@petergleick Just a thought: The motor-vehicle part might be a bit misleading. Because the more car centered your environment becomes, the less people – and especially child – unfriendly it becomes. And so you go out less, and the line goes down just because you have fewer possibilities of a crash. So the line would maybe go up if it accounted for loneliness, sedentary lifestyle and all the health effects that have, pollution of the air and water, etc, but it doesn't.