If you're not using that car, can I rent it?

Paul A. Eisenstein, CNBC Contributor
Kelvin Ma | Bloomberg | Getty Images

If you want to get a hint about the potential for car-sharing programs, consider that Hertz, Avis and other traditional auto rental firms have purchased pioneers in the short-term rental market, such as ZipCar, over the past couple of years.

Apparently, such moves will be well-rewarded. A new study by Navigant Research predicts that the number of car-sharing subscribers will grow from 2.3 million in 2013 to more than 12 million in 2020. Global revenues should make an even bigger leap, from an expected $1 billion to $6.2 billion.

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Car-sharing services offer an alternative to the traditional rental approach by targeting those who might need a car for as little as an hour. Customers usually access vehicles at unstaffed lots or reserved parking spots.

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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