Within an activity-based framework, the hypothesis that carpooling imposes a constraint on the way individuals organize their activities was tested, with resulting impacts on traffic peak congestion and trip-chaining behavior. The hypothesis was tested by estimating the joint probability density functions (PDFs) of subsistence, maintenance, and discretionary trips made by carpool and single-occupancy vehicle (SOV) users. Results show that whereas SOV maintenance and discretionary activities are linked to subsistence trips in a joint undertaking, carpool activities suggest discontinuity in the formation of trip chains. A comparison of the joint PDF of subsistence and discretionary activities reveals that trips are conducted either before or after the commute schedule; this results in a temporal shift that reduces peak-period traffic congestion and emission pollution. Marked differences are found to exist between SOV and carpool trip-chaining behavior. Carpoolers are more likely to engage in a greater number of cold-start trip chains; this behavior uncovers a potential negative impact on emission pollution. These findings suggest that a comprehensive approach to the evaluation of carpool programs must take into account the benefits as well as any ensuing negative externalities.