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By Phil LeBeau

Car sharing companies, which started gaining traction in cities about a decade ago, are increasingly becoming a threat to new vehicle sales according to a new study.

Alix Partners, a consulting and business advisory firm, says a half million vehicle purchases in the U.S. have been avoided due to the growing popularity of car sharing programs like Zipcar and Relay Rides.

Furthermore, the study forecasts that by 2021 car sharing could replace an additional 1.2 million purchases of new vehicles.

"Our study suggests that Americans' willingness to avoid vehicle purchases due to growing car-sharing options is higher than many have thought," said Mark Wakefield, managing director at AlixPartners and leader of the firm's Automotive Practice in North America.

Wakefield believes the growing popularity of car sharing companies is a trend auto makers cannot afford to dismiss.

Car sharing in cities accelerating
Unlike many previous studies about the popularity of car sharing, the Alix Partners study specifically looked at how quickly these programs are impacting auto purchases in ten cities.

The study suggests that one car-sharing fleet vehicle displaces 32 vehicles that would have otherwise been purchased.

That replacement rate is greater than other studies have found when looking at car share programs on a national level.

Wakefield adds, "This study suggests that car sharing nationally could scale up as these ten markets have, and if that happens, the impact on the traditional automotive market could be explosive."

Several automakers, including Mercedes-Benz and BMW have already launched their own car sharing programs.

Mercedes launched its Car2go program with the idea giving clients in cities the opportunity to rent a Smart Fortwo compact car.

—By CNBC's Phil LeBeau. Follow him on Twitter @LeBeauCarNews.

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