Bike share programs might seem like the ultimate environmentally-friendly mode of urban transportation. As more people hop on bikes, the thinking goes, the use of cars will drop.

But researchers have found that the math isn’t quite so simple. According to a new study, London’s bike share program actually increases the number of automobile miles driven per year, partly because trucks are needed to ferry bikes between stations.

Bike sharing systems are growing more popular around the world, with major programs now established in the United States, Europe, and Asia. Proponents argue that bike shares provide a cheap, convenient, healthy, and green way to get around the city.

But bike shares aren’t automobile-free. These programs need to ensure that each station is well stocked with bikes. That means that support vehicles such as trucks must “rebalance” the fleet by moving bikes from nearly-full stations to those that are running low. The question is: Does the reduction in car travel caused by bike sharing make up for the extra vehicle travel added to maintain the program in the first place?

To find out, the researchers examined programs in Minnesota, Washington DC, London, and Melbourne. The data included surveys of participants, who were asked what form of transportation they would have taken if they hadn’t used the bike share — for instance, a car, public transit, or walking.

In Minnesota, Washington DC, and Melbourne, the automobile miles added through fleet rebalancing were outweighed by those lost through bike sharing. Overall, motor vehicle use dropped by a combined 421,895 kilometers per year. But in London, the team found the opposite. Bike sharing reduced car use by about 633,000 kilometers per year, but fleet rebalancing added another 1.4 million kilometers per year.

London probably didn’t reap as many environmental benefits as the other cities because few people commute by car anyway. If the program hadn’t existed, most people would have taken public transit, walked, or used their own bike instead. The bike share still does some good — for example, it might reduce crowding on buses and the metro. But to boost its green cred, the program will need to lure more drivers away from their cars.

Roberta Kwok is a freelance science writer. Her article was originally published in Conservation Magazine and is reprinted with permission.

Study: Fishman, E., S. Washington, and N. Haworth. 2014. Bike share’s impact on car use: Evidence from the United States, Great Britain, and Australia. Transportation Research Part D doi:10.1016/j.trd.2014.05.013.

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Jul 25, 2014