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Innovations in Travel Demand Modeling, Volume 2: Papers (2008)

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Suggested Citation:"T57054 txt_006.pdf." National Academies of Sciences, Engineering, and Medicine. 2008. Innovations in Travel Demand Modeling, Volume 2: Papers. Washington, DC: The National Academies Press. doi: 10.17226/13678.
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graphics, travel activities (activity- based), and land development activity. The interface between these occurs at the represented markets, where elasticities in both the provision of labor by workers and the use of developed space by activities allow for consistent market clearing. The situation with activity- based models is likely to warrant similar combined treatments in some instances in which an assignment process is used to identify an equilibrium state between supply and demand for the network. Expectations of the quantities of household travel demand are the quantities being loaded to the net- work, with those developed by use of a component of the activity- based model run multiple times within each iteration of the assignment process. Through use of the definitions established earlier, such a combined treat- ment would constitute less of a process simulation and more of a disaggregation of the household demand side of the network equilibrium. To date, there has been little in the literature about the issues arising with such a com- bined treatment; at least, it should be acknowledged that there is still a reliance on the concept of equilibrium when an activity- based model is being used in this way. In this light, the debate over equilibrium versus process simulation seems misdirected, and effort should be focused on establishing how to gain the possible ben- efits of each and how to use the two in combination most appropriately. CONCLUSIONS In model systems, the level of disaggregation and the degree of aggregate constraint are two separable charac- teristics. Separation of the two is useful when one con- siders the properties of these systems, as it can help in the understanding of the model system dynamics and in the identification of alternative, more suitable modeling approaches and the aspects of the solutions provided by these approaches. Much of the practical work in transportation system modeling tends to sit very broadly along a diagonal in these two dimensions that runs from the combination of aggregate and equilibrium to that of disaggregate and process simulation. A range of other approaches, includ- ing some more- novel ones that sit off this diagonal, is also available, as are examples of their use in practical work. One such example is the Cambridge Solutions model system, in which an equilibrium approach is used in combination with a representation of individual agents at the disaggregate level. The practical implementation of model systems that use combinations of process simulation and equilibrium techniques now under way is being guided largely by intuition and some potentially relevant previous experi- ence. Little is understood from a more general theoreti- cal perspective. Some more generalized theoretical research is required so as to consider the issues involved, including these: • The uniqueness of converged solutions, • The extent to which equilibrium properties apply, • The possible combinations and variations in tech- niques and the advantages and disadvantages arising with each, • The relevant attributes to use in the definitions of the relevant categories properties, and • The potential linkages among the “strange attrac- tors” in chaotic systems. The results of this more generalized theoretical research then need to be converted into appropriate guidance for practical modeling work. Practice is mov- ing ahead with activity- based modeling (although per- haps not as fast as some would like), and theory needs to catch up. The definitions provided here, and the result- ing taxonomy for sorting model systems along relevant dimensions, is intended as a starting point for such an examination. The issue of the degree of aggregate constraint on the model system needs to be taken into account more com- pletely in much of the current practical work that is using activity- based models. The focus in such work often seems to be the complexity of the representation of the behavior of the individual agents. Certainly, this aspect of the model is important. But the behavior of the model system in relation to the degree of aggregate constraint is equally important. Practice is moving ahead with activity- based modeling (although perhaps not as fast as some would like), and theory needs to catch up and pro- vide some important support. The region definitions presented here along the two dimensions are intended to illustrate the range and potential resolution of model system properties. While it has been judged that they provide mutually exclusive and collectively exhaustive coverage, they are not intended to be the final, authoritative definitions. It cer- tainly may be the case that somewhat different group- ings and distinctions would be more appropriate in a given instance, and thus it would make sense to modify the ones presented here. REFERENCES Caruso, G. 2005. SOLUTIONS Micro- Scale Modeling: Method and Initial Results. Draft Working Paper. Mar- tin Centre for Land Use and Built Form Studies, University of Cambridge, United Kingdom. www.suburbansolu tions.ac.uk. 6 INNOVATIONS IN TRAVEL DEMAND MODELING, VOLUME 2

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TRB Conference Proceedings 42, Innovations in Travel Demand Modeling, Volume 2: Papers includes the papers that were presented at a May 21-23, 2006, conference that examined advances in travel demand modeling, explored the opportunities and the challenges associated with the implementation of advanced travel models, and reviewed the skills and training necessary to apply new modeling techniques. TRB Conference Proceedings 42, Innovations in Travel Demand Modeling, Volume 1: Session Summaries is available online.

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