Innovations in Travel Demand Modeling, Volume 1: Session Summaries (2008)

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analysis using the framework. Volume 2 contains a paper on the topic.2 The following points were covered in her presentation. • There continues to be increased interest in land use and urban form policies, including the ability to influence travel behavior through the design of the built environ- ment. Smart growth strategies and new urbanism con- cepts reflect this interest. These approaches to land use and transportation planning are more proactive and pol- icy oriented than those in the past. Questions related to the effectiveness of built environment policies still need to be examined, including the causal thread versus statistical association and the magnitude of the causal effect. • The relationship between the built environment and travel behavior has been examined recently in dif- ferent studies. The causal effect argument has been con- sidered in some studies, including those addressing the new urbanism and smart growth. Groups that favor these approaches suggest that automobile dependence- reducing built environment strategies will lead to tangi- ble reductions in motorized vehicle use, as well as providing friendlier and socially vibrant neighborhoods. The associative effect argument suggests that certain types of people choose to live in particular built environ- ments and that the automobile- dependent orientation of the population is due to demographic shifts and lifestyle preferences. • Previous literature addressing the relationship between the built environment and travel behavior has highlighted both mixed and inconclusive results. Some studies have found significant elasticity effects of built environment attributes on travel demand variables, oth- ers have identified significant effects of the built environ- ment on one or more dimensions of activity and travel behavior, and others have found no significant effects of the built environment on activity and trip frequency and nonmotorized mode use. • There appear to be two major issues in under- standing the relationship between the built environment and travel behavior. First, the relationship between the built environment and travel behavior can be very com- plex. This relationship appears to be multidimensional in nature and may be affected by the moderating influ- ence of the individual trip decision maker characteristics. The spatial scale of analysis may also influence the relationship. – The first element of the complex relationship is that both the built environment and travel are mul- tidimensional in nature. Questions related to what dimensions of the built environment impact differ- ent dimensions of travel are not easy to answer. Further, some built environment measures act as proxies for other built environment measures. Also, focusing on the impacts of the built environ- ment on narrow dimensions of travel may miss other elements of the overall effect on travel. – The second element of the complex relationship is the moderating influence of the characteristics of individual traveler decision makers on travel behavior. Characteristics of the decision maker, which include individuals and households, relate to sociodemographic factors, travel- related and environmental attitudes, and perceptions regard- ing different attributes of the built environment. There may be two kinds of moderating influences. One relates to direct influence on travel behavior and the other relates to indirect influence on travel behavior by modifying the sensitivity to built envi- ronment characteristics. Additional studies are needed to control for these observed and unob- served influences. – The third element of the complex relationship between the built environment and travel behavior relates to the spatial scale of analysis. Determining the shape and scale of neighborhoods or other geo- graphic areas needs to be considered. Most studies use predefined spatial units based on census tracts, zip code zones, or traffic analysis zones. Individu- als may not perceive neighborhood shape and scale by these units, however. The spatial extents of influence on travel choices may be different for var- ious built environment attributes. The second major issue in the built environment and travel behavior relationship relates to residential sorting based on travel behavior preferences. Most early research assumed there was a one- way causal flow from built environment characteristics to travel behavior. This approach assumes that individuals and households locate in neighborhoods and then determine their travel behavior based on the attributes of the area. This assumption does not consider that individuals and households select neighborhoods based on travel prefer- ences and the availability of different modes. • Possible approaches to account for residential sort- ing include controlling for trip decision maker attributes that jointly impact residential and travel choices, using instrumental variable methods, and using before–after household relocation data. – A first approach is to control for the demo- graphic and other travel- related attitudes impact- ing the neighborhood location choice of individuals and households. This approach can be accomplished by incorporating decision- making characteristics as explanatory variables in models. The remaining effect of built environment mea- 47EMERGING MODELING CONSIDERATIONS 2 See Bhat, C. R., and J. Y. Guo. An Innovative Methodological Frame - work to Analyze the Impact of Built Environment Characteristics on Activity- Travel Choices. Volume 2, pp. 137–141.