Melbourne's Lord Mayor, Robert Doyle, says the trial of a dedicated bike lane across the Princes Bridge has been a success.

One northbound lane between St Kilda Road and Flinders Street Station has been set aside for cyclists for a three-month trial.

Lord Mayor Doyle says their research shows traffic on the bridge is about a minute slower in the morning peak, but cyclist movement is up 40 per cent.

He maintains the bike lane is vital for the safety of cyclists.

"I am absolutely convinced, looking at that video of the old days, if we hadn't done something then we'd be facing what they face in London and that's a fatality about every three weeks," he said.