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