It is found that there exists the metastability and hysteresis effect of traffic flow in the neighborhood of critical density under different initial conditions . a computer simulation system named cats was fulfilled with all celluar automata models in java language 利用面向对象的java语言，设计实现了cats仿真系统，模拟了各种一维周期性边界条件下高速公路上车流运动，显示了交通流的各种现象。
