square signal

例句：

1. 1. The squaresignal was used to indicate the end ofa line.
   方信号用于指示线路的结束。
2. 2. The square signal helped to coordinate trafficflow.
   方信号有助于协调交通流量。
3. 3. Thesquare signal was designed to prevent accidents onthe road.
   方信号设计用来防止道路上的事故。