Investing in big urban transport infrastructure projects, especially motorway and rail projects, is the common response to the growth pressures faced by our major cities. But big transport projects are extremely expensive, and governments at best only budget for one every five years or so. It’s also not clear that all big projects provide a significant improvement outside their immediate area.

Consider, for example, stage one of the East-West Link motorway proposed for Melbourne’s inner-northern suburbs. It’s expected to cost $6 billion to $8 billion to construct and may monopolise new transport infrastructure spending for some years. Data East-West Link Traffic Impact Assessment, a report prepared by consultants GHD for the Linking Melbourne Authority, shows that building the East-West Link would increase the total number of trips on a weekday in metropolitan Melbourne by 6000. It would also increase total metropolitan travel by 800,000 kilometres per day and reduce aggregate travel time by 4300 hours per day. These aren’t trifling changes; for example, if time is valued at (say) $50 an hour, the economic value of the forecast saving in travelling time would be nearly $80 million per year by 2031.

But when looked at in the context of total metropolitan travel, the impact of the East-West Link would be decidedly modest. The number of trips in Melbourne would increase by just 0.04% (i.e. by four one hundredths of one percent); kilometres of travel by 0.4%; and aggregate travel time would reduce by 0.11%.

There would be much bigger changes at the regional scale, but they’re relatively modest too. The East-West Link would increase traffic flows and average speeds at key locations within the orbit of the proposed motorway by only 2%-4% relative to the no-build case. Speeds on the East-West Link itself would be much faster in the off-peak (e.g. average 20-minute saving from Chandler Highway to CityLink, with reduced variability), but the motorway would hardly be the “game changer” for the city as a whole the Premier claims it will be.

This isn’t just a roads issue; there’s a similar story with some proposed rail lines. Consider the Rowville rail line promised for Melbourne’s outer south-eastern suburbs in new metropolitan strategy Plan Melbourne. It’s likely to cost at least $4 billion to construct but by 2046 would only increase public transport’s mode share in the metropolitan area by 0.1%. Moreover, 57% of patronage would be siphoned away from other rail lines and it would reduce the number of car trips on a typical weekday in 2046 by a trifling 15,000. Modelling for the Doncaster rail line, which is also promised in Plan Melbourne, indicates it would have virtually no impact on public transport’s mode share at either the local or metropolitan scales.

Governments need to find ways to increase the infrastructure build rate without threatening other important areas like education. They also need to ensure they build the best projects.

Most importantly, though, they need to think much harder about how to make the huge stock of existing metropolitan-wide infrastructure work better, e.g. by implementing road pricing. Just building major infrastructure projects isn’t likely to be enough.