Elementary processes governing the evolution of road networks

TL;DR

This study analyzes 200 years of road network evolution near Milan, revealing that urban growth is driven by densification and exploration processes, with stable high-centrality roads forming the backbone of urban structure.

Contribution

It empirically identifies two fundamental processes—densification and exploration—that govern urban road network evolution, providing insights for modeling urbanization.

Findings

Urbanization leads to homogenized cell shapes.

High-centrality roads remain stable over time.

Growth driven by densification and exploration processes.

Abstract

Urbanisation is a fundamental phenomenon whose quantitative characterisation is still inadequate. We report here the empirical analysis of a unique data set regarding almost 200 years of evolution of the road network in a large area located north of Milan (Italy). We find that urbanisation is characterised by the homogenisation of cell shapes, and by the stability throughout time of high-centrality roads which constitute the backbone of the urban structure, confirming the importance of historical paths. We show quantitatively that the growth of the network is governed by two elementary processes: (i) `densification', corresponding to an increase in the local density of roads around existing urban centres and (ii) `exploration', whereby new roads trigger the spatial evolution of the urbanisation front. The empirical identification of such simple elementary mechanisms suggests the existence of general, simple properties of urbanisation and opens new directions for its modelling and quantitative description.