1、在十字路口上，交通信号灯的变化是固定的：

1）放行：绿灯亮15秒放行，然后黄灯亮5秒，红灯亮禁止。

2）禁止：红灯亮20秒禁止，然后绿灯亮放行。

3）两条线路交替进行，实现定时交通控制。

2、扩展部分要求

1）交通灯变化时，同时用数值控件进行20秒（15秒，或5秒）的递减时间显示。

2）特殊情况下，允许急救车辆优先通过。当特殊情况发生时，路口的信号灯全部变红，以便让车辆优先通过。假设车辆通过时间为10秒，急救车辆通过后，交通灯恢复先前状态。可以用按键代表车辆到来。

-1, at the crossroads, the traffic lights change is fixed:

1) release: green light for 15 seconds release, and then yellow light for 5 seconds, red light ban.

2) Prohibited: red light for 20 seconds forbidden, then the green light is released.

3) two lines alternately, to achieve regular traffic control.

2, extended part of the request

1) When the traffic light changes, use the value control for 20 seconds (15 seconds, or 5 seconds) of the decreasing time display.

2) In exceptional circumstances, first aid vehicles are allowed to pass priority. When a special situation occurs, the intersection of the lights all red, so that the vehicle priority through. Assuming that the vehicle passes for 10 seconds, the first aid vehicle passes through the traffic lights before returning to the previous state. You can use the keys to represent the arrival of the vehicle.