Relationship between transit's usual and actual mode shares

One major source of transit mode share at the national level is the journey -to-work data from the U.S. Census Bureau's Decennial Census surveys conducted since 1960. These data can be used to determine transit's usual mode share, that is, the share of workers who state that they usually use transit for commuting. The relation of transit's usual mode share to its actual mode share, that is, the share of work trips made by transit as revealed by respondents to daily travel surveys, was studied. Theoretically, a simple aggregate model of workers who commute to work by transit or by nontransit means is built to establish a theoretical relationship between transit's usual and actual mode shares. This model establishes a necessary and sufficient condition for transit's usual share to be greater than its actual share. Empirically, the FHWA's 2001 National Household Travel Survey is used to measure transit's usual and actual mode shares for 34 transit market segments defined by 14 personal, household, and geographical characteristics. For each of the 34 transit market segments, the empirical results show that the necessary and sufficient condition is met and that transit's usual mode share is greater than its actual mode share. Furthermore, the degree to which transit's usual mode share is greater than its actual share is significant, ranging from 14.7% to 87.3%. Differences of such magnitude are too large to be ignored.