The Weston Creek Community Council has criticised Greens Minister Shane Rattenbury's hesitancy to support the duplication of the Cotter Road.
Mr Rattenbury, a former roads minister, said the government needed to break Canberra's dependency on cars and should question whether the $25 million duplication was necessary.
His remarks came after the Canberra Liberals announced they would complete the duplication of the Weston Creek road, closing a "missing link" through a new 1.7 kilometre upgrade.
"I don't intend to support a motion that simply says 'duplicate a road' without presenting any evidence, especially when I know there is a more sophisticated debate about transport planning to be had," Mr Rattenbury said.
But Weston Creek Community Council chairman Tom Anderson said the unduplicated section of the road had become a major bottleneck and was impacting surrounding roads.
"As Molonglo grows, congestion will further increase," he said. "Duplication needs to happen now, not years from now when it'll be too late."
"This is not just duplication for the people in Molonglo and Weston Creek but also those who use Cotter Road to travel from Tuggeranong, Belconnen and Gungahlin to reach various southside destinations."
Mr Rattenbury called on the government to "show some resistance" to public pressure for roads and criticised the Canberra Liberals' description of his views as anti-car, anti-family and anti-Weston Creek.
"The new federal Liberal government has shown a willingness to shift its infrastructure focus, recognising that public transport and active travel are necessary to create sustainable and liveable cities," he said.
"In contrast, in our local assembly we have an ultra-conservative, Tony Abbott-style Liberal Party.
But Mr Anderson said the government had not done enough to encourage public transport use in Weston Creek and Molonglo.
"To encourage public transport uptake, both Weston Creek and Molonglo need bus routes which go directly down Cotter Road to the parliamentary triangle and the city," he said.
"The very buses that Minister Rattenbury says we should catch are already caught up in the current traffic jams on the Cotter Road.
"You can't have one without the other – we need a better bus service and for the Cotter Road to be duplicated."
Opposition transport spokesman Alistair Coe has said the duplication would end bottlenecks between McCulloch Street in Curtin and the Tuggeranong Parkway.
"There's no doubt that there is considerable congestion, especially in the morning peak, going north-east bound, on the Cotter Road," he said. "It is getting worse as more cars enter the road from the Molonglo Valley."
Mr Rattenbury questioned the need to build and duplicate roads in the ACT and whether the money could be better spent on public transport upgrades.
"The Canberra Liberals have a myopic vision that would lead to all kinds of problems for Canberra in the future: congestion, pollution and a loss of our prized liveability.
"It's embarrassing that the local Liberal Party is fixated on denigrating sustainable transport policies, insisting road building and car-reliance is the way of the future."
Minister for Roads Mick Gentleman said the government had already done the preparatory work on the Cotter Road duplication, and a decision on funding would be made by the cabinet.
"It's a matter of timing and those decisions are made in regard to funding and what the community wants to see," he said.