Why The Stop Sign Is My Favorite Traffic Control Sign


Recently, I’ve been trying to slow down and become more present in the small moments of daily life. It’s funny how just being more aware of the world around you opens up additional levels of thought and opportunities for curiosity and reasoning. So, as I was walking home the other day, I happened to glance at a stop sign as I was passing it. This reminded me of an interesting question: “Why are stop signs octagonal?”

This is a question I've seen posed a number of times. It typically gets raised in discussions of ‘interesting’ job interview questions typically attributed to tech firms and includes examples such as:

  • Why are manhole covers round?
  • How would you move Mt. Fuji?
  • How many ping pong balls would fit in a 747?

The purpose of such questions is not so much for the interviewee to get the answer correct but more for the interviewer to get a feel for how the interviewee thinks and how he handles the pressure of a more abstract challenge.

The benefits of these types of questions as indicators of future job performance have been debated at length and I’m not here to support either side of that argument. That being said, I do think that the question poses some interesting food for thought around the design of something that plays an important role in everyday life.

The stop sign is perhaps the most important traffic control sign there is. The consequences of failing to either see or obey this sign could have the most directly harmful effects—matters of life and death.

When I googled ‘Why are stop signs octagonal?’ the most frequent answer I’ve seen is that this decision was pure economics. Sign designers wanted a unique shape for each sign, octagons require the most material and, since the stop sign is the least frequently occurring traffic control sign, less sheet metal would be needed overall if the stop sign was represented as an octagon.

I think there are other interesting possibilities for why this shape was selected. In the design of the stop sign as a whole, there are three defining characteristics:

  1. Color
  2. Lettering
  3. Shape

Color

Color is perhaps the most obvious choice. Red has come to be associated with urgency and and begs attention.

A while ago, I read the following tweet from Neil deGrasse Tyson (@neiltyson) that I found really interesting and somewhat relevant here:

If our blood used Copper instead of Iron, turning it green, I wonder what color we would have made stop lights & stop signs

Whatever the origin, red has come to mean ‘Stop’.

But if we were to distinguish traffic signs only by their color, there would likely be some confusion.

Lettering

Not being color blind myself, I can’t say for sure what a red-green color blind person sees when he looks at a stop sign. How about a stop sign in front of a green bush? Or a stop sign attached to a green building?

Whatever the color experience for viewing this sign may be, the viewer doesn’t have to rely on color alone. Large, bold letters clearly spell out what the sign means: STOP

I believe that part of the reason the sign is not another shape because of the octagon’s capacity to easily fit these four letters maximally sized without the need to accommodate additional angles, edges, or curves. Which leads to the final defining characteristic of the stop sign.

Shape

We’ve now arrived at the stop sign’s shape—the mystery that was the impetus for this discussion. I have a number of thoughts around why the octagon was such a great choice.

First, as mentioned above in the lettering discussion, is the capacity to maximize the size of the word STOP. A rectangle or square would naturally be the most accomodating, but more than 90% of the other traffic signs are some form of quadrilateral (square, rectangle, diamond). Diamonds and circles would require the word to be smaller than it could be due to the fact that they get narrower from their widest point extending upward and downward. The same could be said for the triangle

But, to me, the most interesting feature of the octagon is that it has no orientation, whereas a quadrilateral when rotated could be indistinguishable; If a bolt came loose and the sign swung to an angle, squares might look like diamonds or vice versa .

There is a very specific use-case for this feature and I take advantage of it frequently—perhaps you do too. When I approach a stop at an intersection, I am able to tell very quickly whether I am the only side of the intersection that needs to stop, or if I am at a multi-way stop. This is because I can tell, even from the back of a sign on an opposing corner, that the sign I am looking at is a stop sign. This is possible regardless of its orientation.

Of course, other shapes would afford this action (identifying the control from either side)—circles, hexagons, pretty much any shape with more than four sides. But as previously mentioned, these shapes introduce interesting tapering issues that might impact the size with which the word ‘STOP’ could be displayed.


Perhaps the shape of the stop sign was dictated purely by economics, but I’d like to think that there was additional thought that went into its design—thought that recognized the importance of its function and addressed the needs of its users. And if that is the case, the stop sign wins my vote for best traffic control sign.

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