To design a traffic light controller, you should identify what kind of road intersection model and connecting module among the lamps you want to build. For instance, if you want to build a traffic light controller for a four-road intersection, you need 12 lamps. This model needs at least 12 pins on Raspberry Pi GPIO.
In the real implementation process, lamps in a traffic light controller use AC as a power source, so you can't connect them to your Raspberry Pi board. It can be addressed by using a channel relay. A sample of a channel relay form can be seen in the following figure:
Some channel relay modules provide wireless links, such as radio, WiFi and XBee, which can be used by Microcontrollers (MCUs) to control these modules, so you don't need cables to connect lamps to MCU.
