Human Factors in History
What’s yellow, bedazzled in glass beads, and made out of plywood? If you said a life-size cutout of Glen Campbell wearing a yellow suit, you would only be half right.
Believe it or not, I’m talking about the STOP sign. Interestingly however, the red, reflective, octogonal shaped traffic sign we see so often today did not always have these classic characteristics. This article explores the history of STOP sign designs, and the ways that Human Factors, human vision and perception, and manufacturing processes affected each design change over time.
Start your engines
In the early years of the automotive industry, the motivation for many to own an automobile was born from the need to extend one’s efficiency in farming and business practice. Your average farmer equipped with horse and wagon could only travel a practical distance of 10 – 15 miles at a time before exhausting the horses too much. As a result, farmers beyond that distance from the city faced inherent transportation and business limitations. For the few farmers that possessed the means to afford an automobile, however, this technology increased their mobility potential by two or three times what a horse and wagon offered. The net gain for these fortunate individuals meant the ability to sell to different marketplaces, expanding business potential and profitability.
More often however, the automobile was a technology limited to the wealthy (http://l3d.cs.colorado.edu/systems/agentsheets/New-Vista/automobile/). Indeed, these individuals could afford the steep initial purchase cost, the time and tools needed to learn how to maintain the vehicle, and the parts required when a repair was needed. Plus, if they crashed it, who cares?! We’re rich! *money fight!*
Due in part to these monetary limitations, automotive clubs — with relatively few active members compared to today’s standards — were the first groups responsible for the development and dissemination of the traffic signs. Members would post signs, warnings, and notes to fellow motorists at various points on early roadways. Not surprisingly, since there was no clear standard on how these signs should be designed, or where they should be posted, there was a great deal of inconsistency, confusion, and safety concerns among motorists at that time.
In fact, according to the New York Times, one of the pioneering texts responsible for introducing the notion of traffic signage was, Reforming Our Street Traffic Urgently Needed, written for a magazine called Rider and Driver. The author’s simple plea included placing stoppage signs at intersecting roadways. Mind you, this request came about at a period in history where most roads were unkempt gravel or dirt, had no line markings, no standard for the travel speed, no designation between pedestrian and motorist areas, and no traffic lights of any sort (it wouldn’t be until 1914 that American Traffic Signal Company installed the United States’ first traffic light in Cleveland, Ohio — oh, red and green only, by the way).
Over time however, with increases in automotive production efficiency (i.e., assembly line) and material consistency, the automobile transformed into a financially viable option for many households by the early 1920’s. Indeed, the Bureau of Public Roads — formerly known as the Office of Public Roads — recognized this rise in automobile popularity and sought a solution to the growing safety issues experienced by motorists.
Interestingly, a key issue the Bureau of Public Roads faced was the fact that a large proportion of American motorists were relatively illiterate (U.S. Department of Transportation). This meant that traffic signs had to be designed in ways that could be universally understood — with or without the use of English words. Factors such as the shape, size, and color were of particular importance in these formative years of traffic sign design.
Infinitely Idiotic: The Rationale for Shape
Among the early factors that engineers toyed with was what shape to make the STOP sign. Based on little more than intuition and some feedback from railroad engineers, the Mississippi Valley Association of State Highway Departments decided that the number of sides a sign has would indicate its “danger level”. And, being the Engineering-type, they perceived that it was common knowledge that the circle had the most number of sides — an infinite number, in fact. Thereafter, circular traffic signs were used to denote areas with the highest level of danger for motorists.
And of course, since an octogonal shape has a whopping eight sides to it, well, that should be used to indicate the second-highest levels of danger! Over time, circular signs began to be used (almost) exclusively among railway settings. Whereas, the octogonal shape of the STOP sign remained rooted in motorway settings.
Detour: take a moment to appreciate the lunacy in that decision. Here we have a motorist population who has very little schooling (if any), is largely illiterate, and whose mathematical capabilities is on par with today’s 1st or 2nd graders. In what world would these individuals know that a circle has an infinite number of sides to it? Yet even if they did know, would the average person think to apply that logic to all traffic signs they encountered?
Was that a sign back there? The Rationale for Color
Now that the issue of shape had been resolved — and never again revisited since 1923 — the STOP sign design now had to consider color. Surely, red — right? Believe it or not, the Bureau of Public Roads began production of yellow STOP signs in 1915 in major automotive regions such as Detroit, Michigan (Moeur, 2010; http://www.trafficsign.us/yellowstop.html).
Why yellow? Pioneering research in early Human Factors and physiology had revealed that the human eye was most efficient at detecting colors within the yellow and green spectrum, especially when scotopic vision (i.e., high use of rods, low use of cones) is used in low light (e.g., dusk; night time). Despite common belief, the color red is not detected as easily and quickly — particularly in low light conditions. In fact, the color red is almost impossible to discern at night without the assistance of an artificial light source (e.g., backlighting, headlights).
