Multiple abrupt phase transitions in urban transport congestion
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During the last decades, our view of cities has been deeply transformed by new approaches combining engineering and complexity sciences. Network theory is playing a central role, facilitating the quantitative analysis of crucial urban dynamics, such as mobility, city growth or urban organization. In this framework, betweenness, a well-known centrality measure, represents a fundamental tool, standing out as a proxy of traffic density and congestion, among others. In this work, we focus on the spatial aspects of congestion. To help unveiling such relationship, we introduce a simple model composed of a grid connected to a set of trees. This structure, coined as the GT-model, allows us to analytically describe in terms of betweenness, how and why congestion emerges in particular geographical areas, and more importantly, how it may co-evolve with city growth and mobility pattern shifts. We observe the emergence of several congestion regimes, with abrupt transitions betwenn them, related to the entanglement of arterial and urban local roads. The existence of these transitions is corroborated in a large amount of real city networks, thus representing an important step towards the understanding and optimization of traffic dynamics over coupled transportation networks.
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