Background: The Integrated Transport and Health Impacts Model (ITHIM) is a scenario-based risk assessment tool that quantifies the health benefits and harms of physically active travel (walking and cycling), road traffic injuries, and fine particulate air pollution in urban transportation systems.

Methods: Descriptive statistics on travel patterns, physical activity, traffic injuries, and car emissions were derived from statewide travel and health surveys, collision databases, and outputs from regional travel demand and emissions models. The change in disease burden was measured in deaths and disability adjusted life years (DALYs) based on dose–response relationships from meta-analyses and the distributions of physical activity and traffic injuries. Alternative scenarios were measured against baseline travel patterns experienced in each major California region. Alternative scenarios included increases in active travel from baseline to 20 median minutes/person/day, apportioned entirely to walking (“all walk”), cycling (“all cycle”), and or transit-related active travel (“all transit”). The health benefits and greenhouse mitigation of these scenarios were compared to those of the preferred scenarios regional transportation planning agencies. These agencies are mandated to demonstrate greenhouse gas reductions in their transportation plans (“Sustainable Communities Strategies (SB375)”, which emphasize transit expansion to achieve this goal.

Results:The preferred scenarios increased statewide active transport from 41 to 54 min/person/week, which was associated with an annual decrease of 890 deaths and 15,053 DALYs. The ambitious, maximal alternatives increased population mean travel duration to 283 min/person/week for walking, bicycling, or transit and were associated a reduction in deaths and DALYs from 2.5 to 10 times greater than the California preferred scenarios. The alternative with the largest health impact was bicycling, which led to 8,349 fewer annual deaths and 141,597 fewer DALYs, despite an increase in bicyclist injuries. With anticipated population growth by 2040, no alternative achieved decreased carbon emissions, but bicycling had the greatest potential for slowing their growth.

Conclusions: Expansion of transit confers important health benefits through active transport and meets important societal goals for destination accessibility. However, expansion of walking and cycling, independently of transit, can play a larger role in improving population health.