The absence of a structural relationship between trip generation and transportation supply characteristics has been seen as a major drawback of conventional transportation demand models. We respond to this criticism by developing a model of household activity demand that underlies and generates transportation demand. Time is considered as an argument in an activity utility function that the individual attempts to maximize, subject to a time-budget constraint. Transportation system improvements relax the time-budget constraint, providing opportunities for additional nonhome activities, with a corresponding generation of trips. Trip generation is found to be insensitive to transportation system performance in the short run. However, our model provides a framework in which the impacts of large transportation improvements could be modeled in the future.