Seattle ? Facing rising gas prices, high insurance costs and hefty parking fees, Shana Lehar came to the conclusion that she simply couldn’t afford to own a car.

But Lehar didn’t 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 actually 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 Boston and elsewhere, says business is up 90 percent in a year, to around 25,000 participants as of August.

Rising gas 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’s a combination of financial factors that gets them to sign up.

Scott Griffith, chief executive of Zipcar, said many new members cited rising parking fees and insurance costs. He says car-sharing programs also are becoming popular just because more people are aware of them.

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

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

Car-sharing services work like this: In exchange for hourly and/or mileage-based fees, the service generally foots the bill for everything from gas 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 his Seattle neighborhood.

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

“I can’t imagine wanting a car,” he said.