association chief executive Michael Bradley said he hoped the report would contribute to solutions to address congestion.

"This report confirms what most people living in our major cities know all too well. But we hope it also help stimulate discussion and problem-solving so that Australia can develop smart measures to address our worsening congestion," Mr Bradley said in a statement.

The association said that in 2015 congestion cost Australia $16.5 billion and that without major policy changes, congestion costs were projected to reach between $27.7–37.3 billion by 2030.