Departure-time choice and mode choice for nonwork trips alternative formulations of joint model systems

Modeling travel demand by time of day is gaining increasing attention in the practice of travel demand forecasting. The relationship between time-of-day (departure-time) choice and mode choice for nonwork trips is investigated. Two alternative causal structures are considered: one in which departure-time choice precedes mode choice and a second in which mode choice precedes departure-time choice. These two causal structures are analyzed in a recursive bivariate probit modeling framework that allows random error covariance. The estimation is performed separately for worker and nonworker samples drawn from the 1999 Southeast Florida Regional Household Travel Survey. For workers, model estimation results show that the causal structure in which departure-time choice precedes mode choice performs significantly better. For nonworkers, the reverse causal relationship, in which mode choice precedes departure-time choice, is found to be a more suitable joint modeling structure. These two findings can be reasonably explained from a travel behavior perspective and have important implications for advanced travel demand model development and application.