By 1924, all “warning”-type signs were produced in a standardized, bright yellow color. Likewise, all other signs were given a white background to differentiate the two categories of traffic signs: warning and informational. Both of these lighter colors provided the best degree of contrast (with black letters) in conditions with low light (e.g., nighttime, raining, fog) that manufacturing methods could produce at that time.
Yet problems persisted.
Despite their best efforts, the number of accidents and issues on the (deteriorating) roadways continued to increase from one year to the next. Three central factors to this issue was the fact that a.) automobiles were being designed with greater power and speed potential, b.) more and more motorists were on the road, and c.) headlamps and streetlights were not efficient enough to illuminate STOP signs — among other areas of interest — on their own.
Headlamps at that time only offered about 100 ft. of illuminated distance in front of the automobile. However, today we know that your average (undistracted, unfatigued) motorist traveling at, say, 30 MPH requires approximately 115 ft. – 187 ft. of stopping distance, taking into account average brake reaction times. As a result, if motorists traveled any faster than 30 MPH, there simply wasn’t enough distance for them to be able to react to an unfamiliar or unexpected STOP sign, and respond appropriately.
The need at this point in the evolution of the STOP sign design was to enhance its visibility and luminance.
Extending Our Sight
Multiple design strategies were considered. The one that initially won out during the early 1930’s was the incorporation of glass “beads” into the outer edge and numbers on traffic signs. In the case of the STOP sign, glass beads less than 1 inch in diameter were glued along the perimeter of the sign as well as (in some cases) along the word, STOP itself. The purpose of these beads was to incorporate an element of diffuse reflection to enhance visibility at night when the motorist’s headlamps were turned on.
Unlike mirror reflection, diffuse reflection operates by “taking in” a source of light and distributing a portion of that light in all directions simultaneously. For example, the same way that a car’s paint reflects the sunlight during the day. The net effect of a diffuse reflective STOP sign was that it enhanced visibility by (about) an additional 50 ft. – 100 ft. at night.
However, the downside to this design approach was that it was inherently costly and time consuming to produce. Think about what goes into this process: the glass factory has to make each bead, these beads need to be packaged together for shipping, each crate is shipped to the factory where traffic signs are made, the factory has to store the beads until they are ready to be used, and eventually, each bead must be securely glued onto the STOP sign itself. What’s more, downstream effects included the fact that stop signs were now “thicker” — meaning, fewer could be packaged together and shipped to construction sites in one crate.
The major design challenge at this point was reducing cost and production time. (The functionality was fine — well, at least better than it was).
By the late 1930’s, material scientists at 3M introduced a revolutionary retroreflective sheeting material that changed the design of the STOP sign forever. Unlike the diffuse reflective beads used until this point, this retroreflective surface worked by “taking in” a source of light and reflecting back a portion of that light directly to the source itself.
3M’s design, revolutionary as it was, wasn’t all that different than the glass bead approach. Like previous designs, they too used glass beads. However, the key difference was that they opted to crush the glass beads down to a slightly larger than sand-sized level. Afterwards, a resin was used to adhere the crushed glass material to the entire face of the STOP sign.
This design approach worked — for a few years. Yes, it did indeed enhance the visibility of the STOP sign in comparison to the glass bead approach. And yes, it was (a little) cheaper to produce. However, the rough surface of the STOP signs quickly collected dust, dirt, and debris in its microscopic crevices. Over time, STOP signs took on a dirty, dull appearance — which, reduced their visibility to some extent. Likewise, during inclement weather, moisture would cling to the STOP sign’s surface, making it challenging to read.
This issue was resolved within a few years by covering the STOP sign with a transparent, smooth film. This solution offered the best of both worlds: being transparent, it allowed the STOP si
gn to retain all of its retroreflective properties — simultaneously, the smooth surface eliminated (or reduced considerably) the accumulation of dirt and moisture.
This type of sheeting is referred to in the industry as “flat-top sheeting” — also known as “engineering grade sheeting”. It offers an enhanced viewing distance of up to 500 ft. for motorists. (Engineering grade sheeting is still in production, and is used in environments with light traffic and speeds lower than 35 MPH).
To recap: at this point in history (circa 1940 – 1950s), all new STOP signs produced for use in the United States had the following properties:
- Octogonal shape
- Yellow background color
- Black “stop” text
- Black border (inlaid slightly from the perimeter)
- Retroreflective material (viewable up to 500 ft.)
So, when (and why) did the STOP sign turn red?
