Premier Denis Napthine's pledge that the east-west link will be a ''congestion-busting'' project has been undermined by the government's own figures showing traffic on the Eastern Freeway will soar by as much as 50 per cent once the $8 billion tunnel is built.

As the Coalition embarks on a $3.2 million advertising blitz to spruik the controversial road, official forecasts reveal that inbound and outbound traffic on the suburban arterial will continue to rise, even after the project has been completed.

Figures are likely to prove contentious, because the government has repeatedly said the east-west tunnel would 'reduce' traffic. Credit:Rob Gunstone

''The strategic modelling results indicate that traffic volumes increase along the Eastern Freeway by between 40 and 50 per cent. The volumes increase along both carriageways by approximately the same percentage,'' says the comprehensive impact statement for the east-west link.

Asked about the figures, the Linking Melbourne Authority attributed the growth to more motorists using the Eastern Freeway after the tunnel is built because they will have another option to cross the city. In a bid to accommodate the extra traffic, the government plans to widen the freeway between Hoddle Street and Yarra Bend Road, building two additional lanes on part of the median strip that was reserved for a Doncaster rail line.