A new study from the University of Florida asserts that because Illinois instituted an alcohol tax increase in 2009 and the rate of alcohol-related traffic fatalities have declined 26 percent since 2009, the tax must certainly be responsible for the decline in deaths. Of course, news outlets have begun touting the study as evidence that increasing taxes results in fewer deaths. Are they right?
A team of UF Health researchers discovered that fatal alcohol-related car crashes in Illinois declined 26 percent after a 2009 increase in alcohol tax. The decrease was even more marked for young people, at 37 percent.
So, was it the alcohol tax increase that led to the state’s declining alcohol related traffic deaths? To answer that question, one need only examine the rate before and after the tax increase went into effect and compare it to the rest of the United States. Looking at these numbers (provided by DISCUS), it becomes clear that the rate of alcohol related traffic fatalities was declining faster in the year before the tax increase went into effect. Furthermore, since the state jacked up the alcohol taxes, Illinois has experienced a slower decline than the rest of the nation.
Repeat after me: correlation does not equal causation. The study’s authors claim that since the 2009 tax increase went into effect, Illinois saw a 26 percent reduction in traffic fatalities. That certainly is an impressive decline. But just because two things correlate, doesn’t mean that there is a causal relationship. For example, just because the divorce rate in Maine correlates almost one-to-one with the rate of margarine consumption in the U.S., it doesn’t mean one caused the other.
Furthermore, the whole nation saw a decline in alcohol-related traffic deaths between 2001 and 2011.
The authors’ results are made more impressive by their selective inclusion of data. They include traffic information as far back as 2001 to create a “baseline” for their “pre-tax” data set, but only include 28 months of data in their after-tax data set. I’m sure they have a good scholarly explanation for why they chose to look so far back and not so far ahead. Whatever their reasons, doing this significantly downplays the decline that was already in progress before the tax. It also has the effect of exaggerating the after-effect of the tax on traffic deaths. As the first figure shows, Illinois alcohol related traffic deaths declined most significantly between 2004 and 2008. Combining those years with the less-impressive declines that occurred in 2001 to 2004 makes the total decline seem smaller. Furthermore, not including data from 2012—when the decline in Illinois alcohol-related traffic deaths was significantly smaller than the rest of the U.S.—makes the post-tax effect look much more significant than it was.
Last, it’s worth pointing out that the source of the data for the study, number from the National Highway Traffic Safety Administration, aren’t exactly precise. As NHTSA itself notes, the Fatality Analysis Reporting System which NHTSA and this study rely on to come up with numbers of alcohol-related traffic deaths are estimates. Only 40 percent of people killed in traffic accidents are tested for drugs. As NHTSA put it on their website, “To address the missing data issue, NHTSA has developed and employs a statistical model to estimate the likelihood that a fatal crash-involved driver or nonoccupant was sober (BAC of zero), had some alcohol (BAC of 0.01-0.09), or was intoxicated (BAC of 0.10 or greater) at the time of the crash. The statistical model is based on important characteristics of the crash including crash factors, vehicle factors, and person factors. The published estimates are a combination of actual BAC test results and estimates based on this model” (emphasis added).
Whether the 2009 alcohol tax did or did not affect the rate of alcohol-related traffic fatalities in Illinois isn’t something we can determine based on this study.