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