Metro rail systems such as Auckland's proposed $2.4 billion link from Britomart to Mt Eden do nothing to reduce congestion in the long run, says a visiting international transport expert.

"With metro, all you do is create extra capacity," Dr Dinesh Mohan told the Traffic Institute at its annual conference in Auckland today.

"Then, after two years, all the roads are congested again - and the metro is full.

"You just increase transport, you don't reduce congestion."

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Dr Mohan, who specialises in transport safety and pollution research and has published more than 200 scientific papers, said a study from his Delhi-based Indian Institute of Technology found underground rail systems generated more carbon dioxide emissions than diesel buses when tallying up the amount of energy needed to manufacture their infrastructure.

Only 25 per cent of the "life-cycle" energy costs of underground passenger trains went on running them, but that left the production of concrete, steel and other infrastructure components contributing the remaining 75 per cent.

"Putting anything underground increases carbon dioxide," he said.

"The only way to reduce carbon dioxide is to reduce road area, there is no other way."

One way to do that was to allocate a lane along every road for buses, and another for cyclists and pedestrians.

Squeezing the amount of road space available for cars, and reducing parking lots, was also the only way to encourage mass public transport.

"You must have congestion for the public to use public transport - if you don't have congestion, you would be very stupid to use public transport, because you could get there faster by car."

Dr Mohan told the Herald after his presentation that Auckland was not unique in putting its aspirations for underground rail ahead of other transport projects.

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"That's what they [cities around the world] always do - they delay all these other projects for one fancy project."

Told about Auckland Transport's goal of making trains circulate through the central business district rather than having to back out of Britomart, he wondered whether the planners had considered running buses in a circuit instead.

Asked where London would be without its Underground, he said that was an unfair question as the system was built in the 19th Century when there were no buses, which did not become efficient people-carriers until the 1950s.

London was now making up for that by adding bus lanes to many of its roads, narrow though they were.