The ACT government says a new report warning of booming congestion on Canberra's roads has bolstered its case for a light rail line, with gridlock in some places to increase as much as 20 per cent by 2031.

An Infrastructure Australia audit report released on Friday showed Canberra Avenue, the airport-to-city corridor and Northbourne Avenue will all reach 70 per cent or more of morning peak capacity within 15 years.

Gridlock on some of Canberra's busiest roads will increase as much as 20 per cent by 2031 Credit:Rohan Thomson

Growing congestion will cost the territory $700 million by 2031, as Canberra Avenue reaches 72 per cent of its morning capacity on weekdays and drivers around the airport and city roads face 79 per cent capacity between 7 and 9am.

The Barton Highway-Northbourne Avenue corridor - currently expected to see tram services commencing by 2019 or 2020 - will have reached 68 per cent of capacity in morning peak times by 2031.