  Making the connection between the function and structure of networked systems
is one of the fundamental issues in complex systems and network science. Urban
traffic flows are related to various problems in cities and can be represented
as a network of local traffic flows. To identify an empirical relation between
the function and network structure of urban traffic flows, we construct a
time-varying traffic flow network of a megacity, Seoul, and analyze its global
efficiency with a percolation-based approach. Comparing the real-world traffic
flow network with its corresponding null-model network having a randomized
structure, we show that the real-world network is less efficient than its
null-model network during rush hour, yet more efficient during non-rush hour.
We observe that in the real-world network, links with the highest betweenness
tend to have lower quality during rush hour compared to links with lower
betweenness, but higher quality during non-rush hour. Since the top betweenness
links tend to be the bridges that connect the network together, their
congestion has a stronger impact on the network's global efficiency. Our
results suggest that the spatial structure of traffic flow networks is
important to understand their function.
