By coincedence, I'm in Barcelona when this study hit the news. It shows the health benefits of a (well-planned, helmet law free) bike share programme. The Barcelona Bicing programme is attributed to saving 12.46 lives a year, each year since it's launch, as well as reducing carbon dioxide emissions.
Results Compared with car users the estimated annual change in mortality of the Barcelona residents using Bicing (n=181 982) was 0.03 deaths from road traffic incidents and 0.13 deaths from air pollution. As a result of physical activity, 12.46 deaths were avoided (benefit:risk ratio 77). The annual number of deaths avoided was 12.28. As a result of journeys by Bicing, annual carbon dioxide emissions were reduced by an estimated 9 062 344 kg.
Conclusions Public bicycle sharing initiatives such as Bicing in Barcelona have greater benefits than risks to health and reduce carbon dioxide emissions.
What is also important to consider is that a bike share system encourages private bicycle usage. It serves to reestablish the bicycle on the urban landscape. For example, in Paris, 2 million private bicycles have been bought since the city launched it's bike share programme, Vélib'. Which is why the comment by Simon Birkett, founder and director of Clean Air in London, in the Guardian article about the study shows the usual lack of vision regarding Bicycle Culture 2.0. A bike share programme won't save a city but it is one of the most important tools towards that end and worth every penny.
Here in Barcelona you see many Bicing in action but there are even more private bicycles on the streets and bike lanes.
Here's the link to the study on The British Medical Journal's website - The health risks and benefits of cycling in urban environments compared with car use: health impact assessment study