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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.)
