I have a piece in the new issue of I.D. that considers what influence the visual displays of speedometers might have on our driving behavior. Story is here or after the jump…

Every time I drive my 2001 Volvo, I find myself staring at a design problem: the speedometer.

It’s not the placement, illumination, or typography. It’s the numbers. The maximum speed on the dial is 160 mph. I know from experience that this car, even with its turbo engine, doesn’t do anywhere near 160. It even begins to get a little wobbly around 90 mph—not that I’m at 90 very often.

So why is the maximum speed displayed more than twice the legal limit in the U.S. and almost twice what the car can handle? And what is this speedometer doing on a Volvo, a company whose heritage is predicated on safety?

Car companies actually have an ambivalent relationship with safe driving: One survey in Canada found that nearly half of all car ads depicted unsafe driving acts. I suppose that the speedometer’s high numbers are placed there to stimulate our imagination in the way that radio knobs allow us to crank up the volume far beyond the level where sound is distorted.

But even knowing that these numbers bear little relationship to reality, we’re affected by the visual display. No matter how fast we drive, the needle is always less than halfway up the dial, indicating there’s still plenty of room for acceleration. That remaining space may even goad us into testing the limits by going faster.

The speedometer presents an example of “choice architecture,” a coinage of University of Chicago professors Cass Sunstein and Richard Thaler that refers to the way context can affect behavior. In their book Nudge: Improving Decisions About Health, Wealth, and Happiness (Yale University Press, 2008), Sunstein and Thaler give the example of a school cafeteria in which administrators tried to encourage children to eat healthier food. It was found that simply by displaying the healthier stuff first on the lunch line, consumption of junk food was decreased by as much as 25 percent.

Would topping speedometers at, say, 100 mph influence the way we drive? This question was actually broached as long ago as Carter’s presidency, when the National Highway Traffic Safety Administration briefly enacted a cap of 85 mph on speedometers (with the speed limit of 55 mph marked). The measure was done for energy savings, not safety, but it was undone by the anti-regulatory Reagan Administration—one of whose first acts was to remove the solar panels that had been placed atop the White House.

Capping the speedometer would remove a theoretical and illegal max to test on public roads. Influenced by the so-called anchoring effect, people are induced to eat more when portion sizes are larger and to drink more when the range of beverage options is increased. (Many consumers eschew the biggest and smallest drink choices at fast-food restaurants, so companies have supersized their “large” choice, thus making the “medium” more palatable, even though it’s bigger than ever.) In the same way, the value of 160 on my speedometer has been shown to influence decision-making.

Even more troubling is the speedometer’s dumbness. The device gives a simple reading that lacks context. It tells speed, but it doesn’t convey other useful information. How does the car’s speed compare to the posted limit? How much time is saved by driving faster, and how does it compare to the added fatality risk of a crash (which rises exponentially at higher speeds)? What’s the minimum stopping distance at a certain speed? What is the fuel consumption (expressed in dollars per hour or some such) and rate of harmful emissions at one speed versus another? As the Prius’s dashboard monitor displaying fuel efficiency shows, when people receive live feedback about the consequences of their actions, they are more likely to change their behavior.

Stanford University researchers Manu Kumar and Taemie Kim have proposed a “dynamic speedometer” that would highlight the posted speed limit of whatever road you’re on. In simulator trials, they were able to reduce instances of unintentional speeding and convince subjects to drive more slowly in general. By adding the information previously mentioned, people might be “nudged” to make smarter decisions about their driving. For now, most don’t have a clue.

Tom Vanderbilt, a contributing editor at I.D., is the author of Traffic: Why We Drive the Way We Do (And What It Says About Us) (Alfred A. Knopf, 2008).

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