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137 An Innovative Methodological Framework to Analyze the Impact of Built Environment Characteristics on ActivityâTravel Choices Chandra R. Bhat, University of Texas at Austin Jessica Y. Guo, University of WisconsinâMadison There has been increasing interest in the landuseâtransportation connection in the past decade,motivated by the possibility that design policies associated with the built environment (BE) (land use, urban form, and street network attributes) can be used to manage and shape individual traveler behavior and aggregate travel demand. It is important to determine whether the empiri- cally observed association between BE and travel behav- iorârelated variables is a reflection of underlying causality or whether it is attributable to the relationship between BE and the characteristics of people who choose to live in par- ticular BEs. Literature debating the causal versus the associative nature of the relationship between the BE and travel behavior, including whether any causal effect is enough to cause a shift in travel patterns, is inconclusive. This relationship is the focus of design policies manifested in new urbanism and smart growth concepts. A review by Ewing and Cervero (2001) describes studies that have found elasticity effects of BE attributes on travel demand variables. Other recent studies have also found signifi- cant effects of BE on one or more dimensions of activ- ity/travel behavior (see Rajamani et al. 2003; Krizek 2003; Shay and Khattak 2005; Bhat et al. 2005; Bhat and Singh 2000; and Rodriguez et al. 2005). However, several studies reviewed by Crane (2000) and some other works (see, for example, Boarnet and Sarmiento 1998; Boarnet and Crane 2001; Bhat and Lockwood 2004; Bhat et al. 2005; and Bhat and Zhao 2002) have found that BE measures have little to no impact on such dimen- sions of travel behavior as activityâtrip frequency and non motorized mode use. However, because of different estimation techniques, units of analysis, empirical con- texts, travel behavior dimensions, and BE characteristics and their scales used across the studies, it is difficult to compare results. Academia agrees that it is premature to draw any conclusions about the impacts of BE on activ- ityâtravel behavior. Further, two issues need to be addressed: (a) The relationship between BE and travel behavior can be complex, and (b) the true causal impact of BE on travel behavior can be assessed only if the asso- ciation due to demographics- based residential sorting is controlled for. These issues are discussed in the next two sections (see also Boarnet and Crane 2001; Crane 2000; Krizek 2003; and Handy 1996). COMPLEX NATURE OF THE BUILT ENVIRONMENTâTRAVEL BEHAVIOR RELATIONSHIP Three elements characterizing the complex relationship between BE and travel are discussed below. Multidimensional Nature BE and travel are multidimensional in nature. That is, there are many aspects to BE, including accessibility to transit stops, presence and connectivity of walk and bike paths, land use mix, street network density (such as aver- age length of links and number of intersections per unit area), block sizes, and proportion of street frontage with buildings. Similarly, there are many dimensions of travel,
