Challenges for Europe and North America
Edited by Karst T. Geurs, Kevin J. Krizek and Aura Reggiani
Chapter 5: The connections among accessibility, self- selection and walking behaviour: a case study of Northern California residents
New urbanism, smart growth and neo-traditional communities have been promoted to address the automobile dependence associated with conventional suburban developments. Despite different terms, these communities generally consist of medium-to-high density, mixed-use and pedestrianoriented development. Such development increases the nearby opportunities that residents have within reach to meet their daily needs; it also reduces residents’ travel impedance by reducing the distance between home and destinations and/or by increasing the attractiveness of the pedestrian environment. Overall, it enhances residents’ accessibility to employment and services. Accessibility measures the ease of reaching activity destinations (Levinson and Krizek, 2008). Accessibility is generally a function of the number of opportunities at the destinations and travel costs between origin and destinations (for example, cumulative-based and gravity-based accessibility), although the function may also include components related to individuals’ travel tastes (for example, utility-based accessibility) and time constraints (for example, constraint-based accessibility) (El-Geneidy and Levinson, 2006). Previous studies have found that residents living in high-accessibility neighbourhoods tend to drive less and walk more than those living in low-accessibility neighbourhoods (Cervero and Duncan, 2003; Chatman, 2009; Crane and Crepeau, 1998; Joh et al., 2008). These findings in turn provide empirical evidence for state and local governments to employ land-use policies to reduce vehicle miles travelled (and hence the negative consequences on the environment) and promote the use of transit and non-motorized modes (for example, California’s Senate Bill 375: Redesigning Communities to Reduce Greenhouse Gas). However, the statistical association found in most studies does not necessarily mean causality; scholars widely agree that research has yet to establish the predominant causal link: do people living in accessible neighbourhoods walk more because the built environment itself ‘invites’ them to do so, or because people who like to walk tend to choose residential neighbourhoods conducive to exercising that preference? The latter phenomenon is referred to as ‘self-selection’. It is a potential explanation for observed differences between groups whenever individuals ‘select’ themselves into those groups rather than being randomly distributed between them (Mokhtarian and Cao, 2008). Ignoring selection bias may misestimate the impacts of the built environment on travel behaviour. Further, many studies used cluster sampling or multistage sampling approaches to collect their data. Researchers have often treated such samples as simple random samples when presenting descriptive and modelling results. This practice may produce biased descriptive results and erroneous statistical inferences. Such flawed findings are likely to mislead land-use and transportation policies.
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