555 timer and 4017 decade counter - traffic lights circuit
Here's a simple circuit which simulates traffic lights. 4017's ten outputs are used in such a way so the red and green lights stay on for longer than the yellow one. (twice as long, in this case)

On the 4017, don't forget to connect the "clock inhibit" pin to the ground, or you may get erratic behaviour. In my case, the circuit would sometimes freeze on the red light. Of course, while fixing the problem, I was amused by what would happen if that were a real traffic light.

The OR gates are built easily using 4072 (4-input OR), and 4071 (2-input OR). I'm sure that this can be done more elegantly, but if, for some reason, you want traffic lights, here's a circuit you can use.