The Infrastructure Australia analysis has also raised two weaknesses with the business case for the WestConnex motorway, which has not been released by the NSW government. The business case reviewed by Infrastructure Australia does not account for extra traffic trips taken because of the motorway, nor for how traffic could change as a result of the motorway. According to Infrastructure Australia, these "induced trips" were a major negative aspect of the proposed East West Link motorway in Melbourne, which was ultimately revealed to have a benefit cost ratio returning just 45 cents for every dollar spent. "The proponent intends to refresh its analysis allowing for induced and redistributed traffic in an update to the business case," Infrastructure Australia said of the WestConnex proposal. Another weakness in the business case is that it does not make the usual allowance for cost blow-outs. Infrastructure Australia reviews business on what is called a P90 estimate, which means that there is a 90 per cent chance the actual cost will be below the estimate. But the WestConnex case reviewed by Infrastructure Australia was prepared on only a P50 estimate.

The federal body says it is confident the project should still be of overall benefit, even when it is prepared with tougher cost estimates. "There is a degree of confidence that following an adjustment to the BCR for P90 and any negative adjustment due to induced trips, the BCR will remain positive," Infrastructure Australia's assessment says. The secrecy surrounding the business case is one aspect of the fierce hostility to the motorway proposal that has emerged in pockets of inner west Sydney. The Labor opposition has said the case should be released and, furthermore, it would not build the third section of WestConnex, a tunnel linking St Peters and Rozelle. The Infrastructure Australia assessment says parts of the project do not have as much benefit as the motorway as a whole. "This means that benefits are mainly realised from 2023 onwards following the completion of the entire project," the assessment says. Greens transport spokesman Mehreen Faruqi said it was irresponsible of the federal government, which has already handed over $500 million for the motorway and which is promising another $1 billion grant and a $2 billion loan, to approve the money before completing an assessment of the motorway.

"By estimating costs at P50, rather than Infrastructure Australia's guidelines of P90, there is a huge risk that the project costs will blow out by hundreds of millions of dollars," Dr Faruqi said. "The evidence is stacking up that basic principles of cost estimation and traffic modelling are being thrown aside by both the Federal and NSW Government in their obsession to build toll roads at any cost," she said. A spokeswoman for WestConnex said modelling for "induced demand" was not required under state or national guidelines, and the P50 analysis was consistent with the approach taken on comparable projects. "We are pleased with the recognition that the business case for the project is sound," the spokeswoman said.