BACKGROUND: We used the introduction of free bus travel for young people in London in 2005 as a natural experiment with which to assess its effects on active travel, car use, road traffic injuries, assaults, and on one measure of social inclusion, total number of trips made. METHODS: A controlled before-after analysis was conducted. We estimated trips by mode and distances travelled in the preintroduction and postintroduction periods using data from London Travel Demand Surveys. We estimated rates of road traffic injury and assault in each period using STATS19 data and Hospital Episode Statistics, respectively. We estimated the ratio of change in the target age group (12-17 years) to the change in adults (ages 25-59 years), with 95% CIs. RESULTS: The proportion of short trips travelled by bus by young people increased postintroduction. There was no evidence for an increase in the total number of bus trips or distance travelled by bus by young people attributable to the intervention. The proportion of short trips by walking decreased, but there was no evidence for any change to total distance walked. Car trips declined in both age groups, although distance travelled by car decreased more in young people. Road casualty rates declined, but the pre-post ratio of change was greater in young people than adults (ratio of ratios 0.84; 95% CI 0.82 to 0.87). Assaults increased and the ratio of change was greater in young people (1.20; 1.13 to 1.27). The frequency of all trips by young people was unchanged, both in absolute terms and relative to adults. CONCLUSIONS: The introduction of free bus travel for young people had little impact on active travel overall and shifted some travel from car to buses that could help broader environmental objectives.