Neither is safe but statistics tell us that stoned driving is safer than drunk driving. Marijuana-impaired driving is of particular concern not just because of its inherent danger, but because of its increasing prevalence and a common but falsely-held belief that stoned driving is not dangerous. Marijuana-impaired driving is less deadly than drunk driving, just as a .22 caliber bullet is less deadly than a .45 caliber bullet. But all four can and do kill.
More people are killed by drivers on multiple drugs (polydrug users) than those on either alcohol alone or marijuana alone. This was made quite clear in last month’s report from the Washington Traffic Safety Commission.
The Commission reported that DUI is not just about alcohol – Polydrug cases (880) exceeded those impaired by alcohol alone (759). It’s clear that drugged driving is not just about marijuana – Other drugs, including narcotics, stimulants and depressants (246) exceeded marijuana alone (118). Drivers using marijuana or other non-alcohol drugs were highly likely to be polydrug users (>70%) whereas <40% of alcohol users were also polydrug users. It’s complicated.
Scientists rely upon two types of evidence to understand the impact of marijuana use on safe driving: experimental evidence and epidemiological evidence.
Experimental evidence comes from carefully-controlled experiments, typically on fewer than a couple dozen test subjects and control subjects. Nearly all published experimental evidence proves that marijuana impairment is real. Even educated members of the marijuana lobby admit this is true. But some in the marijuana lobby frequently deny that the observed level of impairment makes drivers unsafe. That’s where epidemiological evidence comes in.
Epidemiological evidence comes from observations of large populations, and therefore cannot be as carefully controlled as experimental evidence. There are literally thousands of epidemiological reports of the impairing effects of marijuana on driving safety, the vast majority of which conclude that marijuana-impaired drivers kill more people than ordinary drivers. Results vary widely from one study to another, so several meta-analyses have been performed to “average” the results. Here’s a popular report on one such analysis concluding that marijuana use within three hours of driving doubles the risk of a fatal crash.
Alcohol is much riskier than marijuana and its crash risk varies depending on the amount of alcohol consumed, the driver’s age, gender, and other factors. For example, someone at the “legal limit” of a .08 Blood Alcohol Content may have a five times increased risk of a fatal crash.
Someone on both alcohol and marijuana, that the Washington Traffic Safety Commission says is extremely common, will have a ten-fold increase in risk (two times for marijuana multiplied by five times for alcohol).