  By introducing a simple model based on two-dimensional cellular automata, we
reveal the relationship between the routing strategies of individual vehicles
and the global behavior of transportation networks. Specifically, we
characterize the routing strategies by a single parameter called
path-greediness, which corresponds to the tendency for individuals to travel
via a shortest path to the destination. Remarkably, when vehicles tend to
travel via the shortest path, a congested-flow state emerges between the
conventional free-flow and congested states, where traffic flow increases
slowly with vehicle density in the presence of congestion. We also found that a
high individual tendency to travel via the shortest path does not necessarily
shorten the average journey time, as the system may benefit from less greedy
routing strategies in congested situations. Finally, we show that adaptive
routing strategies outperform controlled strategies in the free-flow state, but
not in the congested state, implying that controlled strategies may increase
coordination among vehicles and are beneficial for suppressing traffic
congestion.
