News that the government is expected to announce trials of dedicated motorway lanes for cars with more than one occupant has sparked criticism.
Too many cars with too many empty seats?
Some environmentalists believe the government should concentrate on improving public transport; others have welcomed the idea as a sign of the government's commitment to cutting congestion.
Stephen Joseph, the director of the environmental pressure group Transport 2000, welcomed the initiative, but urged the government not to neglect other aspects of transport and not to use car-share as an excuse for motorway widening.
"The problem with general motorway widening is that traffic generally just fills up," he told BBC News 24.
"On the M25, when they widened it from six to eight lanes, traffic increased by a third in a year.
So I think by widening, but only for high occupancy vehicles, you can actually lock in the benefits of widening so that you actually don't have pressure for wider and wider motorways. That's what we are hoping anyway. "
He added that for the initiative to be successful, authorities had to encourage car-sharing through employers.
"They [the government] have done quite a bit on school transport recently, we've heard less about work with employers," Mr Joseph said.
"Yet we do know, because we have done some work with them, that there's a number of big employers... which have been promoting car sharing and have achieved quite a lot of success in reducing pressure on the company car park."
Ali Clabburn, founder of Liftshare, which arranges car shares for 56,000 members across the UK, said High Occupancy Vehicle (HOV) lanes were a "great idea".
He told BBC News Online Liftshare had been lobbying the government to support or at least pilot HOV lanes, but had had no idea such an announcement was in the offing.
There has been no support for people wanting to share cars and finally things are starting to happen. It's quite exciting."
"Anything that encourages more efficient use of the car is a good thing."
Mr Clabburn said surveys showed many commuters would be willing to share lifts to work if they could find someone suitable to share with.
He said it was time the government started to think about making more efficient use of cars rather than simply pouring money into public transport which might be used by only a fraction of the population.
But commuter Phil Jones, of Neath, said he thought the plan was just another way for the government to "hammer" motorists.
He said: "In theory, it's actually a good idea but in practice, many people are forced onto the roads in single occupancy vehicles purely because the public transport systems are so bad.
"For example, if trains ran to time, my journey to work would probably be 10 minutes less than it would be on the road but they're just too unreliable."
Mr Jones said he thought improving public transport should be the
Friends of the Earth spokesman Tony Bosworth told BBC News 24 it was important to encourage people to share cars.
He said the government should make the best of the space we have already got, because wider motorways would attract more traffic.
Friends of the Earth is also calling for car-sharing to be made easier for people, for example by employers helping to match workers who live in the same area.
But Paul Watters of the AA Motoring Trust said the plans would only affect a few hundred miles of the worse affected roads, and would not be cutting into vast areas of green land.
"It's simply to expand the motorways we already have, so in that context it shouldn't be too much of an environmental issue," he said.