Driving impaired isn't cool, regardless of what substance you're on. But a Columbia University study of alcohol and drugs in fatal crashes shows that marijuana impairment is among the least common factors, while alcohol and prescription drugs remain the highest.
According to the, 31.9 percent of all drivers involved in fatal crashes come up positive on drug screens. Add alcohol to the mix, and you become 23 times more likely to get in an accident than someone who is just solely drinking.
According to the study, the odds ratio that you get in an accident while on depressants including prescription pills, 3.57 while on stimulants like meth and cocaine, 3.03 while on narcotics, and 1.83 for cannabis (* there's a caveat to this, keep reading).
Alcohol, meanwhile, remains the worst culprit. Nearly sixty percent of all fatal crashes involved alcohol. The summary/press release from Columbia doesn't breakdown the rates of crashes that involved both alcohol and drugs by each drug category. But according to the report "the odds of fatal crash involvement increased by more than 13 times for those who were alcohol-positive but drug-negative, more than two-fold for those who were alcohol-negative but drug-positive, and 23 times for those who were positive for both alcohol and drugs."
The study did have it's flaws, though. Notably, it was done using data collected from the 2007 National Highway Traffic Safety Administration 2007 Roadside survey of Alcohol and Drug Use by Drivers. As the researchers at Columbia point out: the drug tests only gave a positive or negative - not specific amounts that could help researchers determine whether or not it truly had anything to do with impairment.
For example, someone who used cannabis a week ago at a party then was involved in an unfortunate fatal accident today would test positive for marijuana even though they had clearly not been stoned at the time of the accident.
Dr. Guohua Li, professor of epidemiology and director of the Center for Injury and Epidemiology and Prevention at Columbia's Mailman School of Public Health, puts it more succinctly:
"First, a positive test indicates that the driver had used the drug detected but does not necessarily mean that the driver was impaired by the drug at the time of the crash or survey," he writes. "Second, variations in individual tolerance and pharmacological characteristics of different drugs make it difficult to determine drug impairment. Also, there is no uniformly accepted definition of impairment for different drugs."
While not directed specifically at marijuana use, Li's statements certainly seem to apply to pot that situation more so than other drugs which don't stick around in the body's chemistry for near as long and therefore are more likely to be causing impairment if found in tests. Cocaine and other stimulants can be out of your system in a day and a half, for example. And alcohol can be gone within hours depending on your body weight and how much you consume.
Li's research did lead him, however, to caution strongly against alcohol and prescription drug use.
"While alcohol-impaired driving remains the greatest threat to traffic safety, these findings about drugged driving are particularly salient in light of the increases in the availability of prescription stimulants and opioids over the past decade."