There are a number of variants on the theme, however.
Pioneering firms such as ZipCar offer their own vehicles, much like a traditional rental company. But in a different take on the so-called sharing economy, RelayRides, for example, partners with individual owners who want to earn some money by renting their vehicles.
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RelayRides recently joined with OnStar to make it easier for a customer to gain access to a car, which might be parked in an owner's driveway, a commuter train station or some other spot. An OnStar app allows an authorized customer to unlock the vehicle and drive it away, keeping track of billing in the process.
"The advent of mobile apps and vehicle connectivity is driving growth in car-sharing services," said Navigant, which is based in Boulder, Colo.
Lisa Jerram, Navigant's senior research analyst, said car sharing's broader appeal is that it "offers members the ability to enjoy mobility without the expense and hassle of owning a car, or the need to frequently rent a vehicle from a traditional car rental agency."
The market's growth is expected to accelerate significantly, according to the study. A handful of brands now dominate the market, and traditional companies have rushed to stake out a position. For example, Avis bought ZipCar in March.
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According to its website, ZipCar has more than 810,000 members and more than 10,000 vehicles in an expanding global network that now includes operations in the U.S., Canada, the United Kingdom and Spain.
Because the services' members largely rely on mass transit, said Jerram at Navigant, "car sharing is viewed by both public and private entities as a powerful tool to reduce urban congestion and lower emissions of greenhouse gases."
—By CNBC Contributor Paul A. Eisenstein. Follow him on Twitter @DetroitBureau or at thedetroitbureau.com.