Zip along with shared cars, bikes

Zipcar and Flexcar. They sound like a couple of cartoon characters. But last week's merger of these member-based, by-the-hour car-rental companies points to a noteworthy development in transport: car-sharing as a way to replace car-owning and to cut costs, energy use, and congestion.

Subscriber-based bike-sharing is also gaining traction, thanks to a popular program that started in Paris in July. The City of Light has had such success with its 10,000 rental bikes (the first half hour is free), that it's doubling the program – and catching the attention of Chicago, San Francisco, London, and Montreal.

This trend in personal transit-sharing may appeal to only a minority of people. With 200 million cars in the US today, car-share users are projected to top out at about 2 million (Zipcar, the name of the merged company, now has only about 200,000 users).

So the effects on US vehicle congestion and pollution are limited. And with an expected explosion of car sales in China and India (get ready for Tata Motors' $2,500 four-door, the world's cheapest automobile), the world needs many solutions to improve energy efficiency and reduce oil use.

On the other hand, automakers have yet to deliver a safe and affordable hydrogen-fuel cell car, and Congress has yet to force higher prices on fossil fuel to curb its use. "Just-in-time" cars and bikes can be available to individuals now, or in the not-so-distant future, in cities, at businesses, and at colleges (a growth market) – with a minimum of investment and inconvenience.

Car-sharing works best in urban areas and as a complement to mass transit. For now, Zipcar is profitable only in its established markets. That the big-name car rental companies are following it, though, shows it's on to something.

The big appeals to users are the handy locations, cost-savings, and guaranteed parking. Members pay an annual fee for gas, insurance, parking, and repairs. They reserve cars – many of them hybrids – over the Internet or by phone, paying rates starting at $7.65 an hour. They unlock the cars through smart cards, and pick them up and return them at the same location.

A study by Susan Shaheen and Adam Cohen at the University of California, Berkeley, found car-share use growing in Europe, North America, Asia, and Australia, and yielding reductions in vehicle ownership, distance traveled, greenhouse gas emissions, and costs for users.

The Paris bikes work on similar principles – after users subscribe, they access a bike from a locked stand via card-swiping and can later drop off the bike at any of 450 stations. In the first two months, the city logged 3.7 million rides.

Both of these sharing trends have their challenges: safety issues when many more bikers are involved (and bike gluts at the bottom of hills!); securing enough parking spots and paying the high cost of insuring young drivers for car-sharing companies. But responsive city governments can designate and expand bike lanes and set aside more parking for car-sharing.

In India and China, up-and-comers want to turn in their bikes and scooters for cars. But congestion could encourage car-sharing. And who knows? Perhaps Manhattan's corridors, over time, will more closely resemble Beijing's bike-filled boulevards of 25 years ago.