Objective and subjective constraints that trip makers face are analyzed by using a trip-by-trip multivariate unbalanced panel analysis. These constraints emerge from trip makers' stated reasons and dispositions for why a given mode was not used for their trips. A finite set of behavioral dimensions (situations), which are general, system, and service constraints, lack of information about the modes, negative disposition toward a mode, and time, comfort, and cost considerations were derived from open-ended questions on the respondents' stated reasons. The presence of these situations, for each trip a person made in a day, is explained in terms of social and economic characteristics of the trip maker, place of residence, household resources, and trip characteristics. As expected, stated reasons for not using a specific mode vary with respect to the mode chosen, and they change within a day in a nonlinear manner depending on a person's schedule. Illustrated is another facet of unobserved heterogeneity represented in the radically different individual perceptions, which have been recognized as important components in dynamic behavior simulation studies.