On the identification of the effect of prohibiting hand-held cell phone use while driving: Comment

In a paper recently published in this journal (Nikolaev, A.G., Robbins, M.J., Jacobson, S.H., 2010. Evaluating the impact of legislation prohibiting hand-held cell phone use while driving. Transportation Research Part A 44, 182–193.), Nikolaev et al. (2010) provide evidences on the effect of hand-held cell phone bans on driving safety. More specifically, they analyze the impact of a state-wide ban on hand-held cell phone use while driving on the number of fatal automobile and personal injury accidents per 100,000 licensed drivers per year and conclude that the ban had a significant negative impact on both the mean fatal accident rate and the mean personal injury accident rate. In this paper I argue that they lack of a good identification strategy that enables them to correctly identify the causal effect of the ban. I also provide evidence that the effect they find is a combination of the ban effect and of unobservable variables not accounted for in their analysis. Finally, I provide a way where one can control for unobservables when estimating the causal effect of the ban and find that indeed that ban appears to have a negative effect on fatal automobile accidents.