Four major findings emerged from the present study. First, neighborhood walkability was related to higher levels of physical activity and lower risk of being overweight or obese, but not to social or psychological outcomes. Second, neighborhood income was not related to any measure of physical activity, but lower-income adults had less favorable weight status, physical QoL, neighborhood satisfaction, and social cohesion than higher-income participants. Third, there was only one significant interaction between neighborhood walkability and income, indicating walkability had a stronger positive association with walking for transport in high-income than in low-income participants. Fourth, after adjusting for potential self-selection bias (i.e., “reasons for moving here”), all significant associations of outcomes with walkability and income remained significant, except walking for leisure. However, associations with mental quality of life and depression score became significant, indicating slightly poorer mental health in residents of high-walkability neighborhoods, particularly for those in low income areas.
Adults living in high-walkability neighborhoods had higher objectively measured total physical activity as well as higher self-reported walking for transportation and leisure than did participants from low-walkability neighborhoods. The weekly difference in objectively measured physical activity was about 47 minutes per week for the higher-income group and about 34 minutes for the lower-income group. On average, living in a high-walkability neighborhood was associated with meeting the 30 minute per day physical activity guidelines (Haskell, Lee, Pate, et al., 2007
) at least one day more per week than those in low-walkability neighborhoods. Present findings confirm previous results of higher total physical activity in high-walkability neighborhoods (Frank et al., 2005
; Saelens, Sallis, Black, & Chen, 2003
). These results extend the evidence by demonstrating the effect generalizes to both higher- and lower-income groups, and the walkability effect appears to be stronger for objectively-measured than self-reported physical activity. Walkability associations with physical activity were not explained by self-selection into neighborhoods based on predisposition towards activity-friendly environments, a finding consistent with recent studies (Frank et al., 2007
; Handy et al., 2006
; Handy, Cao, & Mokhtarian, 2008
). Nonsignificant differences in total physical activity by neighborhood income were unexpected, because higher activity levels among higher-income participants have been reported (Crespo et al., 2000
; United States Department of Health and Human Services, 2000
), but studies reporting SES differences in objectively measured physical activity are rare and generally agreed with present results (Troiano, Berrigan, Dodd, et al., 2008
It appears walkability differences in walking for both transportation and leisure contributed to observed differences in total physical activity. It is well-established that adults walk more for transportation in walkable neighborhoods (Heath et al., 2006
; Transportation Research Board & Institute of Medicine, 2005
; Frank et al., 2004
), but the few studies that examined leisure walking or total self reported physical activity usually reported no walkability effect (Owen, Humpel, Leslie, Bauman, & Sallis, 2004
; Saelens & Handy, 2008
). The walkability—leisure walking association was weaker than the relation with transport walking, and after adjustment for self-selection, the walkability—leisure association became nonsignificant. This was expected because the walkability index was designed to explain transport walking.
There were no significant income differences on walking for transport or leisure, but there was an interaction between walkability and income on walking for transportation. The walkability—walking for transport association was weaker for adults living in lower-income than in higher-income neighborhoods. This is an important finding because it suggests lower-income residents may not experience all of the benefits from living in a walkable neighborhood unless other needs are met. Perceived danger from crime, which is higher among lower-income adults (Loukaitou-Sideris, & Eck, 2007
), could reduce their willingness to walk for transport even in high-walkability neighborhoods (Doyle, Kelly-Schwartz, Schlossberg, & Stockard, 2006
). After adjusting for self-selection, the walkability by income interaction became nonsignificant. Self-selection may not apply equally to lower- and higher-income groups, since higher-income groups may be able to satisfy more personal criteria when selecting neighborhoods (Levine & Frank, 2007
Previous studies found walkable neighborhoods protected against overweight and obesity (Papas et al., 2007
), but the present study extends previous work. There was a highly significant walkability effect for percent overweight or obese. Though the walkability by income interactions were not significant, living in low-walkability neighborhoods was associated with about a 50% increased risk of being overweight or obese in the higher-income group (OR=1.53), and the odds ratio was somewhat lower in the lower-income group (OR=1.20). Adjusting for self-selection had virtually no effect on the odds ratios, raising questions about claims that the walkability—obesity association is due to self-selection (Handy et al., 2006
; Frank et al., 2007
; Eid et al., 2007
Hypothesized QoL, social, and psychological benefits of living in walkable neighborhoods received no empirical support. Despite using high-quality measures and examining a variety of outcomes, there was no evidence residents of walkable neighborhoods had benefits beyond physical activity and weight status.
