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miles, with a population of approximately 20 million. There are approximately 4,000 traffic assignment zones, six trip purposes, and 10 motorized modes in the model, as well as four types of urban development. The four urban development types range from the very dense cen- tral business district (CBD) to rural areas. ⢠The NYMTC model has four major consecutive modules. The first module is tour generation that includes household synthesis, automobile ownership, and journey frequency choice models. The second module is tour mode and destination choice that includes premode choice, primary destination choice, entire tour mode com- bination choice, stop- frequency choice, and stop- location choice. The third module is the time- of- day choice and preassignment processor that includes tour time- of- day choice for outbound and inbound directions, trip mode choice, and construction of mode- specific and time- of- day period- specific trip tables. The fourth module is traf- fic and transit simulation that is implemented by time- of- day periods. The first three modules are imple- mented as fully disaggregate microsimulation procedures working with individual records for the synthesized pop- ulation, which includes households, persons, and tours. The fourth module is based on standard aggregate zone- to- zone assignments implemented in TransCAD. ⢠A tour- based model was used because it was the best approach to addressing complex mode and time- of- day choice in a consistent manner. The model also pro- vides the best method to understand and forecast the highly diverse demographic and travel patterns in the region. ⢠There are numerous stakeholders in the New York region. NYMTC staff undertook an outreach and edu- cation program to explain the need for the new model and to build support among different groups. These efforts included technical outreach to staff at other agen- cies, as well as meetings with policy makers. ⢠The model has been used in a number of applica- tions, including air quality conformity analysis, major investment studies, and local planning activities. Exam- ples include the Tappan Zee Bridge study, the Goethals Bridge study, and the Manhattan area pricing study. The experience to date with the use of the model has been positive. ⢠Work is under way to make the model accessible on the Internet. A new wave of data collection will also be starting. This data collection will include a major household travel survey, a work place survey, and obtain- ing information on visitor trips. The results of the data collection efforts, which are anticipated to take approxi- mately 3 years, will be used to update the model. ⢠It is important that models are calibrated to ensure that they replicate reality. A nontrivial and often not explored question relates to validating and calibrating a model chain rather than a single model. A key aspect in the calibration process is to properly identify the source of any discrepancy. It is important to identify the prob- lem first before rushing to make adjustments in the model. Another suggestion is not to overadjust. A good approach is to begin with the largest discrepancies. ⢠Reporting and analysis are also important ele- ments. Most staff who operate the models are not involved in all aspects of the planning process. Reporting the results represents the link between the modelers, planners, and policy makers. Take care to distinguish between reporting options per se and reliability of the forecasts at a fine level of typological, spatial, or tempo- ral detail. Activity- based models are more exposed than aggregate models because more detailed reports may be generated from these models. THE SAN FRANCISCO MODEL IN PRACTICE: VALIDATION, TESTING, AND APPLICATION Maren Outwater and Billy Charlton Maren Outwater and Billy Charlton described the valida- tion, testing, and application of the San Francisco County Chained Activity Modeling Process (SF- CHAMP), which was developed for the San Francisco County Transporta- tion Authority (SFCTA). They discussed the development of the model, the model validation process, applications of the model, and comparisons of the model to the four- step model. Volume 2 includes a paper on this topic.3 The following points were covered in their presentation. ⢠SF- CHAMP was developed in 2000 and 2001. The model was developed to provide detailed forecasts of travel demand for various planning applications in the county. These applications include countywide plans, corridor and project- level evaluations, transit plans, and neighborhood plans. The objective was to accurately represent the complexity of the destination, temporal, and modal options, as well as to provide detailed infor- mation on travelers making discrete choices. A tour- based model using synthesized population as the basis of decision making rather than zonal- level aggregate data sources best met this objective. ⢠The development of the model was influenced by limited resources and time availability. These limitations were considered in the development process. First, no transit onboard survey data were available to validate the mode choice elements of the model. This limitation would have been an issue for a trip- or a tour- based model. New onboard survey data have recently been col- lected and are being used to update the model. The peak- 15TOUR-BASED MODELS 3 See Outwater, M. L., and B. Charlton. The San Francisco Model in Practice: Validation, Testing, and Application. Volume 2, pp. 24â29.
