An artist's impression of a proposed B-Line stop at Collaroy. Credit:NSW Government. But the project's cost has since risen to almost $600 million. On numbers announced in the July budget, the program will cost at least $512 million. An "assurance review report" of the bus program's business case, prepared last year, sheds some light on why costs of the project have continued to increase. Nine stops to the city The proposed Northern Beaches B-Line bus service with added car park spots is scheduled to start operating at the end of 2017.

"The $305 million assumes nil cost for land acquisition for commuter car parks," the document, prepared by engineering and cost experts Brian Watters, Danny Graham and Peter Gill, says. "The [expert review panel] considers this assumption to be questionable." The report is also critical of the failure of authorities to make detailed estimates of the works required. "As a result, the ERP is not confident that value for money has been demonstrated for the proposed capital expenditure," it says.

The report obtained by Fairfax Media is a rare insight into how transport authorities prepare business cases and cost estimates for major projects. Even when the government releases summaries of business cases, they tend to be sanitised documents that reflect little of the substantive analysis that has gone into decision making. The cost increase on the B-Line bus project is just one of a number of a string of transport escalations. The WestConnex motorway, for instance, was first promoted as a $10 billion project but will now cost at least $16.8 billion. The light rail line through the city to the eastern suburbs was expected to cost $1.6 billion, but will now cost at least $2.1 billion. Fairfax Media revealed last week although no official price tag had ever been put on the project, the predicted cost of tram lines around Parramatta had risen to about $3.5 billion. Transport Minister Andrew Constance said the government wanted to give northern beaches residents good travel options, and the ability to get a bus home late at night.

"I will not apologise for giving commuters who sit in some of the worst traffic in Australia an alternative to being stuck in their car, and improving the lives of northern beaches residents," Mr Constance said. "The northern beaches has no train line, motorists are stuck in traffic for hours on end, so every single dollar we spend on this project is worth it," he said. "We have been finalising the scope over the last 12 months. It's obvious you can't have a final cost until you have a final project and that work continues." The B-Line project includes the construction of a number of commuter car parks along the northern beaches, some road works, and extra bus services. Buses are planned to run every five minutes in peak hour, and every 10 minutes during other times. The expert review panel report, however, questions the early promise of "rapid" transport times, pointing out that an early business case did not demonstrate how travel times would improve.

"In particular, it does not demonstrate improved outcomes in the congested section between Spit Bridge and Warringah Freeway," the report says. The report also said the needs and benefits of investing in commuter car parks along the corridor were "not adequately demonstrated". "The modelling does not appear to demonstrate that this initiative will generate more passengers on the BRT," it says. A spokesman for Transport for NSW said the project would deliver bus travel time improvements, "with the biggest benefit experienced between Seaforth and Neutral Bay". "The main benefit of the B-Line service, however, is reliability and frequency throughout the peak and off peak periods, where road infrastructure improvements and initiatives target congestion, reducing the variability of journey times to the CBD and overcrowding at bus stops during the peak," the spokesman said.