Rand has come up with some interesting ideas on how to increase it.
An excerpt, with several more links after the jump.
Most Americans fail to meet national recommendations for physical activity, and inactivity is associated with numerous health problems, including obesity and diabetes. The majority of Americans live in areas that have parks and recreational facilities—both of which represent promising settings for physical activity—but research suggests that parks are underutilized, especially by seniors and adults. Park use is declining, too.
Increasing physical activity in parks could help improve public health, but promoting physical activity may be difficult in a tight fiscal environment in which local parks departments are likely to see their budgets cut, leaving them with very limited resources. With this constraint in mind, a RAND research team worked with 50 Los Angeles parks and recreation centers to determine if low-cost outreach and marketing could promote physical activity.