So now we've switched over to LED traffic lights, why are they still all circular and differ by colour only? It would be really easy to make them different sha

So now we've switched over to LED traffic lights, why are they still all circular and differ by colour only? It would be really easy to make them different shapes and colours. e.g. go = green + circular, stop = red + octagonal (like a stop sign), caution = yellow + triangular.