Asked why the government had not achieved its aim, set out by John Prescott in 1997, of having more people using public transport and fewer using cars, Mr Darling said a buoyant economy was the reason.
'There is a tension between a growing economy, our wish to travel, whether for business or for pleasure, and making sure that we minimise the environmental damage,' he said.
But there was more use of public transport because more people are travelling by train, he said.
Mr Darling rejected Neil Kinnock's call for the UK to be brought into line with the rest of Europe and start using kilometers.
UK Metric Association report
'Of all the problems we face on transport, spending nearly£700m on converting our roadsigns to kilometres doesn't seem a number one priority, frankly,' Mr Darling said.