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Status Report, Vol. 42, No. 10 | October 13, 2007 Subscribe

Risk compensation theory pops up where it's completely irrelevant

Every now and then someone resurrects the idea of risk compensating behavior to claim that this or that highway safety program costs lives instead of saving them. The idea is that anytime a safety program is introduced — airbags are added to cars, for example, or belt use is mandated — drivers will compensate by taking more risk.

The latest resurrection of this idea is in a recent issue of the statistical journal Significance, in which John Adams claims that Britain's safety belt law "failed to achieve the life-saving benefits claimed for it" because its proponents, including the British Medical Association, Royal Society for the Prevention of Accidents, and Royal College of Surgeons, weren't "aware of risk compensation."

Institute president Adrian Lund counters that "they probably were aware, but they knew, like everyone else who follows this subject in the scientific literature, that risk compensation in response to occupant protection innovations repeatedly has been debunked. This doesn't mean drivers never change their behavior in response to safety technologies. Sometimes they do when vehicle designs are changed in ways that give motorists direct and immediate feedback — for example, when vehicle acceleration capabilities are boosted or brakes improved."

Such features change the driving task, and some motorists respond by changing their behavior. This could explain why antilock brakes have so little effect on crashes and why benefits from studded snow tires have been less than expected. Some people drive faster because of how their vehicles handle.

There's no evidence to support the notion that people drive any differently when safety features like energy-absorbing steering columns, high-penetration-resistant windshields, or airbags are introduced (see "More junk science for the Institute to refute," Feb. 25, 1995). Nor do they change when belt use is required. These don't alter how cars drive or how people drive them.

To believe Adams you'd have to believe that people have a certain tolerance for risk and that their levels of risk are regulated by a homeostatic mechanism so that, if forced to "consume" more safety than they voluntarily would choose, people will balance the safety increase by taking more risk. It's a stretch, isn't it?

But Adams isn't the only one to make this stretch. People have been misapplying the idea of risk compensation to driver behavior since at least the 1970s. For example, economist Sam Peltzman claimed there would have been no change in crash deaths, or perhaps fewer deaths, if occupant protection hadn't been enhanced by safety regulations. But Peltzman didn't provide credible evidence that drivers compensated for the regulations.

Studies show that deaths are reduced when states enact belt laws, and Adams doesn't provide any evidence to the contrary. But don't expect claims like his to cease. Someone else is likely to come forth with stop-the-presses insights based on the same misapplication of risk compensation.

"Don't believe them," Lund warns, "not until they produce credible evidence that people compensate for safety and not for factors that change the driving task."

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