Lowering Parkes Way could cost $460 million, according to a report to government in December 2014, which would make it nearly $200 million more expensive than the Majura Parkway.
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The report from consultants Smec Australia and using costs estimated by John Holland, assessed an alternative, truncated, option for bridging Parkes Way near the city, at $275 million.
The 11 kilometre Majura Parkway cost $288 million. The 9km Gungahlin Drive Extension cost $200 million.
The government is trying to find an affordable way of bridging Parkes Way to link the city more directly with the lake and overcome the barrier of the arterial road.
But the December 2104 report from Smec makes it clear what kind of costs decision makers are up against.
Smec said the calculations were based on recent projects, including Kings Avenue and the Majura Parkway. But they were only based on a "preliminary and skeletal" concept, with many unknowns, including signals, lighting, planting, pavements, stormwater and other details.
"It is not considered appropriate to be used as a basis for financing of the project or for any investment decisions made in the development of adjacent commercial blocks or as a commitment to proceed with or abandon the project," the consultants said.
The costs did not include earthworks for lakeside development and roads in West Basin. Nor did it include earthworks for other commercial and development sites in the area, including around City Hill.
The report, called Re-engineering Parkes Way and Civic's Southern Road Network, estimated the cut at 226,000 square metres and the fill at 190,000. Smec estimated a total project cost of $462 million.
With "the need to investigate a reduced scale version of the project", the project team had looked at a number of variations, reducing scale and cost "whilst limiting compromises to road network performance and urban design outcomes".
They had settled on a preferred option called "variant 2c", which proposed a much shallower Parkes Way, reducing costs for excavation, piles and "dewatering". The length of roadworks was also reduced, so instead of trenching about 1100 metres all the way from Edinburgh Avenue to Coranderrk Street, an 850-metre trench would stretch only as far as Allara Street.
Deleting a proposed ramp from Commonwealth Avenue for heavy afternoon rush hour traffic saved 400 metres in tunnelling and trenching. The cheaper option also envisaged a 400-metre tunnelled section instead of an open trench.
The changes cut almost $190 million off the cost, bringing it down to about $275 million.
It is unclear how the options in the December 2014 cost report relates to the ISG rapid appraisal handed to the government seven months later, in July 2015.
ISG assessed six options, with the top-scoring option involving a 650 metre-long tunnel. But when costs and benefits were calculated, the option performed poorly, with costs significantly outweighing benefits. The Land Development Agency has released the ISG report but blacked out all cost estimates.
The government is now considering alternatives, looking for cheaper ways of bridging the divide that Parkes Way imposes between the city and the lake.
Land Development Agency chief executive David Dawes said, "Work is under way to further refine options and costs and the government will openly consult with the community on the options to inform development of a full business case."
