Understanding the mechanisms leading to the formation and the propagation of traffic jams in large cities is of crucial importance for urban planning and traffic management. Many studies have already considered the emergence of traffic jams from the point of view of phase transitions, but mostly in simple geometries such as highways for example or in the framework of percolation where an external parameter is driving the transition. More generally, empirical evidence and characterization for a congestion transition in complex road networks are scarce, and here, we use traffic measures for Paris (France) during the period 2014–2018 for testing the existence of a jamming transition at the urban level. In particular, we show that the correlation function of delays due to congestion is a power law (with exponent η ≈ 0.4) combined with an exponential cut-off ξ. This correlation length is shown to diverge during rush hours, pointing to a jamming transition in urban traffic. We also discuss the spatial structure of congestion and identify a core of congested links that participate in most traffic jams and whose structure is specific during rush hours. Finally, we show that the spatial structure of congestion is consistent with a reaction–diffusion picture proposed previously.

了解导致大城市交通拥堵形成和传播的机制对于城市规划和交通管理至关重要。许多研究已经从相变的角度考虑了交通拥堵的出现，但主要是在简单的几何形状中，例如高速公路或在外部参数驱动过渡的渗流框架中。更一般地说，复杂道路网络中拥堵过渡的经验证据和特征是稀缺的，在这里，我们使用 2014-2018 年期间巴黎（法国）的交通措施来测试城市层面是否存在拥堵过渡。特别是，我们证明了由于拥塞导致的延迟的相关函数是幂律（指数η≈ 0.4) 与指数截止值ξ相结合。这种相关长度在高峰时段显示出差异，表明城市交通的干扰过渡。我们还讨论了拥堵的空间结构，并确定了参与大多数交通拥堵的拥堵链路的核心，其结构在高峰时段是特定的。最后，我们表明拥塞的空间结构与之前提出的反应扩散图一致。