The least polluting cars on sale at the end of this decade will be so efficient that they match the cleanest of today's high-speed electric trains, Richard Parry-Jones told an intelligent transport conference in London this morning.

Parry-Jones, one of the industry's most-respected engineers, predicted that by 2020, the car industry is targeting just 40g/km of tailpipe carbon, which translates to 25g/km per passenger when the average number of occupants in cars - 1.6 - is factored in.

That number is comparable with today's most-efficient electric trains; "that's a remarkable development when you think that the typical average was 140g/km in 2000," said Parry-Jones.

He also predicted that intelligent cars that inter-communicate with each other and traffic management systems are the future of the car industry.

"Networked vehicles under digital control are going to become the future of car manufacturing," he said.

The former Ford engineer is now head of the industry-government Automotive Council and chairman-designate of Network Rail.

Also speaking at the conference, transport minister Justine Greening, said the British road network needed upgrading to cope with a potential 44 per cent increase in traffic.

"A traffic increase like that would put our country under a crippling strain," Greening said. Around £1.5bn will be spent on national and local road improvements with a further £2.3bn earmarked for major roads.

However, future road improvements may hinge on public-private finance initiatives. "With the Treasury we are looking at attracting more finance from private investors,' Greening added.

The government is trying to pursuade British pensions funds to invest in infrastructure in return for reliable, long-term financial returns. "We also need to use these roads more intelligently," she said.