The population of the Greater Toronto-Hamilton Area (GTHA) is projected to grow from 6.2 million people in 2001 to 9.6 million in 2031. Existing commute times are the country's highest with annual costs of congestion to reach $15 billion. A proposed 25-year, $50 billion Regional Transportation Plan (RTP) would build a regional rapid transit network, expand active transportation (AT), and build communities that are AT- and transit-supportive. Medical Officers of Health of GTHA communities issued a call to action for government investment based on the estimated health-related benefits of implementing the RTP. To calculate increases in transport physical activity, the RTP's projected increase in public transit use was combined with conservative estimates of increases in AT to work and school by adults (5%), as well as a substitution of 5% of local vehicle trips ≤7 km with AT. The associated number of prevented deaths and incident cases of diabetes were calculated using the Health Economic Assessment Tool and the Diabetes Population Risk Tool, respectively. Data inputs included regional travel data from transportation and census surveys, crude mortality rates, and literature-based walking estimates with transit use. Adjustments were included for the duration of the academic year and seasons. A value of a statistical life (VSL) was $6.5 million and the lifetime excess medical costs of a diabetes case of $24,500. Previous Toronto-based estimates for annual premature deaths and hospitalizations attributed to traffic-related emissions were extrapolated to the GTHA based on population. Full implementation of the RTP would prevent 338 deaths per year in the current population with a VSL-based cost of $2.2 billion. The number of projected prevented deaths due to increased physical activity (184) and reduced vehicle emissions (154) were similar. The annual number of prevented cases of diabetes due to an increase in physical activity was 1,061 with a projected lifetime excess healthcare costs of almost $26 million. The projected health impacts of the RTP are substantial, yet conservative since they consider only mortality and diabetes incidence. The results provided an additional health-based argument for government investment, as well as signaled public health's interest in building healthier communities. In addition to initial media coverage, the report has been repeatedly cited at local, provincial and national levels. While impossible to directly attribute causality, subsequent government policies have included strengthening the accountability for integrated land use and transportation planning, as well as substantial investment in the RTP.