What are digital system and switching circuits?

This depend on the way where you want to place your design you have to design a timing circuit first.(u can use IC 555 ) after that according to your variable and expression give this output of timing circuit to LOGIC gates serially after every one minute the output of timing circuit will change and gates are also change their position and indicate traffic light.There are many ways you can design a simple traffic light circuit. I recommend you to use a 4017 IC. Please check this site as reference. It's a two way traffic light circuit.