In 1954, a big decision had to be made about traffic signs. Many signs on the roadways were yellow. As described earlier, yellow traffic signs indicated some form of warning or caution to motorists. But there are tons of things that motorists have to be cautious of while driving. For example, “SLOW”, pedestrian crossing, traffic light ahead, horse and wagon, “DIP”, “NO OUTLET”, “STREET CLOSED”, “DEAD END”, deer crossing, construction ahead, etc…
Bear in mind, the list of traffic signs was much, much shorter in the 1950’s than it is today, but you get the point. We’re still talking about dozens upon dozens of signs. All yellow. All black text. It gets confusing. And confusion on the roadway is a precursor for accidents.
The STOP sign seemed to become red almost overnight. According to revisions made to the Manual on Uniform Traffic Control Devices in 1954, the STOP sign should be “red” with “white letters” — which, followed similar practices and color-coding systems adopted by traffic signals and railroads. According to a New York Times interview with Professor Gene Hawkins from Texas A&M University, “red” has always been associated with the idea of “stopping” (Greenbaum & Rubinstein, 2011: http://www.nytimes.com/2011/12/11/magazine/stop-sign.html?_r=0). After all, red had been used at traffic lights for the same purposes for nearly 40 years. Implicitly too, the color red has strong associations with themes such as “danger”, “blood”, and “bad” — all things that potentially stem from choosing to run a STOP sign. But if this were the case, why didn’t the STOP sign switch over to red much, much sooner?
The issue was production. The color yellow wears quite well in heavy traffic environments with rocks and sleet and sunlight and exhaust and rain. Red, on the other hand, degraded too quickly; signs would have had to be replaced (which adds to cost, time, etc.).
Early red STOP signs are a testament to this issue. Even after guidance from the Manual of Uniform Traffic Control Devices, manufacturers still weren’t quite sure how to color the sign in a red that would resist fading and slow, but gradual wear. As a result, it would still be a couple more years until STOP sign manufacturers got the “recipe right” for the red color they needed.
To help you keep perspective on all of this — consider the following.
These design iterations came about almost a decade before many of today’s government agencies and organizations aimed at traffic control were firmly established. For instance, National Highway Traffic Safety Administration (NHTSA) didn’t exist until 1970, when it was introduced as an offshoot of the Highway Safety Act. Yet even at this point in history, engineers, scientists, and motorists understood the need to promote safety on the roadway.
STOP Sign Design Considerations for the Future
Today, we face new challenges in STOP sign design. According to Retting, Weinstein, & Solomon (2003), there are approximately 700,000 police-reported motor vehicle crashes each year at STOP signs. Furthermore, approximately one-third of these reports involved one or more injured motorists or passengers (Retting et al., 2003).
To compete with these challenges, new STOP sign design concepts have begun to see production in high-risk areas (e.g., school zones, heavy pedestrian traffic). For instance, STOP signs with LEDs at the perimeter of the sign have begun to be incorporated into intersections with high-risk for driver distraction. Areas with busy downtowns, increased nightlife activity, and heavy pedestrian traffic are ripe for these types of STOP signs because they stand out better amongst flashing lights from advertisements and the constant battery of headlights and taillights.
However, as more technology is incorporated into the STOP sign design, the cost of production increases as well. Indeed, these high-visibility, LED STOP signs cost approximately $450 per sign to produce. Compare that to your standard STOP sign, which usually costs less than $100 per sign.
Beyond technological consideration however, there are several dilemmas that need to be better understood by Human Factors Engineers, Sociologists, and Traffic Scientists. As one example of these dilemmas, imagine people in your neighborhood are unhappy with the number of motorists speeding down your roads. Through a lot of effort, they get a petition signed by local residents and have a couple STOP signs installed at key intersections to slow things down (e.g., four-way stops).
Problem solved, right?
Well, no. Just the opposite, in fact. Numerous studies from the Institute of Transportation Engineers have demonstrated that having an excessive amount of STOP signs in a residential neighborhood can lead to increased speeding behavior amongst motorists. The reasoning behind this is that when — or, if — motorists come to a full stop at the STOP sign, they attempt to make up for “lost time” by speeding up between signs. Especially when motorists are familiar with the road — ex. I know there are 4 STOP signs down this stretch of road — they become focused on looking for the traffic signs, rather than looking for pedestrians, cyclists, and fellow motorists.
Detour: if you think a series of well-placed speed humps would work better than STOP signs, you might want to think again. While speed humps and other traffic calming measures do lead to slower vehicle speed during ingress of the speed hump (Johnson & Nedzesky, 2004), motorists are likely to “compensate” after egress by speeding up considerably — similar to the issue of having too many STOP signs.
This is one of the assortment of dilemmas regarding STOP sign design that we must resolve in the future. Topics such as promoting safety, awareness, and compliance are just a few of the topics that Human Factors Engineers can play a part in over the coming years. Understanding how and why motorists interpret and respond to STOP signs in various contexts, driving conditions, and social environments will be a challenging, but critical piece of the puzzle to put together prior to making informed, smart design recommendations.