A more complex example is a traffic controller. In the simplest case, there are just two streets, and the light is green on each street for a fixed period of time. It then goes to yellow for another fixed period and finally to red. There are no inputs to this system other than the clock. There are six outputs, one for each color in each direction. (Each output may control multiple bulbs.) Traffic controllers may have many more outputs, if, for example, there are left-turn signals. Also, there may be several inputs to indicate when there are vehicles waiting at a red signal or passing a green one.