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Why We Think White Stripes on the Highway Are Shorter Than They Really Are

Photo by jackdog2508/Flickr

In Traffic I mention one of the most common, and surprising, ways we are fooled by what we see on the road: White stripes. I was asked by one engineer to guess how long they are, and I was more than a bit off in my estimation.

It turns out I’m not alone. A fascinating new study, headed by Dennis Shaffer, assistant professor of psychology at Ohio State University’s Mansfield campus and appearing in Perception & Psychophysics, asked a group of subjects to recall from memory — as well as look at a recreated “stripe” and during an actual drive — the length of the dashes. The most common answer was two feet — which is interesting considering the federal guideline calls for ten feet.

The finding holds implications for traffic safety. Each dashed line measures 10 feet, and the empty spaces in-between measure 30 feet. So every time a car passes a new dashed line, the car has traveled 40 feet. But in this study, people consistently judged the lines and the empty spaces to be the same size, claiming that both were two feet.

“This means that to most people, 40 feet looks like a lot less than 40 feet when they’re on the road,” Shaffer said. “People cover more ground than they think in a given period of time, so they are probably underestimating their speed.”

Interestingly, Shaffer began his pioneering research when the federal guideline was for fifteen feet, which has since shrunk. But no matter.

“Wherever the researchers went, they found all lines to be close to the federal guidelines of the time. In Arizona in 2000, for instance, some lines were 16 feet long instead of the expected 15.

But even back then — when the federal guideline was 15 feet — people still thought of lines as measuring only two feet.

What’s going on?

One possible explanation: as we drive, we look out far ahead the car for safety reasons, so the only lines we really see are faraway lines that look small.

Even though lines appear to expand as a car passes by, drivers can’t safely notice that effect. Rather, the first line we can comfortably look at while driving safely is some 120 feet ahead — the fourth line ahead on the road. So perhaps we think that all lines are as small in reality as that one faraway line appears to be.

But why are so many people consistently wrong, in exactly the same way?

As to why everyone’s estimates were consistent in every experiment, Shaffer suspects that the answer has something to do with how our brains perceive geometry. Engineers design roads, buildings, and public spaces using Euclidian geometry — the system of lines and angles first described by the ancient Greek mathematician Euclid. But this study and previous ones suggest that our brains perceive objects in a non-Euclidian way.

Might this cause a paradigm shift in the schools of highway engineering, a ‘non-Euclidian’ revolution? Maybe we shouldn’t use lines at all, and instead use random geometric patterns — Mandelbrotian fractals? — to delineate highway lines. In any case, the study is useful in quantifying what most engineers, and readers of Traffic, already know. Shaffer, meanwhile, carries on.

In the future, Shaffer will examine how people perceive the size of lines that are oriented at different angles — as if seen by a driver approaching a bend in a road — and how our perceptions affect our ability to judge the steepness of hills.

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This entry was posted on Tuesday, February 3rd, 2009 at 9:27 am and is filed under Drivers, Roads, Traffic Engineering, Traffic Psychology, Uncategorized. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.

2 Responses to “Why We Think White Stripes on the Highway Are Shorter Than They Really Are”

  1. Bossi Says:

    Just a quick addition (though it may already be on your mind) — the effect is also quite profound for signing, be it along the side of the road or overhead. I had never known how large those directional signs are above a freeway until I walked up beside one and discovered it was twice as tall as I. …Some are 3x taller, with each letter being about my height. I suppose that would mean that billboards are “supersized”.

  2. Alan MacHett Says:

    When I first read this passage in the book, I estimated the lines to be about 5 feet long. I completely disbelieved that they were 10 feet. Reading it here again, I still utterly disbelieved; surely they are 3-5 feet long! …so I jogged, tape-measure in hand, the few blocks to the nearby highway to see for myself! (It’s 2:30am here in Chapel Hill, and the traffic is all but absent; so this was a relatively safe thing to do.) Lo and behold! the lines really are 10 feet long!!

    Perhaps the general estimate of 2 feet speaks also of a general inability to judge distance; I believe you mentioned something to that effect in the book. I say this because, even standing on the line just now, I still didn’t imagine it was 10 feet. Or perhaps it is only I who sucks at measurements…? ;-)

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Traffic Tom Vanderbilt

How We Drive is the companion blog to Tom Vanderbilt’s New York Times bestselling book, Traffic: Why We Drive the Way We Do (and What It Says About Us), published by Alfred A. Knopf in the U.S. and Canada, Penguin in the U.K, and in languages other than English by a number of other fine publishers worldwide.

Please send tips, news, research papers, links, photos (bad road signs, outrageous bumper stickers, spectacularly awful acts of driving or parking or anything traffic-related), or ideas for my Slate.com Transport column to me at: info@howwedrive.com.

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