We propose a simple cellular automaton to simulate the city traffic in rush hours. Vehicles on road are treated as particles hopping on lattice. In the morning rush hours, downtown parking lots are taken as the sinks for particles moving from suburban; in the afternoon rush hours, the parking lots become the sources for particles moving out of town. The traffic is regulated by the operation of a series of traffic lights. Both the traffic flow (from global perspective) and the travel time (from personal perspective) are studied. As the traffic patterns response to the traffic lights, the phase transition is defined by the pattern shift.