The other day I passed a set of temporary traffic lights. Hardly an unusual occurrence in a modern city! This was a nice modern set, with the lights made up of an array of LEDs. That is of course a great improvement. Apart from the environmental benefits of being low-energy and long-lasting, the light area is evenly bright all over. And there can be no problem with low sun reflecting off the back mirror so it is hard to tell which colour is illuminated.
What struck me, however, was that the LEDs were carefully arranged in a circular pattern, mimicking the traditional look of traffic lights. This is clearly not the most efficient way of arranging them. That would be a hexagonal pattern, but of course you cannot produce what looks like a neat circular outline that way.
There is no reason why in an LED system each colour of light has to be round. They could just as well be hexagonal, or any other convenient shape. A hexagon would clearly match a hexagonal pattern better. Indeed, there could be advantage in making each colour a different shape, to help the colour-blind. They make the lights round because that is how we expect traffic lights to look.