Disaggregate i\.utomobile-Ownership and Mode-Choice Models

In this paper the study of model transferability is extended to n isagq reqate models of automobile-ownership level. Models of automobile ownership and mode to work are estim.ated and transferred among sectors of a metropolitan regton. The transfer effectiveness of these models is evaluateil by using previously developed disaggregate ann aggregate measures of model transfer effectiveness. The automobile-ownership models are found to have a high degree of transfer effectiveness in this context, higher than the transfer effectiveness of mode-choice models in the same context. It is concluded that previous findings about the effectiveness of model transfer, based on studies of modechoice models, can be extended to automobile-ownership models. The application of travel demand models estimated on observed dat-a for preciici::. ion c,f onclitior:~l fu.tl. re behavior in the same or other context is commonly undertaken as part of the transportation systems analysis process (}). The a9plication of a model in a context other than that in which it was originally estimated is described as model transfer. Model transfer is likely to be effective in predicting behavior in the application context if the transferred model will contain useful information about the behavioral phenomenon o f interest in the application context. Models that contain such useful information are d.escribed as transferable. Model -transferability is necessarily conrHtional on similarity of the underlying behavioral process in the estimation and application oontexts and the adequacy of the model to represent that behavior (2). A number of. studies of transferability of ~isa°qgre~ate tcliv~l choice models have been undertaken in recent years. Most of these studies consider mode choice (1-1>, whereas some examine frequency choice (6,7). The goal of this study is to extend the analysis of the transferability of travel choice models to the related choice of automobile ownership. The transferability of automobile-ownership choice models is analyzed and the transferability of these models is compared to that of mode-choice models. These analyses were undertaken in the context of an artificial transfer situation created by dividing the Washington, D.C., region into three geographically distinct sectors. These sectors are distinctly d iffe.rent in terms of the demoqraphic characteristics of their populations, such as houiiehold size, household income, and automobile ownership, and with respect to travel time and cost to the central business district (C8D) by both car and bus transit (1). Automobile-ownership and mode-choice models are estimated for each sector, and the transfer effectiveness of each model to the other two sectors is examined. This analysis was undertaken within a single urbanized area to reduce the confounding e(fect of differences in variable definition, measurement of level-of-service variables, and sampling procedures between metropolitan areas. Previous studies of the transferability of disaggregate modechoice models suggest that the results of intra-area transfer studies are indicative of inter-area transfer effectiveness. MODEL STRUCTURE AND ESTIMATION Models of. Travel and Related Choices Travel behavior is commonly analyzed in the four steps cmbod icd in the rarl · tlonal aggre,;iate urban transportation model system: trip generation, trip distribution, modal split, and network assignment l! .! .,11 • The compatable choices for disaggregate analysis are tri.p fl:equency (whether or no·t to make a trip), destination, mode, and path choice. An important issue in travel analysis revolves around the Rtruc ure of these choices and the models that represent them. Charles River Associates (! .Q.,ll) derived a sequential formulation of the choice process and applied it to estimation of choices of shopping tcip frequency, mode, destination, and time of day. BenAkiva (12) argued that certain of these choices are behaviorally joint and that they should be represented by a joint or simultaneous choice model. He also demonstrated that sequential model estimations may be quite different from t.hose obtained by estimation of the corresponding simultaneous model. However, the differences in parameter estimates reported were not statistically significant at any reasonable level, and the goodness-of-fit measures for the simultaneous and sequent i.al models were essentially Lhe s1:1me. Ocn7\kiva . ind LP.rman (131 extended the individu<tl choice structure to form a hierarchical model of travel and travel-related choices. In this hierarchy mobility choices, including residential location, automobile ownership level, and breadwinner mode chotce to work, are assumed to be made jointly. Decisions on trip frequency, destination, and mode for nonwork tr i.ps are assumed to be made jointly but conr3itional on the higher-level mobility choices. The discussion of choice model structure is basen on behavioral conjecture about the sequence of' the (unobserved) decision process employed by the tripmaker. More recently, McFadden (. _il suggested an alternative theoretical basis for mathematically structuring multidimensional choice models. Specifically, he formally derived the nested logit model that takes account of similarity among alternatives with respect to excluded variables. In this structure , the mathematical foi-m of the cboioe mor! el ii