Introduction: This paper analyses three years’ data from the People and Places longitudinal study. The study examines the travel behaviour impacts of major investments in active travel infrastructure in three Outer London boroughs (the ‘mini- Hollands programme’). Methods: The People and Places survey, conducted annually in May-June, treats the mini-Holland interventions as a ‘natural experiment’. Participants in other Outer London boroughs form a control group. The survey had over 3,000 respondents at baseline (May-June 2016). Three follow-up waves each had over 1400 repeat respondents. A difference-in-differences analysis was used to compare changes in active travel uptake in intervention and control groups. Further analysis examines likelihood of meeting targets for past-week active travel and all physical activity. Finally, the article conducts a health economic benefit assessment based on uptake of active travel at Wave 3. Results: At all waves, living close to mini-Holland interventions (‘high-dose’ areas) was consistently associated with increased duration of past-week active travel, compared with the control group (44.0 extra minutes in Wave 2, 41.0 in Wave 1, and 41.5 in Wave 3). Changes in active travel behaviour were stronger in the high-dose area than in the low-dose area. Most of the increase was in walking. People living in high-dose areas were 13% more likely at Wave 3 to achieve 140 minutes active travel than people in control areas. People living in high- or low-dose areas in mini-Holland boroughs were more likely to be physically active for 5 days in the past week. The 20- year health economic benefit in high-dose areas from three years’ of interventions (costing £80 million) is £724 million. Conclusion: Ambitious interventions can yield substantial health economic benefits from changes in active travel. This includes early uptake of walking as well as cycling. Most of the increase was in walking but the ratio varied by year.