Mobile phone bans and traffic fatalities

Many countries have introduced bans on mobile phone use while driving, in order to reduce distracted driving, traffic accidents, injuries and fatalities. The idea has a fairly simple economic foundation based on rational behaviour - when the cost of something increases, we tend to do less of it. Mobile phone bans introduce penalties (usually in the form of fines) for mobile phone use while driving. This increases the costs of using a mobile phone while driving and reduces the number of people doing so.

But do mobile phone bans work to reduce traffic fatalities? That is the research question addressed in this recent article by Nicholas Wright (Florida Gulf Coast University) and Ernest Dorilas (Cone Health), published in the Journal of Health Economics (sorry, I don't see an ungated version online). Wright and Dorilas look at state-level data on traffic fatalities for 14 states over the period from 2000 to 2015. They implement both: (1) a regression discontinuity design, which looks at daily traffic fatalities for the 90 days before, and after, implementation of the (handheld) mobile phone ban in each state; and (2) a difference-in-differences strategy, which looks at the difference in monthly fatalities between states with and without a mobile phone ban, before and after the ban was introduced.

Using the regression discontinuity design, Wright and Dorilas find that:

...a handheld ban reduced daily traffic fatalities by 0.63 individuals or 0.012 fatalities per hundred thousand population. These reductions account for at least 36% of the mean fatalities over the period.

In the difference-in-differences analysis, they find that:

...the policy reduced motor vehicle casualties by 5.72 individuals on average each month.

The size of the effect is much larger in the regression discontinuity design than in the difference-in-differences analysis (about 18 vs. 5.7 fewer fatalities per month). However, both are statistically significant, providing strong evidence that the mobile phone ban reduces traffic fatalities. However, Wright and Dorilas note that:

...the long-term reduction in total monthly fatalities that we observed in the DID model (5.72) represents approximately one-third of the short-term policy impact... As such, the result indicates that handheld bans are still effective at curbing traffic fatalities over a longer time horizon, although the effect is significantly smaller. One potential explanation for this observed fade-out is that drivers are less inclined to comply with the policy over time...

It would be interesting to explore why there might be reduced compliance over time, if the penalties (and hence the costs) of violating the ban remain the same. Perhaps a policy of gradually increasing penalties over time would maintain compliance with the ban? Increasing the costs of distracted driving would maintain the salience of the penalty in the minds of drivers, and hopefully maintain a lower level of distracted driving (and resulting accidents, injuries, and fatalities) over a longer period of time.