Tom Vanderbilt | Essays

Traffic: Why We Drive the Way We Do

This article is adapted from Tom Vanderbilt’s new book, Traffic: Why We Drive the Way We Do (Knopf).

Once, on a driving trip in rural Spain, I decided to take a shortcut. On the map, it looked like a good idea. The road turned out to be a climbing, twisting, broken-asphalt nightmare of blind hairpin turns. There were few guardrails, just vertigo-inducing drops into distant gulleys. The few signs there were told me what I already knew: peligro. Danger. And how did I drive? Incredibly slowly, with both hands locked on the wheel, eyes boring straight ahead. I honked ahead of every blind curve. My wife, who fears both heights and head-on collisions, never trusted me with a Spanish map again.        

Was the road dangerous or safe? On the one hand, it was incredibly dangerous. The “sight distances,” as road engineers call the span required for one to see a problem and safely react to it (based on a certain travel speed), were terrible. The lanes were narrow and not always marked. There was only the occasional warning sign. Had there been a collision, there was little to keep me from tumbling off the edge of the road. And so I drove as if my life depended on it. Now picture another road in Spain, the nice four-lane highway we took from the airport down to Extremadura. There was little traffic, no police, and I was eager to get to our hotel. I drove at a healthy pace, because it felt safe: a smooth, flat road with gentle curves and plenty of visibility. The sun was shining; signs alerted me to every possible danger. And what happened? Grown briefly tired from the monotony of the highway (drivers have a greater chance of becoming drowsy on roads with less traffic and on divided highways free of junctions) and the glare of the sun, I just about fell asleep and ran off the road. Was this road dangerous or safe?

Of the two roads, the highway was of course the more objectively safe. It is well known that limited-access highways are among the safest roads we travel. There is little chance of a head-on collision, cars move at relatively the same speeds, medians divide opposing traffic streams, curves are tamed and banked with superelevation to correct drivers’ mistakes, there are no bikes or pedestrians to scan for, and even if I had started to nod off I would have been snapped back to attention with a “sonic nap alert pattern,” or what you might call a rumble strip. At the worst extreme, a guardrail may have kept me from running off the road or across the median, and if it was one of the high-tension cable guardrails, like the Brifen wire-rope safety fence, increasingly showing up from England to Oklahoma, it might have even kept me from bouncing back into traffic. 

Those rumble strips are an element of what has been called the “forgiving road.” The idea is that roads should be designed with the thought that people will make a mistake. “When that happens it shouldn’t carry a death sentence,” as John Dawson, the head of the European Road Assessment Programme, explained it to me. “You wouldn’t allow it in a factory, you wouldn’t allow it in the air, you wouldn’t allow it with products. We do allow it on the roads.”

This struck me as a good and fair idea, and yet something nagged at the back of my brain: I couldn’t help but think that of the two roads, it was the safer one on which I had almost met my end. Lulled by safety, I’d acted more dangerously. This may seem like a simple, even intuitive idea, but it is actually an incredibly controversial one — in fact, heretical to some. For years, economists, psychologists, road-safety experts, and others have presented variations on this theory, under banners ranging from “the Peltzman effect” and “risk homeostasis,” to “risk compensation” and the “offset hypothesis.” What they are all saying, to crudely lump all of them together, is that we change our behavior in response to perceived risk, without even being aware that we are doing so.

As my experience with the two roads in Spain suggested, the question is a lot more subtle and complicated than merely “Is this a dangerous or safe road?” Roads are also what we make of them. This fact is on the minds of engineers with the Federal Highway Administration’s Turner-Fairbank Highway Research Center, located in Langley, Virginia, just next to the Central Intelligence Agency.

The first thing to think about is, What is a road telling you, and how? The mountain road in Spain did not need speed-limit signs, because it was plainly evident that going fast was not a good idea. This is an extreme version of what has been called a “self-explaining road,” one that announces its own level of risk to drivers, without the need for excessive advice. But, you protest, would it not have been better for that mountain road to have signs warning of the curves or reflector posts guiding the way? Perhaps, but consider the results of a study in Finland that found that adding reflector posts to a curved road resulted in higher speeds and more accidents than when there were no posts. Other studies have found that drivers tend to go faster when a curve is marked with an advisory speed limit than when it is not.

