Traffic Light

I am a teacher and one of my classes is arduino programming. For this I needed a simple project for the students. A project where you can explane the basics of programming and advance very easily.
A colleague of mine suggested to let them program a traffic light.
It's an easy project to begin with (first red then green then yellow) where you can expand very easily.
The next step was to link all the traffic lights.
Then create a program where, in case of malfunction, the yellow light begins to blink,....

I accounted for the accuracy of the printer (whitch is 0.1 mm on mine).

But the holes for the LEDS (5mm) are to small. I enlarged them by using the tip of my hot solderingiron. And that works great.
You can also drill the hole to the necessary width.