Marijuana usage in America rose 6.9% between 2009 and 2010, when some 17.4 million Americans reported using the drug. Sixteen states now permit the medical use of marijuana for diseases such as glaucoma and multiple sclerosis, but the drug’s growing popularity is primarily due to more recreational users ages 18 to 34, according to the Substance Abuse and Mental Health Services Administration.

While the dangers of drinking and driving are clear, the perils of operating a vehicle under the influence of marijuana are still being debated; groups such as the National Organization to Reform Marijuana Laws (NORML) claim that while risks do exist, they are minimal. Studies that describe marijuana’s adverse cognitive effects have typically been conducted in labs, not in real-world settings.

A 2011 metastudy from Columbia University published in Epidemiologic Reviews, “Marijuana Use and Motor Vehicle Crashes,” compares nine epidemiological studies from six countries published after 1990 on marijuana use and motor vehicle accidents.

Key study findings include:

• Eight of the nine studies reported a statistically significantly increased risk of crash involvement associated with a driver’s marijuana use prior to operating a vehicle.
• “Drivers who test positive for marijuana or self-report using marijuana use are more than twice as likely as other drivers to be involved in motor vehicles crashes.”
• In a study of more than 64,000 insured U.S. drivers between 1979 and 1985, 31% of drivers involved in a motor vehicle crash reported smoking marijuana prior to the accident.
• The study findings were “generally consistent across … different geographic regions and driver populations, [despite] us[ing] different research design approaches, and [being] based on different methods for measuring marijuana use.”

The authors note that the “crash risk appears to increase progressively with the dose and frequency of marijuana use.”

Tags: cars, drugs, safety, metastudy