....and what younger PPRuNers won't remember is that Heathrow - then known simply as "London Airport" - also used to have a similar system for pax! In its first "proper" passenger terminal, then known as Europa and even now still forming the core of Terminal 2, there was a system of ceiling lights - about 4 or 5 colours I think - that illuminated along the route from departure lounge to the gate. Flight announcements went along the lines of:"Flight BE123 to Paris is now ready for departure. Passengers for this flight should follow the
blue lights to the boarding gate".
It was simple and
AFAIK it worked perfectly until the number of flights boarding around the same time overtook the number of colours available!