The financial case for the east-west link hinges on a prediction that toll road use will jump over the next 30 years because of rising wealth and shrinking petrol and CBD parking price rises.

A cabinet-in-confidence document, obtained by Fairfax Media, for the first time details key assumptions used to justify the $6 billion to $8 billion project, which the state government claims will produce a return of $1.40 for every $1 invested.

Hoddle Street overpass looking east showing the new East West Link stage one.

A discussion paper produced by VicRoads, the Linking Melbourne Authority, the Transport Department and Public Transport Victoria, reveals the government was able to boost its predictions for the road by as much as 15 per cent using a controversial assumption that time will be more valuable to future motorists because of rising wealth.

The report, used to prepare the highly secretive business case for the road, says the methodology ''has not been used in any of [the Transport Department's] other public transport projects or program modelling to date''.