Have airbags and anti-lock brakes made us safer? The answer is no, according to a study by Dr. Mannering of Purdue University. And the reason is that people adjust their driving behavior to the safer cars:
The researchers used a series of mathematical equations in "probit models" to calculate accident probabilities based on the motor vehicle data and actual driving records. Using the data, the model enabled researchers to calculate the probabilities of whether drivers in different age and demographic categories would be involved in an accident. The models showed that the safety systems did not affect the probability of having an accident or injury.The study represents the first attempt to test the offset hypothesis using "disaggregate data," or following the same households over time instead of using more general "aggregate" data from the population at large.
"By using disaggregate data, we have added to the credibility that our findings actually reflect offsetting behavior," Mannering said. "And the 2005 National Highway Traffic Safety Administration fatality data released last month indicate that fatalities per mile driven in the United States have actually increased, which adds some aggregate validation of our findings."
I'm one of those codgers who grumble about how when I was kid we didn't even have seat belts, and all the other safety features we take for granted, and somehow survived childhood. Now I'm on firm scientific footing when I do so.
If we make such an adjustment without even thinking while driving a car, think of all the other stuff we simply adjust to so that we are keeping something like risk constant, or even increasing risk while thinking we are lowering it.
