信号系统

例句：

1. 1. The policeman gave a signal to the driver to stop.
   警察向司机发出了停止的信号。
2. 2. The car's engine emitted a loud beep as it signaled the end of the test.
   汽车的引擎发出响亮的蜂鸣声，作为考试结束的信号。
3. 3.The traffic light is a common signal for pedestrians crossing the street.
   交通灯是行人过马路时常见的信号。