One of the benefits of living in a major city is that I rarely have need of a car. Public transit and my own two feet take me nearly everywhere I go. Yet I do keep a car for visits to my family in the suburbs and heavy-duty shopping near home. It hardly seems worth the cost of insurance, but how else to have a car when I need it?

I've discovered there is a way, called carsharing.

This is not the same as carpooling, which involves sharing rides. With carsharing, you get exclusive use of a car during the slots you reserve it for -- not necessarily the same car every time, but a clean, well-maintained one. In this sense, it's a lot like car-renting, except that you don't have to pay for a whole day when you only want an hour.

Nor do you have to travel to the ends of the earth just to pick up the car. Carsharing services locate their vehicles near where people live and work. For instance, Zipcar, the local service in New York, has about 20 cars stationed within a few minutes' walk from my home. Four of them are even closer than the lot where I park my own car!

If you keep a car, but don't use it much, the potential savings from ditching it and sharing instead are substantial. In my own case, a rough calculation shows I'd save $3,500 a year -- on parking, insurance, gas and maintenance. And that leaves out the prorated cost of the car itself. (This is more, I believe, than most people would save, as I drive particularly little and live where the costs of keeping a car are particularly high.)

There are a variety of carsharing services in cities and university towns around the country. Some, like Zipcar, are quite extensive; others are small, with limited fleets (which means less likelihood of a car very near your home). The plans differ in the details, but the bigger ones generally work like this:

1) You pay a deposit and/or small annual fee to join the service and pick a plan based on the number of hours you think you'll drive per month.

2) You receive a personal electronic key in the mail.

3) You reserve a car, online or by phone, when you need it.

4) You go to the location you selected and use your electronic key to identify yourself and unlock the car, where you find the key you need to drive.

5) You return the car to the same location when you're done.

6) At the end of the month, you pay for the time you used the car and/or mileage. The costs of insurance, maintenance and gas are covered by the service.

What's the point, environmentally?

Studies show that households who join carsharing groups end up driving less -- and causing less air and noise pollution as a result. The reason appears to be simple economics. For people who already own a car, driving tends to be cheaper than taking public transit (because the fixed costs of owning don't enter into the equation -- only the variable costs, such as gas, which are relatively low). The reverse is true for carsharing. While the membership fee (which is the fixed cost) is low, the hourly price to use the car is comparatively high. So carsharers tend to cut back on driving.

To the extent that carsharers' reduced driving translates into greater demand for mass transit, there are other beneficial effects. Increased demand can help to increase long-term investment in the transit system, which can help to attract more ridership, which can reduce driving further, which cuts pollution more...all in all, a very positive little cycle.

Should carsharing ever really take off, it could also reduce production of new cars. While manufacturing cars has far less environmental impact than using them does, it does involve mining and the use of significant amounts of energy and water -- as well as a wide range of toxic substances, including lead and PVCs. Keeping unnecessary cars out of the production stream would therefore be a very good thing.

Of course, for the majority of people in the United States today, cars are anything but unnecessary. Poor land use in most communities makes it impossible to get anywhere without them.

But the situation is different in many American cities, from Chicago to Atlanta, Seattle to Boston, San Francisco to Washington D.C., and many smaller towns with good mass transit and healthy downtowns. If you live in such a place and rely on your car very little, I suggest you look into carsharing. You may find it serves your own interests as well as the common good.

—Sheryl Eisenberg

ONLINE RESOURCES SEATTLE POST-INTELLIGENCER Flexcar Gives Many Drivers a Thrifty, Easy Alternative SEATTLE POST-INTELLIGENCER A Drive Toward Fewer Cars CARSHARING.NET Where Do They Have It? DISTRICT DEPARTMENT OF TRANSPORTATION Carsharing Video COALITION FOR SMARTER GROWTH Car Sharing SURFACE TRANSPORTATION POLICY PROJECT Transportation and the Environment WIKIPEDIA Carfree Places

Carsharers end up using alternative transportation more.

Who is carsharing for? It works best for people who drive under 6,000 miles a year (or under, say, 10,000 in cities where the costs of keeping a car are high). Businesses with only occasional need for cars can also benefit.

There are limits. Carsharing services often have minimum age requirements for drivers, which means children of driving age may not be able to use the car -- a deal-breaker for my family, unfortunately. Many services also prohibit pets except in pet carriers.

We need to clear the air. Fifty-three million Americans are exposed to so much particle pollution in the air from cars and other sources that their health is at risk, according to the American Lung Association's 2006 State of the Air Report.

Cities should be for people, not cars -- and some cities are. In Siena, Italy (above), cars are banned. With no car-carrying roads or parking lots, the city is very compact. Everything is in walking distance, kids run free and people of all ages interact more. In American cities, carsharing could reduce the amount of space devoted to cars, leaving more room for walkways, bikeways and parks.