使用vhdl语言设计交通信号灯。由一条主干道和一条支干道汇合成十字路口，在每个入口处设置红、绿、黄三色信号灯，红灯亮禁止通行，绿灯亮允许通行，黄灯亮则给行驶中的车辆有时间停在禁行线外。主干道处于常允许通行的状态，支干道有车来时才允许通行，主、支干道均有车时，两者交替允许通行，主、支干道每次放行时间不得短于30S，在每次由绿灯亮到红灯亮的转换过程中，要亮4S黄灯作为过渡。

-Using vhdl language design traffic lights. By a main road and a branch roads converge into a crossroads, and at each entrance red, green, yellow and white signal lights, red light is closed to traffic, the green light to allow passage of yellow light time to stop moving vehicles forbidden line outside the line. Main road is often state that permits travel only allow the car to pass through the main and branch roads have car, alternating between the two to allow passage, the main and branch roads each release time shall not be less in the 30S branch roads in each 4S yellow light, brighter green light to red light conversion process as a transition.