Cannabis policy and traffic accidents

by Chris Auld

Do reductions in enforcement of marijuana prohibition lead to more traffic accidents?

David Mineta, Deputy Director for Demand Reduction for the Office of National Drug Control Policy, writes in The Tennessean:

Proponents of marijuana legalization often argue it will do everything from fixing our economy to ending violent crime (“Marijuana legalization bill offers safer alternative,” Tennessee Voices, Aug. 15). Yet, the science is clear: Marijuana use is not a benign drug and it is harmful to public health and safety.

No reasonable observer believes that marijuana legalization will “fix the economy” nor, obviously, would it “end” violent crime. Charitably suppose Mr. Mineta intends to counter the common view among drug policy experts that the costs of prohibition exceed the benefits. Mr. Mineta believes that demonstrating that “marijuana use [sic] is not a benign drug” is equivalent to showing that prohibition is sound policy, which is mistaken.  Mr. Mineta believes the science supports his claims:

Decades of scientific study, including research from the prestigious National Institutes of Health, show marijuana use is associated with addiction, treatment admissions among young people, fatal drugged driving accidents, and visits to emergency rooms. Data also reveal that marijuana potency has almost tripled in the past 20 years.

The negative social consequences Mineta claims support the case for prohibition are ER visits and traffic accidents.  But he makes an error in assuming that because marijuana use causes ER visits and traffic accidents, marijuana prohibition reduces ER visits and traffic accidents.

Consider a simple model in which traffic accidents (T) are caused by alcohol use (A), marijuana use (M), holding all other causes constant:

T = \beta_0 + \beta_1 A + \beta_2 M

Suppose \beta_1 > 0 and \beta_2 > 0, that is, assume that all else equal increases in either marijuana use or alcohol use causally increases accidents.

Now suppose marijuana is legalized and let the new consumption levels be A^L and M^L. We expect marijuana use to increase under legalization, so (M^L > M). What happens to alcohol use depends on whether alcohol and marijuana are economic substitutes or complements. If these drugs are substitutes, alcohol use falls when the (full) price of marijuana decreases with legalization, so (A^L > A).

The left hand side is the decrease in alcohol-caused accidents induced by marijuana legalization and the right-hand side is the increase in marijuana-caused accidents induced by marijuana legalization. The sign of the effect depends on demand elasticities and on the the relative dangers of these two drugs.

The evidence (e.g., Bates and Blakely 1999) suggests that it is likely that marijuana use causes accidents but that the effect is much smaller than that of alcohol, so \beta_2 << \beta_1. It follows that even a small increase in alcohol use may lead us to the superficially counterintuitive conclusion that: (1) marijuana use causes traffic accidents and (2) marijuana legalization reduces traffic accidents.

The evidence suggests that reductions in enforcement of marijuana prohibition cause decreases in traffic accidents. Chaloupka and Laixuthai estimate that marijuana decriminalization reduces youth traffic fatalities by 5.5% (p<0.01). Mr. Mineta's claim about ER admissions fails for the same reasons: Model (1993) shows that marijuana decriminalization increases marijuana ER mentions (note that a mention does not mean the drug was the direct nor indirect cause of the ER visit) but decreases
mentions of all other illicit drugs by about 14% (p<0.01).

Mr. Mineta then confuses correlation and causation:

This is especially troubling for use among teens because the earlier a person begins to use drugs, the more likely they are to develop a more serious abuse and addiction problem later in life.

The correlation that Mineta asserts does hold, but it does not follow that early use causes heavier use later in life. Even in the case of nicotine, which is an extremely physically addictive drug, the evidence suggests that exogenously changing age of initiation (such as through policy changes) has little effect on usage patterns later in life, see for example Glied (2003), Auld (2005), and Eisenberg and Rowe (2009). Put another way: the same characteristics which tend to induce a given person to begin smoking early in life also tend to induce that person to smoke more later in life. Since marijuana is much less addictive than tobacco, it would be very surprising to find that exogenous changes in age at initiation into marijuana use cause substantial changes in use later in life.

Mr. Mineta proceeds to argue that regulating marijuana would be bad policy because prescription drugs are regulated in Tennessee but nonetheless Tennessee has higher rates of prescription drug abuse than in other states, which is a senseless argument for several reasons, perhaps most notably that those other states also regulate prescription drugs.