For example, the use of the automobile exacts a predictable toll in human lives—more than forty thousands deaths annually in the United States. But that does not lead us as a society to give up cars. In fact, it does not even lead us to lower the speed limit. During an oil crisis in 1974, the U.S. Congress mandated a national speed limit of fifty-five miles per hour. Although the goal was to save energy, an effect of the lower speed limit was fewer traffic fatalities.