Transurban's $5.5 billion Western Distributor plan under Yarraville and through Footscray is being assessed by the Andrews government. Last month in Melbourne, Transurban and Premier Daniel Andrews unveiled the toll road operator's proposed $5.5 billion Western Distributor. The 12-kilometre toll road would link the West Gate Freeway to CityLink via a new tunnel and elevated freeway through Footscray. The tunnel would have three lanes in each direction. The project would also see the West Gate Freeway widened by two lanes in each direction and improvements made to port access roads. Transurban brought the project to the government under its "market-led proposals" program in March.

The operator of NorthConnex will be paid compensation if not enough trucks use the 9 kilometre motorway The cost of building the project would be one-third funded by Victorian motorists via a 10 to 15 year extension of tolls on CityLink. Another third would come from tolls on the new road, and the remainder would come from Canberra. But Prime Minister Tony Abbott has indicated that Canberra is unlikely to even consider funding the proposal unless Victoria also stumps up money for the project. Transurban's nine-kilometre NorthConnex tunnel Transurban says the Sydney road is so much cheaper than the plan for Melbourne largely because of the difference in geological tunnelling conditions.

"In Sydney, there are ideal tunnelling conditions with Sydney sandstone enabling very efficient tunnelling techniques," a spokeswoman for Transurban said. "In Melbourne, there are complex geological conditions requiring significantly more complex tunnelling techniques." She also said there were many other "significant differences in the two projects, making a direct comparison very difficult". Bruno Aleksic is a former director of the Linking Melbourne Authority, which oversaw the East West Link and Peninsula Link roads. He said Sydney tunnels were built in relatively soft and uniform strength Silurian sandstone, which was "ideal for tunnelling and enables high production rates" because they did not need a tunnel-boring machine. This reason alone would explain much of the cost difference, he said.

"Melbourne geology is vastly different," he said. "The Western Distributor tunnel, as would have been the case with East West Link tunnel, will be in built basalt rock, formed from ancient lava flows, which ranges from extremely weathered, fractured basalt rock to 'fresh', very high-strength basalt rock. Groundwater passes through weak zones in basalt rock. This makes it extremely complex technically to build," he said. Sydney-based actuary Ian Bell has looked at both Transurban proposals and said it was not clear why the Victorian plan was so much more expensive, given the toll road operator could expect about twice as much daily traffic to pay to use the Melbourne road. And Mr Bell said tunnelling usually made projects far more expensive, not cheaper. "So again a reason to question why the cost in Melbourne is so much higher," he said. Jago Dodson is director of the Centre for Urban Research at RMIT. He co-authored a report last year on the likely long-term costs of the East West Link toll road.

He said no public money – whether from Victoria or Canberra – should go into funding the Transurban road. "Transurban is a private business, and it should be able to finance its own toll road projects," Professor Dodson said. "They should wear all of the risk." Another transport expert, Melbourne University's John Stone, said he feared the Transurban plan would not get as many trucks off roads in the west as suggested. "It will be sold to people in the west as the answer to their dire problem," he said, but it was not clear that truck drivers could be forced to use the tunnel and pay tolls. Opposition roads spokesman Ryan Smith said Victorian Premier Daniel Andrews' announcement of the road had placed the state government in a very poor bargaining position. "Andrews has come out touting this project as something that appeases industry groups, and provides thousands of jobs," Mr Smith said. "So it is now politically untenable for him to come out and say 'You know what, it didn't turn out, we are not doing it'. Transurban know that."

A spokesman for Treasurer Tim Pallas said the government was taking "a careful and considered approach to ensure we act in the best interests of Victorian taxpayers". He said the government was "pursuing projects that stack up and create jobs for Victorians". * Roads Minister Luke Donnellan said the government had shortlisted seven construction companies to build the initial stage of a project it promised in opposition, called the West Gate Distributor. The first stage of the project – widening Moreland Street, adding traffic lights on Footscray Road and Moreland Street, and widening and strengthening Shepherd Bridge over the Maribyrnong River – was announced in this month's budget. Mr Donnellan said it was the first step in reducing truck traffic in the inner west. Follow The Age's City Desk on Facebook.