Barking up the wrong tree

The single biggest predictor was not statewide alcohol problems, safety belt use, number of older drivers or wealth, but the murder rate.

The researcher, psychologist Michael Sivak, argues that this is not because people are using cars as murder weapons, but because the murder rate is a proxy for aggression and "the same aggressive tendencies that contribute to homicides also demonstrate themselves, to a certain degree, in interpersonal behaviors on the road".