Complexifying urban expansion: An exploratory, gradient-based approach

Opportunities for innovation in urban expansion research abound given the emergence of longitudinal and spatially explicit data. Scholars now use a broad array of data when analyzing expansion, yet the conceptual approach remains limited. Toward this end, this work extends conceptualization of expansion beyond the relatively simple economic approach that emphasizes population growth and income. Instead, insights from social–ecological theory—focusing on the heterogeneous, nonlinear, and hierarchical nature of cities—provide a pathway for specifying urban expansion gradients. The paper demonstrates how a spatial gradient model that considers urban, periphery, and exurban subregions complexifies the urban regional phenomenon, yielding crucial insights into the processes that drive expansion. Exploratory quantitative analysis of the National Land Cover Database (NLCD) provides a prototype implementation, testing the utility of the gradient-based approach for all urban regions in the contiguous United States. Modeling covers both the full study period, 2001–16, and as separate 2001–11 and 2011–16 panels, assessing how relationships change over time. Results show that social–demographics, land use and transportation, regional economics, and physical drivers exhibit diverse impacts across the expansion gradient and over time. This work concludes with a call for scholars to deepen the approach presented here before conducting predictive or confirmatory modeling. POLICY RELEVANCE Urban expansion takes many forms and understanding those different forms can benefit decision-makers. Rather than a policy framework that treats urban growth as a regional phenomenon, a gradient-based approach is necessary, one that targets how expansion processes operate differently in urban, peripheral, and exurban subregions. By disaggregating the region, this research provides new insights into expansion dynamics such as how higher incomes and GDP growth tie to infill development; concentration of immigrants coincides with reduced land consumption across the gradient; age-based locational preferences yield compact growth; regions with greater natural resource economies and warmer temperatures have more exurban expansion; and steep slopes and open water constrict peripheral expansion. This kind of information allows for different institutions—from regional planning authorities to urban municipal governments to small town mayors—to address the drivers of expansion through more targeted and differentiated policy mechanisms, while also having more information to consider unintended consequences.