The goal of this review was to determine whether smart growth principles are associated with either physical activity (general and walking-specific) or body mass. Few studies reported significant associations between smart growth principles and physical activity or body mass. The exceptions were for physical activity and principle seven (open space preservation), and walking and principles one (range of housing choices), six (mixed land use), nine (development toward existing communities) and ten (compact building design).
Body mass was the least measured outcome among the studies examined. This is often an imperfect outcome to choose when measurement is cross-sectional, as it is the furthest downstream from the predictors, and so there are likely to be many factors which affect it that were unmeasured, such as eating behaviors. All of the studies in this review which examined body mass are cross-sectional. Thus, the almost exclusively non-significant results found here are not surprising. Body mass might be a more informative outcome of the effect of smart growth principles on obesity risk if it were measured longitudinally utilizing a long-term follow-up, for example, a period greater than one year(21
). It is also possible that rather than working directly, smart growth's impact on body mass operates through mediation or moderation, mechanisms which are not addressed in the studies reviewed. Owing to a lack of studies which have this as an outcome, it is still not known whether smart growth can yield changes in body mass.
It is difficult to infer from this body of literature whether the objective or perceived environment is a more important determinant. It is also unclear whether there was objective evidence of the implementation of smart growth principles as part of a deliberate planning process. For the studies reviewed, evidence of planning was inferred either from observational or archival data (e.g. neighborhood audits, GIS) on geographic and structural features, such as the proximity of green spaces, or from self-reported perceptions of residents. None of the studies referred to the use of planning department data, such as development codes or general plans, as evidence of smart growth principles. If perceptions of the environment contribute to the impact of smart growth principles on health, then future interventions may focus on education or marketing campaigns to make residents aware of resources which are available to them, such as parks, greenbelts, or essential services in local shopping areas. If objective environmental features are the more important predictors of activity, then modifications would likely be necessary. This could take the form of sidewalk construction/rehabilitation and improved park playground equipment, or policy changes, such as reduced speed limits and denser housing requirements in the development code. However, it is likely that both presence of environmental features and subjective awareness of those features are important to creating behavior change, and any intervention will need to account for each of them.
There are several implications based on the findings of this review. First, communities which have diverse housing types, mixed land use, increased housing density, development which is directed toward existing communities, and increased levels of or access to natural space and critical environmental areas may show increases in walking and physical activity among residents who are exposed to these characteristics. The second is that if a community has decided to make these priorities, established smart growth literature and materials can guide adjustments to development codes, general plans, tax incentives and other policies designed to foster the above changes. Third, smart growth planning as it currently exists could incorporate a dedicated public health component (17
). With a more conscious focus on health as part of the community planning process, associations between implementation of smart growth principles and health outcomes could increase. A health component could be mapped onto or operationalized for each of the 10 existing principles rather than constitute an additional new principle. Communities could decide what health issues are most important to them, and then tailor each of the principles to address those health goals. In practice this could take the form of requiring health impact assessments for any new development projects, dedicating a portion of transportation funding to bike and pedestrian projects, or the integration of public health officials into decisions about park location, transit services, housing density, etc.
Systematic inclusion of a health component in planning also has ramifications for evolving planning standards that are incorporating smart growth, including the relatively recent U.S. Green Building Council LEED (Leadership in Energy and Environmental Design) standards for neighborhood development (ND) (23
). LEED-ND certifies neighborhoods on a credit system based on their inclusion of features that promote greater walkability, mixed use, public transportation alternatives, and orientation towards the existing community; reduction of vehicle miles traveled; and greater energy efficiency. LEED certification carries incentives in the form of both sales and property tax credits, which means that in the future, communities may be reinforced for following smart growth principles.
