Walking, cycling can have mixed benefits.
The study, which focused on San Francisco, examined health benefits of transportation strategies to cut greenhouse gas emissions. It also looked at travel patterns, injuries, physical activity, air particulate matter and greenhouse gas emissions in the San Francisco area.
Increasing walking and biking from four to 22 minutes cut heart disease and diabetes risk by 14 percent, but it also increased traffic injuries by 39 percent.
The study's authors said increasing physical activity could have a significant improvement on population health, but only if measures can be put in place to minimize pedestrian and bicyclist injuries. (Page 703)