A recent study claims that while bike sharing has some real benefits such as improved health; increased transport choice and convenience; reduced travel times and costs; and improved travel experience; there is "no evidence" that it reduces congestion, carbon emissions or pollution. In addition benefits tend to go to younger, more socio-economically advantaged males. This is particularly troubling since some places, DC in particular, have utilized Congestion Mitigation and Air Quality Improvement money to fund bike-sharing; and Montgomery County has tapped into a Jobs Access and Reverse Commute grant which is designed to help welfare recipients and low-income persons seeking to obtain and maintain employment. If Miriam Ricci, a researcher at the University of the West of England’s Centre for Transport and Society, is correct about these claims, that might put future funding from these programs at risk.
I haven't read the study since it's behind the paywall and I'm not made out of money, but a NextCity article on it has more details than the abstract does. It doesn't appear that Ricci gathered any new data, but rather relied on "existing studies and surveys", which I don't say to make it sound invalid.
Ricci found that bike-share users don’t bike instead of driving so much as they bike instead of taking transit or walking. She writes that, “although Dublin bikes users reported considerable behavioral change, the prevailing trend showed a large modal shift (80.2 percent) from sustainable modes of travel to the bicycle, particularly from walking (45.6 percent) and including transfer from bus (25.8 percent) and rail (8.8 percent).” Still, nearly 20 percent of Dublin bikes users say they now drive less. Other European and American cities saw far lower rates of mode shift. In London only 2 percent of users shifted away from cars. In Lyon, France, and Washington, D.C., it’s 7 percent.
If each bike causes 22-28% as much congestion as a car, depending on how that compares to transit or walking, when a person moves from transit or walking to biking that might increase congestion - of course it's more than offset by each person who moves from driving. Another source of congestion could be the rebalancing:
bike-share can actually contribute to congestion with the vans and trucks they use to redistribute bikes in the system. Those motorized fleets also harm bike-share’s environmental benefit.
I know that in 2011-2012, CaBi reportedly drove their vehicles about 140,000 miles, a lot of it outside of congestion time periods I suspect. So the contribution to congestion and pollution by rebalancing might not be that much. But these are good questions to ask, still when they say "Ricci found zero evidence that bike-share leads to any significant reduction of carbon emissions" I wonder what is "significant," because if there's significant enough benefit from a health or transportation choice standpoint, then those benefits - though "insignificant" - are still gravy. And they might be enough to continue using CMAQ and JARC money (something Ricci probably doesn't care about).
Ricci argues that bike advocates should focus on the benefits for which more compelling evidence is available.
According to Ricci’s paper, a recent survey of London bike-share’s active users found that 78 percent started to ride or ride more as a result of the system. Similarly, 68.4 percent of sampled bike-share users in Dublin claimed, “not to have cycled for their current trip prior to the launch of Dublinbikes” and 63.4 say they purchased a private bicycle after using bike-share.
Ricci also found encouraging evidence that shows bike-share is good for user health...
Which is all well and good, if bike-sharing systems are content to find other funding.
I suspect that a more detailed study - focused on the US, where there is more driving, instead of Dublin - would show that there are modest, but real, congestion, air pollution and transportation equity benefits from bike-sharing.
Update: A related study adds to the common sense claim that active commuting is good for your health
Despite evidence that more compact settlement patterns enable active commuting, only a small share of workers in these areas choose to walk or bike to work. In general, the activity level of residents in more compact cities and residents in more sprawling areas is very similar. But, there is a robust association between active commuting and lower body mass index that is not explained by unobserved attributes or preferences suggests that policies to promote active commuting may be effective. In particular, active commuting has a greater effect on BMI. Consequently, compact settlement appears to be an effective infrastructure for promoting more active lifestyles. The policy challenge is finding ways to ensure that this infrastructure is more widely utilized.