Future research should address the limitations discussed in this review. As mentioned, studies should include a wider distribution of races and genders to increase population representativeness as well as external generalizability. Second, more studies should include both subjective and objective measures of the environment in order to increase both the validity and reliability of results. Third, more controlled, longitudinal studies are needed. A major issue in this line of research is the effect of self selection into neighborhoods more conducive to physical activity by people who already lead active lifestyles (24
). This can bias the outcomes and produce inflated effect sizes. Such bias can only be ruled out if an appropriate control group is utilized that represents randomization or relevant demographic matching. Longitudinal studies will allow us to determine whether there is a time threshold, as far as how long someone must live in an environment in which they are exposed to smart growth principle operationalizations before changes in physical activity are evident. They will also allow us to see whether there are effects on more distal outcomes such as body mass which require longer observation periods to determine if there is truly an effect. Fourth, more research is needed which incorporates the upstream development process, including the topics covered by principles three (encourage community and stakeholder collaboration) and five (make development decisions predictable and fair), which were found to not have been studied at all, at least insofar as they impact our chosen outcomes. Community input into development projects can take several forms, including open town hall meetings, city council meetings with public input, as well as the more specialized charrette process, by which community leaders, administrators, and organizational stakeholders can discuss and give feedback to planners before build out has taken place (25
). Increasingly, stakeholders include health agency directors and community health advocates who are sensitive to planning efforts that could affect air quality, traffic noise and accidents, and other health-related factors (11
). New research could examine whether the inclusion of these health-oriented stakeholders is associated with the incorporation of smart growth principles into a community's development code or general plan, and whether their inclusion is associated with the direct application of these principles to improving targeted health outcomes in the community. Finally, future studies should investigate the complete set of smart growth principles in order to gain a better sense of whether there is a particular set or combination of certain principles, a minimum number or critical mass which need to be deployed, or if there are certain key principles which must be incorporated in order to achieve beneficial health outcomes. Related to this, an understanding of what constitutes a sufficient level or threshold of each principle to modify behaviors is needed. To be sure, the studies reviewed are not the result of deliberate smart growth research. Instead, these studies measure aspects of the built environment that smart growth planning efforts seeks to modify, in order to achieve their goals. These variables vary in value along a continuum and do not necessarily represent the ideal built environment as envisioned by smart growth advocates. As a result of the vague and somewhat arbitrary terms which currently exist in the literature, such as ‘high’ mixed land use versus ‘low’ mixed land use, this review is unable to quantify, for example, exactly how many units of commercial land use per acre are necessary to cause meaningful shifts in physical activity levels. Future research will be more informative in developing policy and behavioral interventions if it is able to establish these minimum thresholds.
There are several limitations to our current review. The first is the inherent difficulty in mapping the data from the studies identified onto the 10 smart growth principles. Ultimately, these were subjective judgments based on existing definitions of each principle that were not as concrete as might be hoped for. To account for this, a multi rater system was used, and it is expected that this minimized the subjectivity to the greatest extent possible. Also, as in any literature search, there is the possibility that relevant studies were missed, either in the initial search, or in the subsequent three-step screening process. However, unintended exclusion was minimized through use of a multi-screener process and by searching diverse databases that encompass a variety of disciplines which relate to the smart growth planning process, including architecture, landscape architecture, urban planning, economics and law. Third, while a number of studies utilized objective methods to assess the built environment, none of them verified the presence of these principles in city planning records or documents. This would have allowed us to determine whether the principles observed were implemented as a direct result of smart growth planning or were pre-existing features of the built environment. Thus, the findings from this review represent conclusions about the potential effectiveness of types and sets of features that are represented in smart growth planning and which could be included in future planning decisions in order to maximize health benefits of community residents.
A caveat to the study of smart growth as a mechanism for promoting healthier communities is that development is a complex process which is shaped by forces beyond the control of any one group, including planners themselves. These include political leadership and will, market demand and the economics associated with developing a tract of land. It would be naive to think that simply shifting urban planning codes and regulations will be enough to alter entrenched development practices and achieve behavior change. The reality is that these legal and policy modifications are likely to be necessary but not sufficient components of a broader campaign to change the way communities are built and re-developed. With acknowledgement of this complexity, the findings from this review suggest that smart growth planning principles hold promise for promoting physical activity, especially walking. Furthermore, it is suggested that the potential impact of smart growth principles on health could be increased by the systematic inclusion of a health component to community planning.