A June 2016 study in the Journal of Safety Research concludes that “polydrug use, rather than marijuana, is the most common cause of drugged driving in Colorado.”
The study, by Ed Wood and Stacy Salomonsen-Sautel, examines driving under the influence of drugs (DUID) based on impairment measures rather than drug tests.
Colorado has over 25,000 driving-while-intoxicated cases (DUI) per year, but they do not record how many of these involved drugged driving. The authors obtained data from the Colorado State Judicial Branch for all charges and judicial outcomes in 2013 for individuals charged with vehicular homicide or vehicular assault. The case files were then studied for written evidence of the cause of the DUI charge and classified in terms of alcohol only, single or multiple drugs other than alcohol, or alcohol plus one or more drugs.
The authors examined 1,263 charges against 229 defendants, but the actual details of the cases resulted in the sample being reduced to 170 cases (for example, a vehicular assault charge due to a DUI was filed as part of a plea bargain or a case did not involve vehicles and intoxication). Of these cases, 70 percent involved alcohol only and 30 percent involved drugs.
For the 51 cases involving drugs, 32 (18.8 percent of the total 170 cases) involved a combination of alcohol and drugs, and 19 (11.2 percent of the total 170 cases) identified drugs only, with no alcohol.
“Although marijuana was the most commonly cited drug (23 cases), it was the sole impairing substance identified in only three cases,” the report states. “However, after alcohol, the combination of alcohol and marijuana was the second most prevalent cause of DUI citations.”
Like any study, this one has limitations (detailed by its authors). Defendants charged with these vehicular crimes may not have the same characteristics as other DUI defendants. Court records in various counties may differ on the extent and accuracy of their detail. Some defendants testing positive for alcohol impairment may not have been tested for other drug use. Nonetheless, the authors believe their findings raises important public policy questions.
Since marijuana’s legalization in Colorado, the legislature has focused on the question of marijuana-impaired driving.
“Even though research has found that the proportion of marijuana-positive drivers in fatal motor-vehicle crashes is increasing in Colorado, it is imperative not to lose sight of DUI cases involving polydrug use,” the report explained.
According to Wood and Salomonsen-Sautel, their data makes it clear that a focus on marijuana alone overlooks the larger problem of drugged driving in which marijuana is but one component—in other words, the real threat to public safety is not marijuana use but polydrug use. People who only use marijuana are not the primary threat to highway safety, it’s people who drink alcohol or those who use several drugs, including marijuana, who are responsible for accidents.
These findings reinforce the need to improve biological testing for drugs after highway safety related incidents occur. This gets complicated with respect to marijuana, and its active ingredient THC, because as the authors of this study conclude from the available scientific literature, “There is no blood level of THC (or any other drug) above which everyone is impaired, and below which, no one is impaired.”
This is also true for alcohol, but it has a narrow range of concentrations that result in impairment to a per se law, setting a specific blood level upon which to base DUI charges, is feasible. This narrow range does not exist for marijuana and other drugs because many users have developed tolerance to impairing effects.
Another approach is to rely on Standardized Field Sobriety Tests (SFST). But these (the ones designed for use with alcohol impairment) don’t work reliably with respect to marijuana impairment.
“Both a 2006 and a 2011 study demonstrated that SFSTs were only moderately successful in identifying marijuana-impaired drivers (66 – 76 percent)” compared with 81 percent to 91 percent with alcohol impaired drivers.
The public policy issue here—determining when a driver is impaired by marijuana—is complex and difficult to resolve. But some facts are clear. The presence of marijuana metabolites indicates recent use, not impairment, so the presence of marijuana metabolites in individuals involved in traffic fatalities tells us nothing about the role of marijuana in the cause of the accident.
That is already well-known.
Now we have another important fact. Marijuana use alone is not a major contributor to vehicular homicide or vehicular assaults in Colorado.
And another important fact: Alcohol and polydrug use present a greater public health threat than marijuana use alone.