Instructables user Baron von Brunk has constructed a fully functional LEGO Traffic light. Not a mini one either, a large scale version. Possibly for a go-kart track or something. It is nearly life sized, but I would guess that it’s not weather proof, and wouldn’t last out doors for long.
Using nearly 1,800 LEGO Bricks, the Traffic light is pretty basic, and the only non-LEGO parts are those that control the lighting. I particularly like that feature of it as so many things are partially constructed with LEGOs, but use special pieces, or modified pieces.
The Instructable itself is very meticulous, especially for only being 4 steps long, but all the files needed to construct this yourself are included, as well as links on where to buy some of the needed parts.