This study evaluates changes in travel mode shares in California over the period from 2012, when the California Household Travel Survey (CHTS) was most recently completed, to 2017, the most recent implementation of National Household Travel Survey (NHTS). Initial review of the data suggests decreases in biking and walking over this time period. This study explores the factors contributing to this change, namely are these apparent changes the artifact of methodological differences between the analysis of the two surveys, or do they reflect real changes in the travel behavior of Californians? This piece also explores external factors, or changes over time that may contribute to mode share changes, such as demographic shifts or system-wide shocks such as the Great Recession. There are many differences in the preparation of the data used in the analysis across the two surveys; for example, the variables included in weighting are not the same for the CHTS and the NHTS. These differences are not found to have an impact on the outcomes of interest; however, they do suggest the need for more coordination among the NHTS and the CHTS to better enable comparative studies.