Modeling personal travel behavior is complex, particularly when one tries to adhere closely to actual casual mechanisms while predicting human response to changes in the transport environement. There has long been a need for explicitly modeling the underlying determinant of travel- the demand for participation in out-of-home activities; and progress is being made in this area, primarily through discrete-choice models coupled with continous-duration choices. However, these models tend to be restircted in size and conditional on a wide variety of other choices that could be modeled more endogenously.