  This letter propose a new model for characterizing traffic dynamics in
scale-free networks. With a replotted road map of cities with roads mapped to
vertices and intersections to edges, and introducing the road capacity L and
its handling ability at intersections C, the model can be applied to urban
traffic system. Simulations give the overall capacity of the traffic system
which is quantified by a phase transition from free flow to congestion.
Moreover, we report the fundamental diagram of flow against density, in which
hysteresis is found, indicating that the system is bistable in a certain range
of vehicle density. In addition, the fundamental diagram is significantly
different from single-lane traffic model and 2-D BML model with four states:
free flow, saturated flow, bistable and jammed.
