The estimated relative risk for all cause mortality associated with physical activity among the residents of Barcelona who travelled by bicycle (Bicing initiative) compared with by car was 0.80, resulting in an attributable fraction of 0.23 avoided deaths in the Bicing population who had shifted mode of transport from the car. An estimated 12.46 deaths were avoided each year. The relative risk of all cause mortality related to the incremental inhalation of particulate matter less than 2.5 µm was 1.002. The corresponding mortality attributable fraction was 0.002, leading to an estimated 0.13 expected annual number of deaths from air pollution in the Bicing population. The relative risk of road traffic crashes for the Bicing population compared with car users was 1.0007, with an associated attributable fraction of 0.0007, resulting in 0.03 extra deaths per year from road traffic incidents in the Bicing population. As a result, 52.15 deaths would have been expected each year, but because cycling was used as a typical means of transport, the number of annual deaths was reduced by 12.28 to 39.87 (table 2).
Table 2 Main results from health impact assessment of Bicing initiative in Barcelona
The results were most sensitive to the variations imposed on the relative risk associated with physical activity and mortality, derived from a previous study,21
the average duration of the trip by bicycle, the number of days travelled by bicycle per year per person in Barcelona, and the proportion of cyclists who started cycling when Bicing was implemented. These four variables, in that order, had the highest correlation with the combined mortality and led to the greatest variations in net number of deaths per year in the new cyclist population (see figure and web extra figure 7). Using black smoke instead of particulate matter less than 2.5 µm as a proxy for air pollution slightly reduced the negative effects of cycling (0.04 v
0.13 deaths/year) (see web extra table 4) despite the greater contrasts between exposures during cycling and travel by car, because of the lower relative risk reported in the limited literature on black smoke. Applying a factor of 5 to account for the assumed higher toxicity of traffic related exposure to particulate matter less than 2.5 µm14
increased all cause mortality to 0.52 people each year (see web extra table 5). When the effects of varying the input variables within the ranges specified in the web appendix were analysed, the effect on all cause mortality ranged between 4 and 22 deaths avoided.
Sensitivity analysis tornado plot. Centiles for relative risk (RR) for all cause mortality associated with physical activity refer to arithmetic increase of confidence intervals
As the results were shown to be most sensitive to variations in the relative risk from the physical activity benefits of cycling, the levels of cycling activity were converted to hours of metabolic equivalent tasks a week and the relative risk functions applied from general physical activity (not specifically cycling) reported in a meta-analysis.28
This analysis showed similar benefits among the current population (−12.46 v
−11.65 deaths). Additional sensitivity analyses were based on age and shift of travel mode scenarios, considering only physical activity, the main driver of the results. Assuming a younger age population distribution of Bicing users (average 33 years) an estimated 7.43 annual deaths would be avoided instead of the 12.46 from the baseline analyses, whereas assuming a higher age distribution (mean 48 years) an estimated 20.55 annual deaths would be avoided (see web extra table 10 and figures 8-10). Data on shifts in mode of travel as a result of the Bicing initiative could not be found, but using an alternative scenario with 60% of Bicing users having shifted from public transport, 30% from walking, and 10% from car, had little effect on the number of deaths avoided (10.46 v
12.46) (see web extra table 9).
The annual reduction in carbon dioxide emissions resulting from implementation of the Bicing initiative in Barcelona was estimated to be 9062 metric tonnes (see table 2).