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Levels of Disaggregation and Degrees of Aggregate Constraint in Transportation System Modeling J. Douglas Hunt, University of Calgary This paper compares modeling approaches used in trans- With the equilibrium approach, a particular state of portation system modeling and in system modeling more the system with certain properties is identified, a calcula- generally. It considers two dimensions: (a) the level of tion process is used to bring (iteratively) the system to disaggregation in the representation of system elements this state (sometimes called the equilibrium solution), and (b) the degree of aggregate constraint on the system. and then other aspects of the system at this state are Furthermore, it incorporate both the equilibrium and the examined and perhaps compared with what they are at process simulation approaches and thereby enters the that same state under other conditions. The standard debate concerning the relative merits and (perceived?) four-step modeling system (with feedbacks) uses the flaws of these two approaches. The intentions of this equilibrium approach. paper are (a) to engender a greater appreciation for the With the process simulation approach, an explicit advantages and disadvantages within the range of avail- reproduction of certain elements of the behavior of the able techniques and the potential for matching technique system is developed, including representation of the with context in a given instance and (b) to present a more separate actions and reactions involved (often involv- complete view of the linkages among techniques and the ing a direct representation of behavior through time). A scope for hybrid approaches. Coverage includes some calculation process is used to work through the new, emerging approaches, including the combination of sequence of combined actions that arise under specific an activity-based model with equilibrium treatments for initial conditions. Aspects of the system through this both land use and network assignment. Therefore, the sequence are examined and perhaps compared with paper provides the framework for a discussion of the what they are through the same sort of sequence under opportunities and challenges arising with the implemen- other starting conditions. The activity-based approach tation of activity-based models and transportation sys- and the latest in traffic microsimulation modeling use tem models more generally, helping progress beyond the this approach. Each approach has advantages and standard positions taken in the debate about equilibrium disadvantages. versus process simulation and aiding the consideration of appropriate directions for further research and devel- opment work. EQUILIBRIUM VERSUS PROCESS SIMULATION For the equilibrium approach, A standard view in transportation system modeling-- almost a central tenet in the orthodoxy--is that two A well-defined system state is considered that is at basic types of modeling approach are available: least identifiable and in many cases unique, allowing rei- equilibrium and process simulation. dentification and reexamination; 3