Distracting Miss Daisy

Why stop signs and speed limits endanger Americans

There is a stretch of North Glebe Road, in Arlington, Virginia, that epitomizes the American approach to road safety. It’s a sloping curve, beginning on a four-lane divided highway and running down to Chain Bridge, on the Potomac River. Most drivers, absent a speed limit, would probably take the curve at 30 or 35 mph in good weather. But it has a 25-mph speed limit, vigorously enforced. As you approach the curve, a sign with flashing lights suggests slowing further, to 15 mph. A little later, another sign makes the same suggestion. Great! the neighborhood’s more cautious residents might think. »

We’re being protected. But I believe policies like this in fact make us all less safe.

I grew up in Great Britain, and over the past five years I’ve split my time between England and the United States. I’ve long found driving in the U.S. to be both annoying and boring. Annoying because of lots of unnecessary waits at stop signs and stoplights, and because of the need to obsess over speed when not waiting. Boring, scenery apart, because to avoid speeding tickets, I feel compelled to set the cruise control on long trips, driving at the same mind-numbing rate, regardless of road conditions.

Relatively recently—these things take a remarkably long time to sink in—I began to notice something else. Often when I return to the U.S. (usually to a suburban area in North Carolina’s Research Triangle), I see a fender bender or two within a few days. Yet I almost never see accidents in the U.K.

This surprised me, since the roads I drive here are generally wider, better marked, and less crowded than in the parts of England that I know best. And so I came to reflect on the mundane details of traffic-control policies in Great Britain and the United States. And I began to think that the American system of traffic control, with its many signs and stops, and with its specific rules tailored to every bend in the road, has had the unintended consequence of causing more accidents than it prevents. Paradoxically, almost every new sign put up in the U.S. probably makes drivers a little safer on the stretch of road it guards. But collectively, the forests of signs along American roadways, and the multitude of rules to look out for, are quite deadly.

Economists and ecologists sometimes speak of the “tragedy of the commons”—the way rational individual actions can collectively reduce the common good when resources are limited. How this applies to traffic safety may not be obvious. It’s easy to understand that although it pays the selfish herdsman to add one more sheep to common grazing land, the result may be overgrazing, and less for everyone. But what is the limited resource, the commons, in the case of driving? It’s attention. Attending to a sign competes with attending to the road. The more you look for signs, for police, and at your speedometer, the less attentive you will be to traffic conditions. The limits on attention are much more severe than most people imagine. And it takes only a momentary lapse, at the wrong time, to cause a serious accident.

Smeed’s Law, or Why Safety Measures Don’t Improve Safety

What matters most for road safety? The quality of the roads themselves? The engineering of the cars that travel them? The speed limit? The answer may be “none of the above.” In 1949, a British statistician named R. J. Smeed, who would go on to become the first professor of traffic studies at University College London, proposed a now-eponymous law. Smeed had looked at data on traffic fatalities in many different countries, over many years. He found that deaths per year could be predicted fairly accurately by a formula that involved just two factors: the number of people and the number of cars. The physicist Freeman Dyson, who during World War II had worked for Smeed in the Operational Research Section of the Royal Air Force’s Bomber Command, noted the marvelous simplicity of Smeed’s formula, writing in Technology Review in November 2006: “It is remarkable that the number of deaths does not depend strongly on the size of the country, the quality of the roads, the rules and regulations governing traffic, or the safety equipment installed in cars.” As a result of his research, Smeed developed a fatalistic view of traffic safety, Dyson wrote.

Smeed’s Law has worked less well since the mid-1960s; traffic deaths have been somewhat reduced by engineering features such as seat belts and air bags. But technical improvements generally matter less than you might expect, because they affect driver behavior. It’s called “risk compensation”: as cars become safer, drivers tend to take more risks. Psychological factors, in other words, appear to play a huge role in road safety, and they often undercut well-intentioned safety initiatives.

I’ve spent my professional life studying adaptive behavior—how changes in the environment lead to changes in the ways humans and animals act. I’d contend that as traffic signs have proliferated in the U.S., drivers have adapted in profoundly unhealthy ways. We may imagine that driver training is something that happens to 16-year-olds in small cars labeled studentdriver. But of course we spend a lifetime on the roads after we get our licenses, and we’re being trained by our experiences every day. Let’s think about what drivers are actually learning on the roads in America.

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