Earlier this week, 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.

Mr Rattenbury, a former roads minister, said the government needed to question whether the duplication was necessary and if the funds could be better spend on public transport.

"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" he said.

Mr Rattenbury said Canberra remained a car centric city and there was constant pressure on territory politicians to upgrade and build more roads.

"As the election approaches, I expect to see both the Labor and Liberal party battle to outdo each other in the promise of road upgrades," he said.

"The costs of these promises are likely to swell and whoever promises to spend the most on roads will likely consider themselves the winner of a political battle."

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."

The plan is the second major spending pledge from the opposition in four months, and comes a year before the election campaign begins.

In June, the opposition committed to spending $146 million the duplication of Gundaroo Drive and a new flyover on the Barton Highway near Gungahlin.

Mr Rattenbury said the government needed to consider whether there were more pressing public transport or road infrastructure projects.

"We should show some level of resistance to upgrading or building roads," he said.

"Certainly we shouldn't just charge out and try to build every new road that someone asks for with special enthusiasm if it happens to be in our electorate."

Opposition leader Jeremy Hanson said Mr Rattenbury's speech was "dripping with ideology" and revealed an anti-car, anti-family and anti-Weston Creek agenda.

"The Canberra Liberals are focused on all of Canberra, he said.

"Mr Rattenbury and this government … have a much greater focus on the three percent of people who will be able walk right to the light rail."

Minister for Roads Mick Gentleman said the government had already done the proprietary 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.

"We want to make sure we stage these works in a process that allows people to have less congestion, but provides the right outcome."

Mr Gentleman said the government remained committed to an integrated public transport with investment in roads, cycling, walking and light rail.

"You have to develop other ways to get people to work and to their commitments that don't pressure the road system too much," he said