With most of the services, customers pay a small annual fee to join, then make reservations over the Web or with a smartphone app. They typically unlock the car by swiping a special laminated card across a sensor on the windshield, and rates are usually by the hour.

Amid all the competition, the rental providers are trying to differentiate themselves.

Zipcar, for example, has long tried to portray itself as part of a young, hip lifestyle, calling its members Zipsters and promoting itself through a Twitter hashtag, #thatswhereiroll. It promotes its vast number of rental locations, including many on university campuses, and its blend of ordinary and prestige cars, from Ford Escape S.U.V.’s to Mini Cooper convertibles.

Enterprise CarShare, which last year absorbed a Zipcar competitor called Mint, builds on Enterprise’s strong tradition of catering to businesses while offering customers the chance to try out newer technologies like Nissan’s all-electric Leaf.

And nonprofit groups are adding their own specialized services to the industry, like City CarShare in San Francisco, which in 2008 created the first wheelchair-accessible car-share vehicle. Called AccessMobile, the program offers minivans that accommodate two people using wheelchairs along with three other passengers and a driver. Ms. Shaheen said variety was a key part of the industry’s growth, like “going to the chocolate shop and having access to 15 types of different chocolate.”

Car2go’s chief executive, Nicholas Cole, said Daimler used the latest technology to provide cars almost instantly to members. For a $35 registration fee, Car2go members can locate and reserve a blue-and-white Smart microcar within 15 minutes. Members pay only a per-minute fee for the rental, and can park free in legal parking spaces in Washington and other participating cities. Car2go also lets members leave the car nearly anywhere in the city it is rented in.

By comparison, Zipcar members pay an hourly or daily rate that typically winds up being cheaper than Car2go’s rates, but they have to return the rented vehicle to the same parking lot where they picked it up. (BMW’s DriveNow service in San Francisco also allows one-way rentals.)

“Our members are not required to tell us how long they’re going to drive or where they’re going, as long as they bring it back to the whole area,” Mr. Cole said.