By Steven T. Jones

At the Share conference that I covered for this week’s Guardian, there were wildly divergent claims for how many vehicles carsharing companies such as City Car Share, Zipcar, and Getaround take off the road. I was also a little skeptical of claims that carsharing dramatically reduces overall driving and greenhouse gas emissions, so I decided to take a deeper look at the issue.

“For every car that is shared, we take seven cars off the road,” Board of Supervisors President David Chiu said during his presentation. The next day Getaround founder and spokesperson Jessica Scorpio cited a study claiming that 32 cars get taken off the road for every shared vehicle.

Luckily, one of the country’s top researchers in this area is right in our backyard. UC Berkeley civil and environmental engineering professor Susan Shaheen heads the school’s Transportation Sustainability Research Center and has been doing peer-reviewed studies on car-sharing for almost 20 years.

Her research, which is consistent with the body of academic research on carsharing from around the world, has found that each shared car takes between nine and 13 other cars off the road, figures that she says are amazingly consistent around the world. That big reduction is because households that have cars tend to get rid of at least one of them when they sign up for carsharing, while car-free households that want access to a car will choose (as Shaheen says is the case for about 25 percent of the people in each group, which adds up to 90,000-130,000 fewer cars on the road nationwide).

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