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