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L E V E L S O F D I S A G G R E G AT I O N A N D D E G R E E S O F A G G R E G AT E C O N S T R A I N T 5 direct demand models Mandelbrot entire aggregate supply-demand curves chaotic drawings population Wardrop2nd Wardrop1st MSA simple Frank-Wolfe mechanical aggregate systems population multi-class Furness segments assignment behavioral unit competing biological TOPAZ MEPLAN, TRANUS population simulations POLIS LOWRY, PECAS-AA sample SATURN enumeration CONTRAM emergent Oregon2 SWARM synthesized simulated annealing Paramics ALBATROSS agents SOLUTIONS population synthesis VISSIM BE-microsim activity-based travel demand Calgary CVM PECAS-SD observed UrbanSIMLand agents UPlan ILUTE aggregate equilibrium path converged stable agent system independence bounded processes optimal degree of aggregate constraint FIGURE 1 Comparison of modeling approaches for level of aggregation (i.e., behavioral unit) and degree of aggregate constraint (i.e., aggregate behavioral construct). TAXONOMY OF TRANSPORTATION SYSTEM is equilibrium based (that uses the MEPLAN framework) MODELING APPROACHES to refine the search for the equilibrium solution and to explore further the aspects of this equilibrium solution at The existing approaches in transportation system model- the level of individual households. A detailed resolution ing tend to sit along a diagonal from upper left to lower is provided without giving up desirable equilibrium right in Figure 1; the recent increasing use of process sim- properties. ulation has arisen in conjunction with a swing to greater 2. Calgary commercial vehicle movement model (Stef- use of explicit representation of individual agents. an et al. 2005): This model uses a tour-based microsimu- Clearly, the ability to handle systems with large numbers lation framework with Monte Carlo simulation in which of interacting agents (with the advent of increasing com- logit choice models provide the sampling distributions to puting capabilities) has led to more attempts at explicit simulate the movements of commercial vehicles in the representation of the behavioral processes involved at delivery of goods and services. It runs in combination the individual level. It appears that more and more with an equilibrium-based model of household travel analysts--at least implicitly--are taking the view that demands. Trip tables from multiple runs of the commer- enough is known about the nature of individual agents' cial movement model are averaged to obtain a trip table behavior (possibly in part because these analysts are such of expected movements, and this table is combined with agents themselves in the real world and thus have insight trip tables from the household demands model and then gained by experience) to result in modeling systems that assigned to road networks by means of techniques for provide more accurate, or at least more faithful, repre- stochastic user equilibrium. The resulting congested sentations of reality. travel times are fed back to both the household demands A range of other combinations off the diagonal are model and the commercial vehicle movements model in available in transportation system and related modeling. an iterative process that runs to a convergence. A con- Some of these other combinations are now being verged system is obtained with a household demands explored so as to gain some of the available advantages. model at equilibrium and a process simulation tour-based Examples of these other combinations are microsimulation representation of commercial vehicle movements. 1. Cambridge Solutions modeling system (Caruso 3. Oregon2 integrated land use transport model 2005): This system uses a bid-choice framework to allo- (Hunt et al. 2001): This model includes a spatially disag- cate individual households to residential locations within gregated inputoutput model that is based on equilib- a particular transportation analysis zone consistent with rium to represent industrial and government activity, the results of a combined land use transport model that process-oriented microsimulations of household demo-