Sci Rep. 2012;2:296. doi: 10.1038/srep00296. Epub 2012 Mar 1.
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.
城市化是一种基本现象，但对其定量特征的描述仍不够充分。我们在此报告对一个独特数据集的实证分析，该数据集涉及位于意大利米兰北部的一个大面积地区近 200 年的道路网络演变。我们发现，城市化的特征是细胞形状的同质化，以及时间上一直保持着高中心道路的稳定性，这些道路构成了城市结构的骨干，证实了历史路径的重要性。我们定量地表明，网络的增长是由两个基本过程控制的：（i）“密集化”，对应于现有城市中心周围道路的局部密度增加，以及（ii）“探索”，即新道路引发城市化前沿的空间演变。对这种简单基本机制的实证识别表明，城市化具有普遍的简单特性，并为其建模和定量描述开辟了新的方向。
