Raspberry Pi—just connect up LEDs and buttons to the GPIO pins, and with a few lines of code you can turn lights on and control things with button presses. Once you know the code to do the basics.
If you know how to flash one light, you can flash three. Pick out three LEDs in traffic light colors and you can code the traffic light sequence. If you know how to use a button to a trigger an event, then you have a pedestrian crossing! Also look out for great pre-built traffic light add-ons like PI-TRAFFIC, PI-STOP, Traffic HAT, and more.
This can be used as an exercise in understanding how real world systems are devised. Computational Thinking is a useful skill in any walk of life.