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⢠The research used the ATUS, which is conducted by the Census Bureau under contract with the Bureau of Labor Statistics. The sample is drawn from a subset of households responding to the Current Population Survey interviews. One individual, age 15 years or older, is selected from each household. A 1-day time- of- use sur- vey is collected from these individuals. An elaborate three- tier activity classification scheme is used. Data are also collected for weekdays and weekend days. The sam- ple for analysis is large. It includes 412,611 episodes from 20,720 persons in the 2003 survey and 279,042 episodes from 13,973 persons in the 2004 survey. One limitation with the use of ATUS for examining joint activity participation is the lack of time use information for the respondentsâ companions. While this limitation does not allow assessing the impacts of time constraints of all individuals on the joint time investment decision, it is possible to examine the impacts of individuals and household socioeconomic characteristics and day of the week and seasonal factors. ⢠The ATUS includes classifications of companions for household members and nonhousehold members. Household members include a spouse, unmarried part- ner, children, grandchildren, and parents. Other house- hold members include siblings, other related people, foster children, housemates or roommates, roomers or boarders, and other nonrelatives. Nonhousehold mem- bers include nonhousehold children, parents or parents- in- law, and other nonhousehold family members. Additional nonhousehold members include friends, coworkers, colleagues, clients, neighbors or acquain- tances, other nonhousehold children, and other non- household adults. ⢠The total number of episodes of each activity type in the sample and the percentage of joint activities by each type were examined for weekdays and weekend days. During weekdays, 32% of all in- house episodes are joint activities, while 35% of all weekend days in- home episodes are joint activities. Some activities, such as care- giving and socializing, are by their nature always joint activities. Other activities, such as personal care, sleep- ing, work, and school, are defined as solo activities. Eat- ing and drinking and watching television and listening to music are the most frequently reported joint in- home activities. These activities include both household and nonhousehold members. ⢠The survey results indicate that out- of- home activ- ity episodes are more likely to be joint activities than in- home episodes. Some 47 percent of all weekday episodes and 71 percent of all weekend activities are joint episodes. Socializing and serve- passenger are considered to be joint activities, while work and school are consid- ered solo activities. Eating and drinking, leisure, and reli- gious, civic, and volunteer episodes are most likely to be pursued with household and non household individuals. ⢠The survey results indicate that 42% of weekday travel episodes and 62% of weekend travel episodes are with other people. Some 60% of all joint travel is under- taken with only household members. Travel with non- household members is more likely to occur on weekends than on weekdays. ⢠A review of the survey results indicates some gen- eral trends. Joint activity- travel participation levels are significantly high on both weekdays and weekends. The levels of joint participation vary by activity type. For in- home episodes, nonhousehold members are more likely companions during weekends than weekdays. For out- of- home travel episodes, nonhousehold members are more likely companions during weekdays than week- ends. Joint leisure activities are more likely to be pursued with nonhousehold members than joint maintenance activities. ⢠The results also indicate that episodes of longer duration are more likely to be jointly undertaken with other persons. Weekday joint episodes are more likely to involve only nonhousehold companions than weekend episodes. Caucasians are more likely to pursue joint activities. Employed persons and students are likely to pursue leisure activities with colleagues. Marital status and presence of children in the household negatively impact undertaking leisure with only nonhousehold persons. ⢠The study results suggest that the social aspect of travel behavior is not currently suitably accommodated in travel forecasting, which could potentially lead to erroneous forecasts of responses to policy actions. The empirical analysis undertaken in this study using ATUS highlights the continued and critical need to explicitly incorporate interpersonal interactions in travel model- ing. Enhancements in travel surveys might include activity- type classification schemes and querying of activity and travel companions for both household and nonhousehold members. AN INNOVATIVE METHODOLOGICAL FRAMEWORK TO ANALYZE THE IMPACT OF BUILT ENVIRONMENT CHARACTERISTICS ON ACTIVITY- TRAVEL CHOICES Jessica Guo and Chandra Bhat Jessica Guo described a methodological framework to analyze the impact of the built environment on activity. She provided an overview of land use and travel demand interactions and discussed some of the issues associated with understanding the relationship between land use and travel demand. She presented a proposed methodol- ogy for analyzing the impacts of the built environment on activity and described the results of an empirical 46 INNOVATIONS IN TRAVEL DEMAND MODELING, VOLUME 1