Will simply building more public transport seriously suppress car use?

The study also concluded it would increase the share of all trips carried by public transport in the metropolitan area in 2046 from 12.6% to just 12.7% i.e. by 0.1% (see Do new suburban rail lines always make sense?).

Melbourne Metro: The latest estimate of the cost to build the planned 9 km tunnel under the CBD is $11 Billion. According to the Government, the project will provide capacity for an extra 20,000 passengers during the peak. That's a significant increase in CBD peak capacity (it's bigger than the Westgate Bridge) and there'll be network-wide benefits in terms of greater reliability. So it's worth doing.

But it's the veritable drop in the bucket in terms of generating mode shift given the size of the total metropolitan transport task.

Although I don't have formal mode change estimates, the proposed $2 - 3 Billion Melbourne Airport rail line would replace the existing SkyBus (note Brisbane's established Airtrain has just 8% mode share). Brisbane's proposed $5.5 Billion Cross River rail line, like Melbourne Metro, is aimed at increasing peak capacity through the CBD.

Why small impact?

My point isn't to deny the benefits mega projects bring to public transport users; most of the current raft of rail projects touted by State Governments are worthwhile improvements. Rather, it's to point out that even very costly rail projects - and this group collectively would cost $25-30 Billion - won't by themselves have a big impact on car use.

Why would so much money have such a small impact on mode split?

In part it's because most new rail projects serve work trips to the CBD, where congestion and high parking charges mean public transport's mode share is already very high e.g. 80% of motorised trips in Sydney.

Most importantly though, it's because Australia's capital cities are highly suburbanised; more than 90% of the population and around 70% of jobs are more than 5 km from the CBD. Moreover, only around 20% of trips are for the purpose of getting to work.

Even shiny new rail lines can't compete well with the car for local and relatively short trips in the suburbs e.g. for shopping, personal business, school.

Given the breath-taking cost of retro-fitting rail it's implausible to suggest that low density Australian cities can effect a significant shift away from driving just by building more and more public transport infrastructure.

Nevertheless, while they wouldn't look much like the Paris Metro, Australian cities can and should have modest but effective metro-style systems that work with our low density cities. These would be based largely on existing train lines networked with a "grid" of frequent bus/light rail services operating with priority in existing road space.