Participation in physical activity reduces the risk of chronic health conditions including cardiovascular disease, diabetes, obesity, hypertension, cancer, and depression . Despite its protective effect many adults do not participate in recommended levels of physical activity (i.e., ≥30 minutes of at least moderate-intensity physical activity on most days) . A combination of demographic, biological, psychological, social environmental and physical environmental characteristics determine physical activity behavior . Of increasing interest is the role of urban form in supporting and constraining physical activity. Urban sprawl – a pattern of urban development which includes large areas of low residential density, expanses of a single land use i.e., residential and low levels of land-use mix, scattered land developments, and commercial strip development –negatively impacts health and the environment and is a major concern for city and municipal planners . Urban sprawl is associated with less transportation-related walking and cycling and increased dependence on private motorized modes of transportation which negatively affects air quality [4, 5]. Consequently, higher levels of urban sprawl are associated with an increased risk of overweight and obesity, pedestrian injury, and chronic diseases [4–6].
Different urban planning practices can be implemented at various geographical scales (i.e., street, neighborhood, county, city, and region). Thus, within vast sprawling cities there are opportunities to improve the built environment’s supportiveness of walking at smaller geographical scales such as within individual neighborhoods. For example, even in sprawling cities, traditional, neo-traditional or new urbanist neighborhoods that are characterized by mix of residential, commercial, and recreational land-uses, well connected street and pedestrian networks (i.e., grid-like street patterns), higher population densities, convenient access to public transit, and walking infrastructure can encourage more local walking . Cross-sectional evidence also suggests that specific components that make up the neighborhood environment are independently associated with walking, such as proximity to and mix of different recreational and non-recreational (e.g., supermarkets, banks, convenience stores) land use types, proximity of street and pedestrian network connectivity (e.g., number of 3 and 4-way intersections, block size), population and employment density, traffic volume and traffic control devices, and aesthetics or appeal [8–10].
The evidence that urban design characteristics are causally related to levels of physical activity is derived mainly from cross-sectional studies. While cross-sectional studies provide no direct insight into temporal cause and effect associations, they provide information about the strength of association and possibility of alternative explanations. Amongst others, these two indicators of evidence provide the epidemiological evidence required to infer causality . However, the strength of association between variables can only be informative in cross-sectional data if the association is non-spuriousa. Especially pertinent here is the suggestion that any association between the built environment and physical activity may be more the result of self-selection: people who like to be active choose to live in neighborhoods that support their preferred activity. To date the majority of cross-sectional studies examining the relations between the built environment and physical activity have not adjusted for neighborhood or residential self-selection [8–10]. Apart from affordability, people consider lifestyle, physical activity and transportation related factors as important when selecting where to live, and these latter factors might influence the amount of physical activity undertaken [12, 13]. The unmeasured contribution of residential self-selection to the association between the built environment and physical activity may result in spurious associations – resulting in an overestimate of the association.
Nevertheless, several studies have found associations between characteristics of the neighborhood built environment and walking even when adjusting for residential selection . For instance, Cao  found self-reported frequency of transportation walking and strolling to be higher in traditional versus suburban neighborhoods controlling for residential preference. Frank et al  found positive associations between a neighborhood walkability index (i.e., a combined index of different environmental characteristics) and self-reported participation in discretionary and non-discretionary walking adjusting for residential selection (preferences for particular neighborhood attributes and reasons for moving to the neighborhood). A US and an Australian study with similar methodologies both found positive associations between a neighborhood walkability index and self-reported minutes of transportation, but not leisure walking, after adjustment for residential selection [17, 18]. Chatman , adjusting for neighborhood preferences related to walking, transit and automobile access, found that self-reported frequency of walking/cycling increased as the neighborhood count of 4-way intersections (i.e., connectivity) increased. These findings suggest that land use planning practices have the potential to mitigate the effects of urban sprawl, encouraging local walking, with concomitant population health benefits.
Despite positive associations between walkability and walking being found, in practice modifying street layouts, land use zoning and types, and residential densities of entire fully established neighborhoods is likely to be logistically, financially, and politically challenging. However, for established neighborhoods, smaller scale and potentially less controversial strategies for improving walkability might be possible, for example installing sidewalks. Several cross-sectional studies using self-reported measures of the built environment have found the presence of sidewalks to be positively associated with walking behavior [20–24]. For example, a meta-analysis of studies that included self-reported measures of the built environment found that those reporting the presence of sidewalks in their neighborhood were more likely to be physically active . A pooled analysis of data from 11 countries found that compared with six other self-reported neighborhood built environment characteristics (i.e., single-family homes, shops near home, transit stop near home, facilities to bicycle, low cost recreational facilities, and unsafe to walk due to crime), the availability of sidewalks was the most strongly associated with achieving recommended levels of physical activity , despite the impact of environmental attributes was cumulative. These findings are encouraging but overall, the evidence from studies examining the association between objectively-assessed sidewalk availability and physical activity is mixed.
While self-reported sidewalk availability in general is found to be positively associated with physical activity and walking, the same conclusion cannot be drawn with regard to objectively-measured sidewalk availability and walking [25–30]. For example, Rodriguez et al.  found no association between sidewalk density within 400 meters of home and time spent in transportation and overall walking after adjusting for other objectively-assessed and self-reported neighborhood environment characteristics. Similarly, Rutt et al.  found no association between sidewalk availability (i.e., ratio of sidewalk to street length) within 400 meters of home and minutes of light, moderate, or vigorous-intensity physical activity. While Lee and Moudon  found no association between sidewalk length within 1-km of home and participation in walking for transportation or recreation, they did find a positive association between sidewalk length and walking for recreation five or more times per week relative to not walking. Despite concluding that the objectively-assessed built environment explained little variation in the way of exercise walking, Lovasi et al.  found a small but positive association between the amount of sidewalk-lined streets within 1-km of home and participation, but not time spent, in exercise walking. Installing sidewalks in existing neighborhoods may be a convenient low cost intervention to increase walking, particularly compared with redesigning street layout, modifying land use zoning, and increasing residential densities. However, evidence regarding the extent to which sidewalks influence walking is inconsistent. This relationship might depend on whether participation or quantity of walking is examined and if context-specific (e.g., neighborhood-based walking) versus general measures of walking are used. Moreover, studies that have reported significant associations between sidewalks and walking have been based on cross-sectional associations that have not adjusted for residential self-selection. Thus, more evidence on the relationship between objectively-assessed sidewalk availability and walking is needed.
This study sought to advance understanding of the associations between sidewalk availability and walking by including neighborhood-based measures of walking. It examines both participation and time spent walking for transportation and recreation, accounts for the influence of other neighborhood built environment characteristics on walking, and adjusts for residential self-selection to reduce biased estimates derived from cross-sectional data. We used an econometric approach to examine the association between sidewalk availability in established neighborhoods and neighborhood-based walking. Specifically, we examine whether or not the total length of sidewalk available in a neighborhood is associated with weekly participation in and minutes of neighborhood-based walking undertaken for transportation and recreational purposes.