Delays on major Canberra roads are tipped to cost $700 million by 2031 as the population of Gungahlin and Belconnen increases, according to a landmark Infrastructure Australia audit.
The audit, which was published on Friday, found the Northbourne Avenue corridor was the most expensive road in the ACT with delays costing $430,000 per lane kilometre in 2011.
But the audit warned delays on the arterial road could cost $1.1 million in 2031, second only to the Canberra Avenue corridor at an estimated $1.22 million.
"The cost of delay on greater Canberra's urban transport network in 2011 was around $0.2 billion," the report said.
"In the absence of any additional capacity – other than projects already under construction or funded – the cost of delay is projected to grow to around $0.7 billion in 2031."
Public transport use on Northbourne Avenue was also tipped to increase with the greatest population growth predicted in North Canberra, Belconnen and Gungahlin.
"Passenger loadings on Northbourne Avenue in particular are projected to rise substantially through to 2031," the report said.
According to modelling by Veitch Lister Consulting, residents in North Canberra had the highest number of bus trips per hour at 14,408 in 2011, which is forecast to increase to 27,066 in 2031.
The Infrastructure Australia audit also found North Canberra was the most popular origin/destination for trips in 2011 followed by Belconnen and Tuggeranong.
"Gungahlin is projected to grow to become the second largest origin/destination for trips by 2031," the report said.
While the audit noted the ACT government's proposed light rail aligned with the Northbourne Avenue corridor, it stopped short of offering an endorsement.
In 2013, an Infrastructure Australia assessment cast doubt on whether the ACT had enough traffic congestion to warrant federal funding for light rail.
Capital Metro missed out on a place on the national infrastructure priority list as a result of the report, which also said the ACT government had not made a strong case for favouring light rail over buses.
In October, Capital Metro Minister Simon Corbell said an increase in bus services along Northbourne Avenue was not an acceptable option for Canberra.
"We know that putting buses down Northbourne Avenue means either taking away two lanes of traffic or building a roadway down the middle of the median strip," he said.
"Neither of those options are acceptable for the city."
The audit, which urged long-term planning and greater transparency, found major investments could also have a significant impact on the ACT economy.
"In 2011, the direct economic contribution of infrastructure in the ACT was $3.5 million, which was around two per cent of the national figure," the report said.
"In 2031, ACT infrastructure direct economic contribution is projected to increase by around $3.3 billion, or 96 per cent, reaching $6.8 billion."
Nationally, the audit argued a massive investment in roads, rail and other infrastructure was needed to avoid congestion in Australian cities.
"It is time for the nation to treat population growth as a fact; a fact our nation should accept and gear up for," Infrastructure Australia chairman Mark Birrell said.
"Without action road travel time in Perth, Melbourne, Sydney, Adelaide, Brisbane and Canberra are expected to increase by at least 20 per cent in the most congested corridors by 2031."
"Major reforms are needed to improve the way we, finance and operate infrastructure to ensure it can underpin gains in Australia's productivity and employment growth in decades ahead."
- With Mathew Dunckley