Background Improving transport infrastructure to support active commuting could help to promote physical activity and improve population health. We aimed to assess whether investment of this kind was associated with an increase in active commuting; determinants of the use and uptake of active commuting; wider health impacts of changes in travel behaviour; and how changes in behaviour were distributed in the population and related to social context.

Methods The intervention was the Cambridgeshire Guided Busway, a new bus network using 22  km of guideway (segregated track) accompanied by a traffic-free path for pedestrians and cyclists, opened in 2011. This quasi-experimental evaluation comprised a cohort study of 1143 adults working in the city and recruited in 2009, along with both nested and supplementary in-depth quantitative and qualitative studies. The primary outcome was the change in time spent in active commuting from 2009 to 2012, using a self-reported measure validated using georeferenced combined heart rate and movement sensor data. Exposure to the intervention was defined using the shortest distance from each participant’s home to the busway, and served as the basis for controlled comparisons.

Results Commuting practices were complex and shaped by various social and environmental factors. Walking and cycling were often incorporated into longer commuting journeys made by car or public transport. In multivariable multinomial regression analyses, exposure to the intervention was associated with a greater likelihood of an increase in weekly cycle commuting time (RRR 1.34, 95% CI 1.03 to 1.76) and with commensurate shifts in the distribution of commuting trips between the car and active travel. There was some evidence that the effect was most pronounced among those who reported no active commuting at baseline. There was a mixed pattern of effects at the individual level, with the intervention providing a more supportive environment for active commuting for some and not for others. Although experiences of the busway were complex, they culminated in meaningful behaviour change for some users, through shifts in the balance between influential factors and planning, trialling and adopting new practices over time. Observational evidence suggested a relationship between active commuting, greater overall physical activity, and improved wellbeing and weight status.

Conclusion These findings provide new empirical support for reconfiguring transport systems to improve population health and reduce health inequalities. They should be cumulated with evidence from natural experimental studies in other settings, using longer periods of observation and controlled comparisons, to support more generalisable causal inference.