The truth is that the road itself tells us far more than signs do. “If you build a road that’s wide, has a lot of sight distance, has a large median, large shoulders, and the driver feels safe, they’re going to go fast,” says Tom Granda, a psychologist employed by the Federal Highway Administration (FHWA). “It doesn’t matter what speed limit or sign you have. In fact, the engineers who built that road seduced the driver to go that fast.” But those same means of seduction — the wide roads, the generous lane widths, the capacious sight distances, the large medians and shoulders — are the same things that are theoretically meant to ensure the driver’s safety.

This is akin to giving a lot of low-fat ice cream and cookies to someone trying to lose weight. The driver, like the would-be dieter, is wont to “consume” the supposed health benefits. Consider a key concept in traffic safety engineering: the “design speed” of roads. This is a confusing concept, not least because engineers are often not so good at explaining their concepts to nonengineers. The so-called Green Book, the bible of U.S. highway engineers, defines “design speed” as the following: “The maximum safe speed that can be maintained over a specified section of highway when conditions are so favorable that the design features of the highway govern.” Got that? No? don’t worry — it confuses traffic people too. An easier way to understand design speed is to think of the speed that most people — what engineers refer to as the “85th percentile” of drivers — generally like to travel (thus leaving out the suicidal speeders and stubborn slowpokes). As we saw in the previous chapters, leaving it up to drivers to figure out a safe speed is itself risky business. Even more confusingly, sometimes this speed matches the speed limit, and sometimes it does not. Once engineers figure out the 85th per centile speed, they try to bring, where possible, the various features of the highway (e.g., the shoulders, the curves, the “clear zones” on the side of the road) into line with that speed.

So does this mean that everyone then travels at the “safe” design speed? Not exactly. As Ray Krammes, the technical director of the FHWA’s Office for Safety Research and Development, explained to me, drivers routinely exceed the design speed. “We know we can drive faster than the design speed,” he said. “We’re doing it every day. We set a design speed of sixty and people are driving seventy. If it’s a seventy-miles-per-hour design, there are a number of people out there pushing seventy-five or eighty miles per hour.” Drivers, in effect, are every day loading twenty-one people on an elevator that has a capacity of twenty and hoping that there’s just that extra margin of safety left. 

As we have seen, traffic engineers face a peculiar and rather daunting task: dealing with humans. When structural engineers build a bridge, no one has to think about how the stress factors and loads of the bridge will affect the behavior of the wind or water. The wind or water will not take a safer bridge as an invitation to blow or flow harder. It’s a different story when engineers design a road. “When the engineers build something,” Granda says, “the question everybody should ask is, What effect will it have on the driver? How will the driver react, not only today, but after the driver sees that sign or lane marking over a period of time? Will they adapt to it?”

To try to answer these questions, Granda, who works in the Human Centered Systems Laboratory at FHWA, spends his days running drivers on test roads in the agency’s driving simulator. “It is hard to know how human beings will react,” he notes. “We can decide to do something, and we think we know how they’re going to react. You don’t really know.” As Bill Prosser, a veteran highway designer for the agency, described it to me, “there are three things out there that affect the way a highway operates: the design, the vehicle, and the driver. We as design engineers can only control one of those. We can’t control the driver, whether they’re good, bad, or indifferent.” 

The best thing engineers can do, the thinking has gone, is make it easy. “You can’t violate driver expectation,” says Granda. Tests of what researchers call “expectancy” routinely show that it takes drivers longer to respond to something they do not expect than something they do expect. People were faster to respond when character traits corresponded to names in a way they expected (“strong John” versus “strong Jane”). Similar things happen in traffic. It takes us longer to process the fact that a car is approaching in our lane on a two-lane highway, instead of, as we would expect, in the other lane. A driver in Maine will brake faster for a moose than for a penguin. As David Shinar, a traffic researcher in Israel, has described it, “That ‘second look’ that we colloquially say we take when ‘we can’t believe our eyes’ may be a very real and time-consuming effort.” 

This is expressed on the highway in all kinds of subtle ways. Highway engineers have long known that a set of curves, seemingly a dangerous road segment, is less dangerous than a curve that comes after a long stretch of straight highway. A similar principle exists in baseball: A batter can more easily hit a curveball if he sees nothing but curveballs than when he is thrown a curveball after a steady diet of fastballs. So engineers strive for what they call “design consistency,” which basically means: Tell drivers what to expect, and then give it to them.

