Facing rising gasoline prices, high insurance costs and hefty parking fees, Shana Lehar came to the conclusion that she simply could not afford to own a car.

But Lehar did not want to give up the luxury of an occasional ride to the grocery store or trip to the beach. So the Seattle resident became one of thousands nationwide to sign up for a car-sharing program, which promises easy occasional access to a car for a fraction of the cost of owning one.

"It just sounded like a really good deal," said Lehar, 23, who lives in the urban Capitol Hill neighborhood and expects to use Seattle-based Flexcar several times a month.

Car-sharing programs across the country like Flexcar say business is booming as more and more city dwellers balk at the cost of car ownership and look for alternatives.

Flexcar, which operates in more than 20 cities, has seen its membership grow to 22,000, from 16,000 a year ago. Zipcar, which operates in the Washington area, Boston and elsewhere, says business is up 90 percent in a year, to around 25,000 participants as of August.

Rising gasoline prices have been a great boon for car sharing, said Eliot Dobris, spokesman for San Francisco's City CarShare, a nonprofit service that has more than 3,000 members. But Dobris thinks it is a combination of financial factors that gets them to sign up.

Scott Griffith, chief executive of Zipcar, said many new members cite rising parking fees and insurance costs. He said car-sharing programs are also becoming more popular simply because more people are aware they exist.

A bad economy, or just a slow recovery, also can be good for the car-sharing business.

"We tend to be a little bit counter-cyclical, so as things get worse for consumers they tend to look for alternatives to save money," said Lance Ayrault, chief executive of Flexcar.

There are also personal circumstances that draw people to car sharing.

Michelle Bourassa, of Kirkland, Wash., needed to sell her car after a recent divorce. She has arranged a ride to work but expects she will use Flexcar for doctor's appointments, and when there is no other way to get her daughter to school. Still, Bourassa hopes to be able to save enough to buy another car soon.

Duc Tran, 32, of suburban Everett, Wash., owns a car but carpools into downtown Seattle for work. He recently signed up for Flexcar in case he needs a car during the workday for personal errands.

Businesses are getting into car sharing, too. Flexcar's business customers include Starbucks Corp. and the North American offices of Nintendo.

Car-sharing services work like this: In exchange for hourly and/or mileage-based fees, the service generally foots the bill for everything from gasoline to tuneups to insurance. Most rely on 24-hour phone and Internet reservation systems and park cars throughout a metropolitan area for pickup and drop-off.

Fees vary depending on the city and the type of service. Flexcar's Seattle service charges a flat fee of $35 a year. After that, users can pay $9 an hour for up to 30 miles, or opt for monthly plans. To use a car for 25 hours and up to 750 miles would cost $200, for example.

Because cars can be used on an hourly basis, car sharing has an advantage over rentals, which typically require a commitment of at least one full day.

For some users, car-sharing has become as necessary as owning a car once was.

David Shaw, 59, sold his car about four years ago when he joined Flexcar. His wife later sold her car and now the two rely exclusively on public transportation, bicycles and the occasional shared vehicle. Shaw estimates that they spend $200 to $300 a month on Flexcar -- about what he says the two would pay for parking alone in Seattle's Capitol Hill neighborhood.

It's not just cost savings that drives Shaw's choice. It's also the convenience of not having to look for a parking space or deal with the other hassles of maintaining a vehicle.

"I can't imagine wanting a car," he said.

David Shaw, a technical support specialist for nonprofit organizations, uses a keycard to unlock a Flexcar shared vehicle while making a service call in Seattle. Shaw enters his PIN on a keypad. Car-sharing programs nationwide are booming.