  The emergence of congestion is a critical phenomenon in transport systems.
Transport is organized along pathways abstracted by links, which connect
different nodes as regions to form the network. The modeling of traffic has so
far mainly been based on one-dimensional lattices or networked queuing systems,
which restricts mechanistic insights and analytical tractability for traffic
dynamics on general networks. An interpretable kinetics model is necessary for
understanding the impact of the network structure on traffic congestion in
large-scale networks. Considering a generalized flow-density relation within
each node, we propose and investigate a general traffic model on networks with
external injections and exits. We observe various phases in terms of node
density that exhibit distinct behaviors under different injection rates. The
onset of congestion along with the coexistence of free nodes and congested
nodes, manifested by the spatial localization of traffic congestion, is induced
by the asymmetry of the flow transitions between neighbors in the network. The
proposed congestion centrality allows us to determine the congestion threshold,
identify congested nodes and predict congestion propagation. Our model and
results are shown to cover a broad spectrum of implications for transport
dynamics on networks as well as offer a practical prospect for traffic
optimizations such as congestion alleviation and targeted diffusion.