The flip side of this is that too much expectancy can be boring. You might feel, for instance, that interchanges, where the on-ramps and off-ramps swirl into the highway, are the most dangerous areas on the highway. They are certainly the most stressful, and they are home to the most crashes. But that’s not where most people lose their lives. “In terms of fatalities,” says Michael Trentacoste, the director of the Turner-Fairbank center, “the highest number is ‘single-vehicle run-off road.’ ” I thought back to my near accident in Spain. “If you look at Wyoming,” he continues, “they have a tremendous amount of single-vehicle run-off-the-road accidents. A few years ago they had the highest percentage of run-off-the-road [accidents] on the interstate. You’ve got long stretches, a lot of night- time driving, people falling asleep.”

This is why road designers will often introduce subtle curvatures, even when it is not warranted by the landscape. One rough rule of thumb for highways is that drivers should not drive for more than a minute without having a bit of curve. But highway curves, most of which can be driven much like any other section, are often not enough to keep a tired driver awake. Which is why engineers, starting in the 1980s, began to turn to roadside rumble strips. The results were striking. After they were installed on the Pennsylvania Turnpike, run-off-road crashes dropped 70 percent in the period studied. 

Those rumble strips would hardly lull drivers into falling asleep, knowing they’ll be startled awake if they drifted off the road. But does something about the highway itself help drivers fall asleep in the first place? The line between safety and danger is not always well defined, nor is it always easy to locate.

Excerpted from Traffic by Tom Vanderbilt Copyright (c) 2008 by Tom Vanderbilt. Excerpted by permission of Knopf, a division of Random House, Inc.

Posted in: Social Good, Technology

Comments [9]

hmmm. this is an interesting article, even though i am a graphic designer; since i am about to finish learning and pass my test *fingers crossed* it has a great relevancy. but just knowing that there are people out there working on and worrying about these things..bloody marvelous!!
Matt Penrose

Amazing write up, completely out of box thinking by road engineers. I never thought of involvement of these kinds of
ideas, psychsolgy, research ... for making a road. I'd say design is the prime consideration for any thing, gosh any thing you can think of and this was one of the best example.
Mayank Keshari

Excellent article. Thanks for posting. It's a fascinating topic and the roadways are certainly a good controlled environment in which to study risk vs. perceived risk and how that affects behavior. I don't envy those engineers whose job it is to work with those factors.

I can personally attest to the effectiveness of the PA Turnpike rumble strips. Not so much for waking me up from sleep, but for letting me know that I'm suffering from 'road hypnosis' and drifting a tad.
Chris Rugen

Nice observations, but signs of "risky vs boring" aren't buckling in my fat American graphic design gut. If you really was to be unnerved/ awakened to highway signage I ask that you take 78 out of the Holland or Lincoln Tunnel and try to find Maplewood, NJ. 5 miles in, where the divider/ intersection splits to the 1 and 9 and the Garden State Parkway is nothing short or graphic design euphoria/ apocalypse.

"Sign Sign everywhere a sign. Blocking out the scenery breaking my mind Do this, don't do that, can't you read the sign."
felix sockwell

Since this is a graphic design-centric site, it might be worth speculating that there are lessons for graphic design here. For instance, it seems likely that reader expectations about difficulty may affect response as much as drivers' expectations affect their response.
Gunnar Swanson

Interesting article... but about that 85th percentile of drivers determining the speed limit... if almost everyone goes 70-75mph and the speed limit is 55, shouldn't the speed limit be raised? It seems dumb that cops can just generate revenue by enforcing a law most of the population is voting against with their right foot.

Rob, I think the answer to that is one of Vanderbilt's main points...a higher posted speed limit makes people feel safer and more comfortable going over that limit, and if it's a wide, flat, and relatively straight road, they will continue to exceed that regardless of how high that number is.

Great article. Here in Australia, we had the Northern Territory recently introduce speed limits on the highways out of Darwin and amazingly, the death toll went up. Prior to this, with open speed limits, drivers drove the highways according to the conditions, their abilities, as well as the abilities of their vehicle.

With the introduction of a 110km/h speed limit on the open road, it appears that drivers tend to nod off (as you stated in your article), instead of either driving at your (comfortable) limit or the sheer conditions attracting all your attention on the driving task.

Sometimes it's out of the designers hands and we get nanny politicians making stupid rules and messing things up.

Its nice having the other perspective not only by drivers but how the road is engineered I give you a plus for this!

But then it boils down to responsible driving.

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