Timms, an environmental activist and education consultant, signed up for her membership before the company even expanded to the Champaign-Urbana area. She uses the cars for errands around town and board-meeting trips to Chicago.

"First comes Tonka Trucks then Hot Wheels," she said. "Before you know it, you're dreaming of a GTO, a Shelby Cobra or a Barracuda. Americans are obsessed with our cars."

Timms is an especially big fan of the Toyota Prius, one of several cars Zipcar has in her area. She said the average Zipcar user saves $500 a month versus owning a car.

"I'm an energy geek from way back," she said. "This is great for the environment. It means a lot fewer people will need to buy cars."

Competition Drives Market

But Zipcar isn't the only car sharing service out there.

Services such as I-Go Car Sharing in Chicago, PhillyCarShare in Philadelphia, and CityWheels Carsharing in Cleveland are just some of the organizations trying to reduce the number of cars on the road.

The effort goes as far as working with cities to ensure private developers incorporate car sharing into planned developments. PhillyCarShare has led the way among non-profits, introducing free memberships, a fleet of more than 50 percent hybrid vehicles and free rides on rail transit for users of vehicles parked at more than 40 stations.

Robin Chase, a Cambridge, Mass., resident who helped co-found Zipcar before leaving the company in 2003, is also offering her own twist on car sharing that taps into social networking.

GoLoco.org is designed to help people save on gas and cut carbon emissions by helping them find rides. In other words, true car sharing.

GoLoco does this by matching people with others going their way and providing drivers with online tools to collect payment for the costs of providing the ride, such as gas and vehicle wear and tear, with a few mouse clicks. GoLoco collects a 10 percent transaction fee to fund the site.

"My goal is to reshape the way people feel about car ownership," Chase said. "We're going to train people that driving alone should make you feel lonely and pathetic. And that sharing a ride with someone is way more fun, you save money and you get an environmental halo."

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