The evolution of road networks during urbanization is characterized by two key processes: ‘exploration’ and ‘densification’. The study, published in the journal Scientific Reports, uses the quantitative tools of network theory to examine the development of road networks in the Groane region in northern Italy. The proportion of the world’s population living in urban areas has recently grown beyond 50% but a quantitative assessment of urbanization processes has been lacking. Marc Barthelemy and colleagues report the empirical analysis of almost 200 years of data on the evolution of the road network in an area north of Milan. Over time, cells of land demarcated by roads have become more evenly sized and square shaped, even though the area has never been subjected to overall large-scale planning efforts. The authors observe a trend towards a greater number of four-way road junctions, whereas dead ends and three-way junctions were more common earlier on. The most central streets in the network in 2007 tend to be the oldest ones, highlighting the role of central roads as a robust spatial backbone that remains stable over time. The authors attribute these findings to two key dynamics, exploration - whereby new roads trigger spatial evolution beyond the outskirts of the town - and densification, or the increase in local road density around existing urban centres. Exploration is more common during earlier historical periods, whereas densification predominates in later years. The two processes appear to operate independently of technological and economic changes over the past 200 years. The results could be useful for urban planning but further investigation of cases in different regions and economic positions is needed to confirm the generality of urbanization processes.
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