The negative finding of walkability in relation to mental health, after adjusting for neighborhood selection factors, is consistent with evidence linking high residential densities with psychological stress (Evans, 2003
). However, scores on the present mental health measures were well within the normal range, so the practical impact of these small differences is unclear. A recent review reported some studies found built environment factors were related to depressive symptomatology, but the evidence base was small for any specific built environment characteristic, such as walkability (Mair et al., 2008
; Clark et al., 2006
). Results were inconsistent, with one study finding walkability was protective of depressive symptoms among older men, but not women (Berke et al., 2007
). Neighborhood population density, a component of walkability, has been found previously to be positively, negatively, or not associated with mental health outcomes (Clark et al., 2006
). The presence/absence or magnitude of a built environment attribute may not be as important as its quality. For example, poorer quality of housing (e.g., state of repair) appears related to greater lifetime incidence of depression (Galea et al., 2005
) and higher depressive symptomatology (Weich et al., 2002
). More and better evidence is needed to improve understanding of built environment effects on mental health.
The present study confirmed the negative effects of low SES on multiple health outcomes. Lower-income participants had less favorable physical QoL, social cohesion, and neighborhood satisfaction. Unfortunately, there was little evidence that living in walkable neighborhoods alleviated these disadvantages, so efforts to improve social and physical environments, enhance health and social services, and empower vulnerable populations need to be strengthened. A recent study found walkable low-income, mostly-minority neighborhoods had lower levels of maintenance, aesthetic, and safety qualities than higher-income neighborhoods (Zhu & Lee, 2008
), so neighborhood built environment attributes beyond walkability should be examined to determine their relation to health outcomes.
A strength of the present study was the design to recruit participants from two regions of the United States that differed in demographic composition, climate, geography, and era of development. Results generalized across the two regions. Other strengths included use of accelerometers to objectively assess physical activity, assessment of walking for multiple purposes, control for seasonal effects, selection of neighborhoods that varied widely on walkability defined by GIS and income, and use of validated measures. The present study is one of the few to statistically adjust for potential self-selection bias (Handy et al., 2006
; Frank et al., 2007
; Handy et al., 2008
; Bagley & Mokhtarian, 2002
An important limitation was the modest recruitment rate and the under-representation of racial-ethnic minority groups and very low SES participants. Thus, present findings should not be generalized to the most disadvantaged populations, and studies of very low income and specific racial-ethnic populations are needed. The cross-sectional design is an important limitation, so prospective designs that follow people who move are needed to determine the relative contributions of personal and environmental influences on physical activity and weight status. Though the validity of the walkability index was supported in this study and several others, it has limitations related to the completeness and accuracy of the multiple data sets required for its computation. In addition, the intersection density variable, based on census block group geography, misses intersections at the boundaries of the blockgroup.
Physical inactivity and obesity are two of the most significant health problems in the United States and globally (Andersen, 2003
; Dishman et al., 2004
; World Health Organization, 2004
), and both outcomes were related to neighborhood attributes which are directly controlled by public policies. Policies to encourage development of more walkable neighborhoods and enhancements to existing neighborhoods could provide health benefits to large proportions of the population, both low- and high-income, on a relatively permanent basis. Policies that favor walkable neighborhood designs have also been related to reductions in driving, greenhouse gases, and air pollution; conservation of open space; and reduced spending on public infrastructure (Frank et al., 2003
; Frumkin et al., 2004
; Frank et al., 2006
; Ewing, Bartholomew, Winkelman, Walters, & Chen, 2007
). Some negative effects have been identified, such as local traffic congestion and concentration of air pollution (Frumkin et al., 2004
). Thus, walkable neighborhoods are not a panacea, and policies promoting walkable development patterns should be combined with other policies to avoid negative outcomes, especially among low-income populations. The potential to produce widespread and long-lasting favorable impacts on physical activity and overweight/obesity should make the creation and improvement of walkable neighborhoods a high priority on the public health agenda. An important next step in research is to identify the shape of the relation of neighborhood environment characteristics to physical activity and overweight/obesity outcomes so recommended levels of walkability attributes can be developed. Other studies are needed to strengthen evidence of causality through prospective and quasi-experimental studies.