Yet Mensa uses it anyway. Not to regulate traffic, though — Mensa uses it to regulate hugs.
Mensa is an international society of high-IQ people; in order to be a member, one needs to have an IQ in the 98th percentile or above. It was founded in 1946 in England with the express rule that the only criteria for admission be that the prospective member meets the IQ (or equivalent) requirement; the organization is apolitical and does not discriminate based on race, religion, etc.
Over the course of the organization’s half-century plus history, Mensa gatherings have adopted a culture to themselves. For example, it is very common to find a large room at any given Mensa convention dedicated to games — board games, typically — and that room is often open and well-attended around the clock. (American Mensa even honors five games each year with an award called “Mensa Select,” given to games which are “original, challenging, and well-designed.”) And for some reason, hugging has become part and parcel of Mensa meetups.
Of course, not everyone has the same tolerance for hugs. So Mensa — most likely organically and informally — developed a red-yellow-green system for hugs. Meetup attendees can adorn their name tags with little circular stickers, as demonstrated above, via blogger Jen McCreight, who wore the above at a Mensa convention she spoke at in the summer of 2011. A green sticker means the wearer welcomes hugs by all comers; a yellow one instructs fellow Mensa-ites to ask first; and a red one means no hugging allowed.
When the hugging custom emerged is not publicly known. And it was not limited to McCreight’s experience. In his 2004 book, The “Know-It-All,” A.J. Jacobs recounts a similar story, available here (with some colorful language).
From the Archives: Radioactive Red: It’s red and you don’t want to hug it.
Related: “The Know-It-All” by A.J. Jacobs, 4 stars on over 250 reviews. Also, Twilight Struggle, the highest rated board game on BoardGameGeek, 10 reviews total (the link goes to the Deluxe edition) averaging 5 stars.