Universal roughness and the dynamics of urban expansion

We present a new approach to quantify urban sprawl using tools from surface growth physics. Analyzing built-up area expansion in 19 cities (1985-2015), we uncover anisotropic growth with branch-like extensions and a piecewise linear relation between area and population. A universal local roughness exponent $\alpha_{\text{loc}} \approx 0.54$ coexists with variable exponents $\beta$, $z$. As modeling urban sprawl remains an open problem, these results offer a necessary empirical basis for theory.