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14 I N N O VAT I O N S I N T R AV E L D E M A N D M O D E L I N G , V O L U M E 1 A key design issue at the person-day level is the A major advantage of microsimulation models is number of activity and tour purposes to be considered. that the analysis is completed at the zone level. The zone The early versions of the Portland model and the San level can be used for origin and destination matrices, Francisco model include three activity purposes. The skims such as zone-to-zone travel times, and travel level other models have at least seven activity purposes. These of service. A more detailed level of spatial aggregation activity purposes typically include work, school, escort, can be used for transit access times, walk times, and shopping, meals, personal business, and social and recre- pedestrian environment factors. ational. Most models do not distinguish between differ- The Sacramento model can predict travel demand ent types of in-home activities, including work and at the parcel level. There are 700,000 parcels in the nonwork activities. model. A building level or parcel level is also being con- A key feature of most of the models is the ability to sidered in the Denver model. A hybrid approach of being model a person's daily activity pattern. This feature able to use both a zone level and a parcel level may be a includes how many trips individuals make for different logical approach. The model design is similar for these trip purposes. In addition, some models include the pres- applications. ence of extra stops by purpose, the allocation of stops to The models also allow accessibility from the land particular tours, and the presence of work-based sub- use and travel system to affect every single decision in the tours. Only a few models include in-home activities. model system, not just mode choice and destination The least consistent aspect of the different models choice, but trip chaining, making more tours, automo- is how tour complexity and trip chaining are measured. bile ownership, and work location. A traditional log- Some models consider these elements at the upper level sum approach, where the whole model system is one of the model and all of the tour and trip decisions cas- large nested decision structure, would not be able to cade off of this level, which results in a lot of substitution accommodate these elements, because it would take a at the top level. Other models predict tours at the upper very long time to run the model. Determining good log- level of the model. These models predict if the individual sum or accessibility measures that can be used at the makes stops and the purpose of the stop. upper-level tour generation day pattern models and car Four types of household linkages are included in ownership models may be needed in some models. Fairly some of the models. First, the main pattern type for each detailed log-sums can be used for work and school desti- person in the household is explicitly linked. A second nations. Precalculated zonal mode and destination linkage is joint tours with household members traveling choice log-sums by segment can be used for transit acces- together, which are generated separately for individual sibility, automobile availability, and household income. tours. A third linkage allocates some activity purposes-- such as escorting or chauffeuring, shopping, and other maintenance--between household members. A fourth NEW YORK METROPOLITAN TRANSPORTATION linkage uses escorting as a tour or stop to pick up or drop COUNCIL TOUR-BASED MODEL DEVELOPMENT someone off with a different activity purpose. These link- ages add complexity to a model. The Columbus model is Peter Vovsha and Kuo-Ann Chiao the only model that includes all four linkages. Time-of-day models include simultaneously pre- Kuo-Ann Chiao and Peter Vovsha described the devel- dicting the time that an individual arrives at work and opment and use of the New York Metropolitan Trans- the time that he or she leaves work. A question is how portation Council (NYMTC) model. They discussed the narrow the time period should be for these models. One need for a new travel model, the key elements of the to 2 hours seems appropriate. At the trip level, where an model, and applications of the model. Volume 2 includes individual may make intermediate stops along the tour, a paper on this topic.2 The following points were cov- there is a need to predict what time the individual leaves ered in their presentation. each stop. In this case, a smaller interval, typically 15 or 30 minutes, is needed. The benefit of these types of mod- Work began on developing a new travel model in els is that they provide consistent tours across the day. the New York region in the late 1980s. One of the limi- Most of the models have fully consistent time windows. tations of the old model was that it stopped at the Hud- Every time an individual is predicted making a tour or a son River. Household travel surveys were conducted in trip, that time window is blocked out so that the indi- 28 counties in three states as part of the model develop- vidual cannot do anything else. Time-pressure variables ment process. The region is approximately 9,700 square may also be used. These variables may result in individ- uals squeezing activities into shorter time periods or 2 See Vovsha, P., and K.-A. Chiao. Development of New York reducing the length of a trip. Time and space constraints Metropolitan Transportation Council Tour-Based Model. Volume 2, can also be introduced into models. pp. 2123.