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O V E RV I E W O F T H E P O L I C Y I S S U E S 3 activity tour structure. All of these elements fit together into the daily travel-activity pattern and must be designed in a consistent and integrative way for each per- son and each household. The Model Other issues associated with activity-travel modeling are the conflict between realism and feasibility. A model Modeler Decision Maker cannot be a perfect representation of the real world; part of the art of modeling is deciding which components can be ignored without seriously undermining the usefulness Other Factors of the model to represent behavior and inform decision makers. An effective structure balances the need to repre- FIGURE 2 Modelers' view of decision making. sent activity and travel components; balances activity loca- tion, scheduling, and tour structure to satisfy timespace A key phenomenon of the past 30 years is a change in constraints; and relates travel pattern and mode choice. the scope of decision making. When I first became Figure 3 highlights the tour modeling dimensions. An involved in travel modeling, the primary decisions con- important observation about this diagram is that the com- cerned investment analysis. This analysis focused on ponent models are generally estimated distinctly. Each identifying what should be built, where it should built, component is estimated separately in both application and and how it should be designed. We have seen a very dra- most research and is linked analytically. An important step matic expansion in the number and the scope of issues in the advancement of activity-based travel models is to included in the transportation decision-making process integrate the information and estimation process; this is a over the years. While capital investment is still a central very difficult, but critical, task to ensure the consistency of issue, a wide range of other decisions have achieved a relationships between the model elements. much higher level of importance. Such decisions include Integrating the activity generation and scheduling system operations and policy decisions for both transit process is important for entire-day schedule consistency. and highway systems, pricing and the impacts on the This process needs to recognize the dynamics of individ- environment, energy consumption, and urban and ual behavior. We tend to reflect behavior based on what regional development. We need to recognize and take individuals have done in a specific period of time. But we account of the linkages among all these components of do not know how much of those activities were planned the context as we develop and implement models. and how much are the result of changes in activities or We have seen many important developments in travel conditions during early portions of the day. modeling over the past 30 years. These developments The major economic developments in this field are can be divided into the broad categories of conceptual, adoption of disaggregate analysis and discrete choice mod- econometric, spatial, computational, transportation ser- eling. The historical argument over disaggregate versus vice, and land use. I will briefly discuss recent develop- ments in each of these areas. Primary and secondary A key conceptual development over the past 10 to 20 activity and tour TOD years is the organization of travel behavior as part of a daily pattern of activity-based travel pattern analysis. This analysis considers all travel by household members Entire-tour mode during a portion of the day, the entire day, or longer time periods. It takes account explicitly of intraperson and interperson consistency as well as joint choice. This Stop frequency imposes a variety of constraints on travel analysis, includ- ing not starting an activity until the preceding activity and necessary travel are completed, and coordination of joint Stop location travel and joint activity participation between individuals and with other travel and activity participation. Further, it ensures that travel resources, primarily cars, are Trip mode assigned to no more than one tour at a time. Model design issues that need to be addressed in activity-based travel pattern analysis include generation Trip departure time of activities, scheduling activities, location of activities in time and space, assignment of activities to individuals FIGURE 3 Tour modeling dimensions. within a household, and development of the travel- (TOD = time of day.)