Radio choice for backstage crew

Moving  away slightly from Talk Back systems but this can possibly help with signaling cues, as a lo-tec method. Eons ago we used to use the 'Trafic Light' system. Which was 3 lights Red, Yellow & Green connected to a 3 position rotary switch. Although I have made a few of these up I used to wonder why we always set up 3 globes & used only the Red & Green. I suppose the theory was Red stop or stand down, Yellow stand by & Green Go, invarably all that was used was Red stand by & Green Go! In most cases the trafic lights were used on the Fly floors & the orchestra pit.  Anyway this might be an easy & cheap solution to transmit cues as a stop gap method & for those groups who only require an elcheapo stationary cue signaling device. Like 'house clear' from FOH To the SM or from SM to the pit pianist. Which might be simpler than using various signaling methods in view of the punters, which still hapens in the odd venue from time to time.

Which brings to mind those Power Line [AC] intercoms, which are plugged into the  [GPO] power piont & use the electical power wires in the building to talk over. I would imagine they could be modified with a headset/mic easly enough, to communicate between the SM & bio box? {just one of those abstact finks - before my lights dim out!}