In an adaptation that is so obvious that it seems almost unworthy of comment, the basic schema of traffic lights has been adopted for most instant messaging and chat programs. Although there has been many different protocols and clients in the two decades since internet chatting became popular, most of them have settled on an iconography derived from traffic signals to show people's chatting status. Green means online and available, red means busy or away, and orange usually means something in-between: online, but idle. There is some variation in this, but the basic layout is based on green/yellow/red.
This probably isn't news to anyone who has used a chat client in the past ten years. But I think it is interesting for two reasons. First, it shows how easily we can immediately recognize the same system of symbols in totally different contexts. A green dot on gchat is not actually an invitation to drive a car into someone's computer, but people intuitively understand that "start driving" can be translated into "start typing", Secondly, it is interesting because at some point internet chatting might outlast automobiles and traffic lights, and it will then become an odd historical relic, that like the QWERTY keyboard has become standardized despite the fact that its original justification is gone.