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Smeed's Law

Reuben Smeed (1909-1976) was a British statistician who analyzed how bombing patterns during World War II affected the likelihood of being shot down. After the war he studied traffic patterns and made a discovery that I find quite amazing.

Smeed discovered that the number of traffic fatalities in a given region depends on two things: the number of people and the number of cars. The fatality rate is not influenced by improvements in automobile safety or by what traffic laws are put in place. Smeed's Law asserts that the number of deaths is governed by this equation, where D is deaths, n is number of cars, and p is the number of people:



This law has held true since the start of the 20th century, when automobiles had no seat belts or safety glass. It is true (or within a factor of two) during any time period in nearly all countries studied carefully.

How can that make sense? Smeed suggested that inherent aspects of human psychology regulate the death rate in a way that does not depend on safety features or speed limits. In a nutshell, people tend to drive recklessly. They will drive as recklessly as possible until traffic fatalities reach a certain level. Once the fatality level is intolerably high, people drive more safely.

The way this might work is that an individual might drive more and more recklessly until someone they know is hurt or killed in an automobile accident. Then that shakes some sense into the person and makes them drive more carefully. These factors balance out on a population scale to make Smeed's Law accurate.

Fascinating!

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