There has been a great deal of concern about obesity, with many calls for Americans to increase physical activity. In spite of all the attention and exhortations, the Outdoor Industry Foundation reported that bicycling is declining, having dropped from 3.9 billion outings in 2004 to 3.1 billion outings in 2005 (Outdoor Industry Foundation, 2006). The most dramatic drop in outings was for American females, who averaged 18 paved road biking outings in 2005 compared to 28 in 2004. Although new bike paths and roadways are frequently being built, there are few studies using objective measures in the United States that prospectively document increases in physical activity in response to environmental changes (Morrison et al., 2004; Killoran et al., 2006). Studies of the association between bicycling and the built environment have typically been cross-sectional (Nelson and Allen, 1997; Dill and Carr, 2003) and when change in the built environment is assessed for its impact on physical activity, studies have included repeated cross-sectional self-reports (Ogilvie et al., 2006), retrospective accounts of use over time (Boarnet et al., 2005), or simulations of what is expected (Niemeier, 1996).