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D E S I G N F E AT U R E S O F A C T I V I T Y- B A S E D M I C R O S I M U L AT I O N M O D E L S 15 DAY-PATTERN TYPE LINKED EXPLICITLY ACROSS even more. One of the key linkages is fairly simple: if HOUSEHOLD MEMBERS each person's full-day activity pattern is classified into three main types--stay at home, go to work or school, or This and the following three sections are concerned with travel for some other purpose--then strong similarities the modeling of explicit linkages between the predicted can be seen between the patterns of members of the same activities and travel of different members of the same household, ones even stronger than the similarities that household. All the models treat such linkages implicitly would be predicted indirectly. The Columbus model sys- through the use of a wide variety of person type and tem includes a sequential model of these linkages, simu- household composition variables, and indeed one of the lating children first and then adults conditional on what main advantages of the microsimulation approach is the the children do. The Atlanta model system includes a ability to reduce aggregation bias by including such case- similar model that is estimated simultaneously across all specific variables. The use of explicit linkages takes that household members, avoiding the need to assume the ability one step further and reduces aggregation bias order in which they are simulated and thus the direction shopping, household and/or personal business, social LEVEL AT WHICH INTERMEDIATE STOP recreational, eat out, and other serve passenger activi- PURPOSE AND FREQUENCY ARE MODELED ties) and three for children (school, joint discretionary activity with parent, and independent discretionary Activity travel patterns are modeled separately for activities). CEMDAP does not distinguish between workers (adults who go to school or work on travel types of in-home activities. day) and nonworkers (adults who neither go to work nor attend school during the day). The daily pattern of workers is characterized by four subpatterns: INTRAHOUSEHOLD INTERACTIONS AND before-work pattern, which represents the activity- EXPLICIT ALLOCATION OF ACTIVITIES travel undertaken before leaving home to work; com- mute pattern, which represents the activity-travel In CEMDAP, the activity generation and allocation pursued during the home-to-work and work-to-home decisions are simulated in the following three sequen- commutes; work-based pattern, which includes all tial steps: (a) the generation of work and school activ- activity and travel undertaken from work; and after- ity participation, (b) the generation of children's travel work pattern, which comprises the activity and travel needs and explicit allocation of escort responsibilities behavior of individuals after arriving home at the end to one of the parents, and (c) the generation of inde- of the work-to-home commute. Within each of the pendent activities for personal and household needs. before-work, work-based, and after-work patterns, Linkage of joint activities, travel, or both is imple- there might be several tours. Each tour, the home-to- mented between parents and children (in single-parent work commute, and the work-to-home commute may and nuclear-family households) in two ways: drop off include several activity stops. In the case of nonwork- at or pick up from school and joint discretionary activ- ers, the activity-travel pattern is considered as a set of ities. Due to data limitations, the nature of these inter- tours, each of them comprising a sequence of out-of- actions is currently restrictive. For instance, CEMDAP home activity stops. does not consider the case of one of the parents drop- The number of tours is predicted at the subpattern ping off or picking up multiple school children at level for workers (pattern level for nonworkers), while multiple locations. There is also an "other serve- the tour mode and the stop frequency are predicted at passenger" activity type recognized in CEMDAP, but the tour level. The activity purpose, activity duration, the activity-travel pattern linkage across household home staywork stay duration before the activity, travel members is not now explicitly implemented for this time to the activity stop, and destination are predicted activity type because of lack of data. for each of the individual activity stops. In essence, the The grocery shopping activity is modeled to be gen- stop purpose is modeled at the stop level, and the stop erated at the household level and is allocated to one of frequency is modeled at the tour level. The purpose and the adults. Joint participation of adults in activities is frequency of stops are modeled conditional on a higher- currently not considered because of lack of good data level choice of each person to undertake activities of to estimate these models in the DallasFort Worth, various types (activity generation models). Texas, area. (continued on next page)