InterventionPhysical inactivity is an important behavioral risk factor for chronic disease in Canada. Individual-level strategies are used in clinical medicine to target individuals for preventive intervention based on one or more risk factors. In contrast, this study examines the impact of a population-level intervention: a municipal policy outside the healthcare sector that influences the built and social environment.Research questionWhat is the preventive effect of a municipal transportation policy to increase active transportation on a chronic disease outcome measure—diabetes incidence—when it is viewed as a population-level health intervention to increase physical activity?MethodsThe impact of increases in active transportation for regular commuting to work in the city of Ottawa, Ontario was modeled to estimate number of diabetes cases prevented over 10 years. As a health-sector comparison, the reduction in incidence was equated to an individual-level approach to prevention targeting those who are inactive, meant to represent a clinical preventive intervention.ResultsThe population-level policy shift could prevent as many as 1620 incident cases of diabetes over 10 years, the largest number prevented by increases in public transit use. This population effect was equal to 17,300 inactive individuals or 12,300 inactive individuals > 45 years old undertaking a clinical preventive intervention to increase physical activity.ConclusionThe results demonstrate why public health matters today as population-level interventions that exist as policies outside the healthcare sector, supported by public health, may have an unrecognized and therefore underappreciated impact on population health.