City traffic congestion is likely to keep getting worse, with the RAC predicting there will be another one million cars on WA roads by 2020.
The estimate is two-and-a-half times higher than previously forecast.
It has prompted RAC head of advocacy Matt Brown to call on the State's policymakers to "confront the issue head-on" or risk congestion "choking the economy".
"These new figures simply confirm that congestion has the potential to become much worse in the near future and if we don't start investing in our roads and public transport now, congestion has the potential to choke our economy by the end of the decade," Mr Brown said.
"If we are serious about growing our economy and protecting the lifestyle we enjoy in WA, roads and public transport must be a top priority."
In 2010, the RAC developed a model for projecting vehicle numbers, based on population estimates and motor vehicle census counts. It projected 400,000 more cars by 2020.
"Since this model was developed, some of the underlying parameters have changed, leading to an understanding that the model may be understating the growth rate," Mr Brown said. "In the past two years alone, vehicle registrations have increased by 281,410.
"To provide a more rigorous and timely estimate of future growth in licensed vehicle counts, the RAC has developed an updated model using recent Department of Transport data on licensed vehicles."
The West Australian revealed this year that there were 143,739 more cars licensed in WA last year, prompting Curtin University's sustainability professor Peter Newman to warn of a pending "Carmageddon" and the need for good public transport and cycling options.
Opposition Leader Mark McGowan said yesterday the only long-term solution to congestion was to provide a public transport network quicker and faster than cars.
"WA Labor's Metronet is an affordable, co-ordinated and integrated plan to change the way people move around our city and suburbs," he said. "It will deliver a heavy rail network to move people across Perth and connect our suburbs and places of work."
Transport Minister Troy Buswell said the State Government recognised the impact of the increasing traffic on WA roads and had responded with a multi-faceted approach to ease congestion.
Based on the new modelling, the RAC predictions include 990,907 more vehicles on WA roads in 2020, a minimum of 88,000 more cars and 12,500 more motorcycles each year and three million licensed vehicles on WA roads in 2019.