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E M E R G I N G M O D E L I N G C O N S I D E R AT I O N S 47 analysis using the framework. Volume 2 contains a paper ent dimensions of travel are not easy to answer. on the topic.2 The following points were covered in her Further, some built environment measures act as presentation. proxies for other built environment measures. Also, focusing on the impacts of the built environ- · There continues to be increased interest in land use ment on narrow dimensions of travel may miss and urban form policies, including the ability to influence other elements of the overall effect on travel. travel behavior through the design of the built environ- The second element of the complex relationship ment. Smart growth strategies and new urbanism con- is the moderating influence of the characteristics of cepts reflect this interest. These approaches to land use individual traveler decision makers on travel and transportation planning are more proactive and pol- behavior. Characteristics of the decision maker, icy oriented than those in the past. Questions related to which include individuals and households, relate the effectiveness of built environment policies still need to to sociodemographic factors, travel-related and be examined, including the causal thread versus statistical environmental attitudes, and perceptions regard- association and the magnitude of the causal effect. ing different attributes of the built environment. · The relationship between the built environment There may be two kinds of moderating influences. and travel behavior has been examined recently in dif- One relates to direct influence on travel behavior ferent studies. The causal effect argument has been con- and the other relates to indirect influence on travel sidered in some studies, including those addressing the behavior by modifying the sensitivity to built envi- new urbanism and smart growth. Groups that favor ronment characteristics. Additional studies are these approaches suggest that automobile dependence- needed to control for these observed and unob- reducing built environment strategies will lead to tangi- served influences. ble reductions in motorized vehicle use, as well as The third element of the complex relationship providing friendlier and socially vibrant neighborhoods. between the built environment and travel behavior The associative effect argument suggests that certain relates to the spatial scale of analysis. Determining types of people choose to live in particular built environ- the shape and scale of neighborhoods or other geo- ments and that the automobile-dependent orientation of graphic areas needs to be considered. Most studies the population is due to demographic shifts and lifestyle use predefined spatial units based on census tracts, preferences. zip code zones, or traffic analysis zones. Individu- · Previous literature addressing the relationship als may not perceive neighborhood shape and scale between the built environment and travel behavior has by these units, however. The spatial extents of highlighted both mixed and inconclusive results. Some influence on travel choices may be different for var- studies have found significant elasticity effects of built ious built environment attributes. environment attributes on travel demand variables, oth- The second major issue in the built environment and ers have identified significant effects of the built environ- travel behavior relationship relates to residential sorting ment on one or more dimensions of activity and travel based on travel behavior preferences. Most early behavior, and others have found no significant effects of research assumed there was a one-way causal flow from the built environment on activity and trip frequency and built environment characteristics to travel behavior. This nonmotorized mode use. approach assumes that individuals and households · There appear to be two major issues in under- locate in neighborhoods and then determine their travel standing the relationship between the built environment behavior based on the attributes of the area. This and travel behavior. First, the relationship between the assumption does not consider that individuals and built environment and travel behavior can be very com- households select neighborhoods based on travel prefer- plex. This relationship appears to be multidimensional ences and the availability of different modes. in nature and may be affected by the moderating influ- · Possible approaches to account for residential sort- ence of the individual trip decision maker characteristics. ing include controlling for trip decision maker attributes The spatial scale of analysis may also influence the that jointly impact residential and travel choices, using relationship. instrumental variable methods, and using beforeafter The first element of the complex relationship is household relocation data. that both the built environment and travel are mul- A first approach is to control for the demo- tidimensional in nature. Questions related to what graphic and other travel-related attitudes impact- dimensions of the built environment impact differ- ing the neighborhood location choice of individuals and households. This approach can be 2See Bhat, C. R., and J. Y. Guo. An Innovative Methodological Frame- accomplished by incorporating decision-making work to Analyze the Impact of Built Environment Characteristics on characteristics as explanatory variables in models. Activity-Travel Choices. Volume 2, pp. 137141. The remaining effect of built environment mea-