This study examines the travel behaviour impacts of major investments in active travel infrastructure in three Outer London boroughs (the ‘mini- Hollands programme’). At all survey 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. The 20- year health economic benefit in high-dose areas from three years’ of interventions (costing £80 million) is £724 million. This is a signifcant return in investment and highlights the value of rolling out similar interventions elsewhere.