I've taken my own advice. About a year and a half ago, I wrote a piece on car-sharing and have since gone over to it myself.

Our old car, which used to spend 95 percent of the time in the parking lot (we're Manhattanites), is now in the hands of Habitat for Humanity, where we trust it is seeing better use. Our new car is any one of some 30 vehicles in the neighborhood (and many more in the city at large), which we share with other New Yorkers who don't drive much either. All are members, like ourselves, of a car-sharing service called Zipcar.

The cars we share include Hondas, Toyotas, Mazdas, Subarus, Volvos, BMWs, Volkswagens, Mini Coopers and Fords. Most are on the smaller side, which is how I like them, but there are a couple of SUVs, too, that might come in handy if I ever have 10 guests in town that need trucking around. The hourly price, including gas, insurance and taxes, ranges from $13 to $15, depending on the car. For longer reservations, there's a day rate.

To arrange for a car, I log on to my Zipcar account online, see what's available, and reserve whichever one fits my fancy for however long I want it -- which can be as little as one hour. At the appointed time (next month or the same afternoon), I walk to the parking lot a few blocks away, unlock the door by swiping my Zipcar ID card over a reader on the windshield, and I'm in. The key is waiting in the ignition.

Availability is rarely a problem. It is Tuesday morning at 10 am right now, and there are more than 20 cars available for 11 am, as well as for the upcoming weekend. The only time I have had cause for worry was the Thursday before Mother's Day weekend when there was one car available in the entire city for Saturday, and I was still uncertain about my plans. The thing to do in such cases is reserve it, as the car can be canceled the night before (or sometimes that morning).

Most Saturdays, however, it's easier to get a Zipcar at the last minute than a dinner reservation.

For someone with an urban lifestyle like mine, which only occasionally requires a car, car-sharing is a near-perfect arrangement:

- It saves money. I don't pay for insurance, inspection, repairs, a permanent parking spot or the vehicle itself.

- It saves hassles. Upkeep and maintenance are someone else's headache now.

- It provides an incentive to drive less and walk, bike and take public transportation more.

- I don't waste environmental resources (the metal, water and energy that went into the manufacturing of a car) on something I hardly use.

- It's a way of advertising my values rather than my income (not that my old car ever really did that!).

Interestingly, despite the fact that car-sharing dispenses with one of the central tenets of the American economy and culture -- private ownership -- it is far from a revolutionary idea. In the words of my mother's husband, Leon, a retired insurance executive with a keen mind for business, "it's like a hotel." When people visit a place very occasionally, it doesn't make sense for them to buy a home there, so they rent a room for the duration of their stay and pay for the amount of time they use it. Sizable fortunes have been built catering to this need, which is a squarely American outcome if ever there was one.

Unlike Leon, I do not have a mind for business, keen or otherwise, but his comparison got me to thinking...does anything else work this way? I thought of Netflix, which led me to muse on whether this business model is primarily a new-fangled thing. Then I remembered Blockbuster, and that reminded me of lending libraries, which go back centuries. So this is an old idea now applied to cars.

But is it an idea whose time has come? I hope so -- because more shared cars mean less traffic congestion, cleaner air, less global warming pollution and fewer wasted resources. And I can now say from experience, rather than research, that it's convenient and cost-effective for the individual, too.

—Sheryl Eisenberg

This Green Life map update: Many readers added their favorite nature spots to our map over the last month. There are some great recommendations in there. I hope you'll view the results below and add a favorite place of your own.

ONLINE RESOURCES YAHOO FINANCE Car Sharing Gains Traction CNN Tech, Profits Drive 'Car Sharing' SLATE Key Party MSN MONEY Should You Share a Car? SMART MONEY Car-Sharing: Hit the Road for Less CARSHARING.NET Where Can I Find Car Sharing UC BERKELEY Circulating Libraries and Video Rental Stores IN THESE TIMES Bike-Sharing Is Caring

For city-dwellers, options abound. Car-sharing services from commercial behemoths like Zipcar and smaller groups, including local non-profits, are available in cities and college towns across the U.S., from Austin to Cleveland to Gainesville.

Bike-sharing also available. Bike-sharing originated before car-sharing, but the early programs ran into trouble with theft and vandalism. Today's programs have gotten around the problems with electronic key systems and deposits that guarantee payment for damaged or missing bikes. Paris has developed the largest program to date with some 10,000 bikes, Beijing is planning one in time for the Olympics and many cities in the U.S. have, or are experimenting with, them.