This paper presents a joint model of walking and bicycling activity duration using a hazard based specification that recognizes the interval nature of time reported in activity-travel surveys. The model structure takes the form of a multilevel hazard-based model system that accounts for the range of interactions and spatial effects that might affect walking and bicycling mode use. In addition to the individual-specific factors, family (household-specific) interactions, social group (peer) influences, and spatial clustering effects are also considered as potential factors that contribute to heterogeneity in non-motorized transport mode use behavior. The model system presented is capable of accommodating grouped duration responses often encountered in activity-travel surveys. A composite marginal likelihood estimation approach is adopted to estimate parameters in a computationally tractable manner. The model system is applied to a survey sample drawn from the recent 2009 National Household Travel Survey in the United States. Model results show that there are significant unobserved family-level, social group, and spatial proximity effects that contribute to heterogeneity in walking and bicycling activity duration. The unobserved effects were also found to have a differential impact on bicycling activity duration, thus suggesting the need to treat and model walking and bicycling separately in transportation modeling systems.