Background In September 2005 London introduced a policy granting young people aged < 17 years access to free bus and tram travel. A year later this policy was extended to people aged < 18 years in education, work or training. This intervention was part of a broader environmental strategy in London to reduce private car use, but its primary aim was to decrease ‘transport exclusion’, and ensure that access to goods, services, education and training opportunities were not denied to some young people because of transport poverty. However, there were also likely to be positive and negative health implications, which were difficult to assess in the absence of a robust evidence base on the impact of transport policies on health and well-being. Objectives To evaluate the impact of free bus travel for young people in London on the public health. Specifically, to provide empirical evidence for the impact of this ‘natural experiment’ on health outcomes and behaviours (e.g. injuries, active travel) for young people; explore the effects on the determinants of health; identify the effects on older citizens of increased access to bus travel for young people and to identify whether or not the intervention represented value for money. Design Quasi-experimental design, using secondary analysis of routine data, primary qualitative data and literature reviews. Setting London, UK. Participants Young people aged 12–17 years and older citizens aged ≥ 60 years. Intervention The introduction of free bus travel for those aged < 17 years living in London in 2005, extended to those aged < 18 years in 2006. Main outcome measures Quantitative: number of journeys to school or work; frequency and distance of active travel (i.e. walking and/or cycling), bus travel, car travel; incidence of road traffic injuries and assaults and socioeconomic gradients in travel patterns. Qualitative: how free bus travel affected young people and older citizens’ travel and well-being. Methods Quantitative component: change-on-change analysis comparing pre–post change in the target age group (12–17 years) against that seen in ‘non-exposed’ groups [for travel mode, road traffic injury (RTI) and assaults]. Qualitative component: interviews analysed using both deductive and inductive methods. Economic evaluation: cost–benefit analysis (CBA). Data sources London Area Transport Survey (LATS) and London Travel Demand Survey (LTDS) (travel mode); STATS19 Road Accident data set (RTI); Hospital Episode Statistics (HES) (assaults); interviews with young people and older citizens; and cost data from providers and literature reviews. Results The introduction of free bus travel for young people was associated with higher use of bus travel by adults and young people [31% increase, 95% confidence interval (CI) 19% to 42%; and 26% increase, 95% CI 13% to 41%, respectively], especially for short journeys, and lower car distances relative to adults (relative change 0.73, 95% CI 0.55 to 0.94); no significant overall reduction in ‘active travel’ [reduction in number of walking trips but no evidence of change in distance walked (relative change 0.99, 95% CI 0.92 to 1.07)]; significant reduction in cycling relative to adults (but from a very low base); a reduction in road traffic injuries for car occupants (relative change 0.89, 95% CI 0.84 to 0.95) and cyclists (relative change 0.60, 95% CI 0.55 to 0.66), but not pedestrians; an overall modest increase in journeys to work or school (relative change 1.09, 95% CI 1.06 to 1.14); equivocal evidence of impact on socioeconomic gradients in travel behaviour and no evidence of adverse impact on travel of older people aged > 60 years. An increase in assaults largely preceded the scheme. Qualitative data suggested that the scheme increased opportunities for independent travel, social inclusion, and a sense of belonging and that it ‘normalised’ bus travel. The monetised benefits of the scheme substantially outweighed the costs, providing what the Department for Transport (DfT) considers ‘high’ value for money. Conclusion The free bus travel scheme for young people appears to have encouraged their greater use of bus transport for short trips without significant impact on their overall active travel. There was qualitative evidence for benefits on social determinants of health, such as normalisation of bus travel, greater social inclusion and opportunities for independent travel. In the context of a good bus service, universal free bus travel for young people appears to be a cost-effective contributor to social inclusion and, potentially, to increasing sustainable transport in the long term. Further research is needed on the effects of both active and other travel modes on the determinants of health; the factors that influence maintenance of travel mode change; travel as ‘social practice’; the impact of driving license changes on injury rates for young adults and the value of a statistical life for young people. Funding The National Institute for Health Research Public Health Research programme.