This study explores bicyclist behavior in San Francisco using data collected before and after major bike infrastructure investments. From early 2011 to December 2013, investments of $3.3 million correlated with a 14% increase in counts of bicyclists, part of a 96% increase in bicyclist counts from 2006 to 2013. To better understand the relationship between these investments and changes in bicycling behavior, the authors build on the successful GPS travel survey conducted in 2010 by the San Francisco County Transportation Authority (SFCTA). The authors used data from the smartphone based GPS data collection method (the CycleTracks application developed by SFCTA) which records bicyclists’ routes. In addition, they administered a detailed web-based survey to CycleTracks users in order to better characterize the factors associated with their bicycling behavior. The authors examine the relationship between bicycle infrastructure and behavior of bicyclists so as to assess the effectiveness of existing investments, and to provide guidance on efforts that are effective at increasing bicycling.