Eric Britton, Robin Chase and others have written eloquently about the urgency for making massive changes in our transportation system. What will offset a big chunk of that car use is pretty clear: a lot more walking and cycling, increased use of public transportation, ridesharing, plus a dash of carsharing!
My guess is this will probably involve some high tech solutions - cell phones and social networking services, electric bikes, etc. - but mostly it's going to be a lot more old fashioned shoe leather and bike tires on the pavement. But right now we have a lot of wonderful solutions and a big gap between what we're doing and where we want to be.
But what combination of factors will motivate a large number of people to change their these transportation habits is a mystery. Carsharing is indeed a "missing link", but it's only one of a number of "missing links" in the chain.
Recently, the City of Portland is sponsored the latest in a series of Sunday Parkways bike rides, attracting thousands of people, families, and kids for a ride through several neighborhoods. Cross streets are controlled and it's a veritable party attracting thousands of people over a 5+ mile circular route. The other part of event is scheduling community groups, food vendors and musicians of all kinds to set up in the parks along the route. It is truly inspiring to see so many people on bikes and it seems can't help create awareness and experience of cycling in a broader section of the population than the "hard core bike commuters". It's a small, but I think important step in the cultural shift in our thinking and transportation habits. You can see a nice Streetfilms video of the first event last year here.
The start of such a shift was reflected in an interesting essay by a Canadian newspaper columnist who characterized the change in the change in their attitude about cars from "object of desire" to "necessary evil". I think this captures one of the cultural shifts perfectly - for so many people cars are now becoming a necessary evil.
Another cultural shift I've been thinking about could be a basic change in the type of cars we own - instead of the maximum car we can afford, we would spend our money on the minimal car we need for 90% of the trips we take. This minimal car wouldn't have to be a "econobox". Daimler has shown there's a market for a $15,000 city car and Europe has got a wide variety of upscale microcars! For now, car ownership in the US is probably too cheap and awareness of the impacts of ownership too limited to see this happening very much.
What's needed now is some good minds to think about the intermediate steps to facilitate this cultural shift and conditions to promote them. With the passage of climate change legislation in the US and a new commitment to addressing environmental problems, the next decade is going to be interesting.