Urban sprawl has been around for ever. Reading recent contributions on measuring this phenomenon [e.g. Elena G. Irwin, Nancy E. Bockstael and Hyun Jin Cho, "Measuring and modeling urban sprawl: Data, scale and spatial dependencies" or Charles Jaret, Ravi Ghadge, Lesley Williams Reid and Robert M. Adelman, "The Measurement of Suburban Sprawl: An Evaluation".] I became interested in urban porosity. While sprawl is primarily a phenomenon of the urban edge, to describe it and to explain it, there is a need to understand the dynamics of cities as 3D physical objects.
At the same time, we are making a bit of progress in formulating the structural relationships that should be at the heart of the relevant modeling effort. See Czamanski, D., Roth, R., “Characteristic time, developers’ behavior and leapfrogging dynamics of high-rise buildings" forthcoming in Annals of Regional Science, 2009.