In the United States, and on this blog, we spend a lot of time talking about the dangers of drunk driving, particularly around holidays like the Fourth of July and New Year’s, and with good reason - about one-third of total traffic fatalities involve alcohol impairment.

However as states have begun to legalize marijuana for medical or even recreational purposes in the last few years, driving while under the influence of marijuana is becoming an increasingly important topic.

It’s a common myth that driving under the influence of marijuana isn’t all that dangerous. But how does marijuana really affect the ability to drive safely?

Marijuana Use Impairs Driving Performance

While the National Highway Traffic Safety Administration reports that marijuana-impaired drivers are sometimes able to “pull themselves together” to concentrate briefly on simple tasks, or may try to make up for their impairment by driving more slowly or cautiously, driving high is by no means safe.

According to the NHTSA, marijuana has been shown to impair driving performance significantly for 1 to 2 hours following use, and residual effects have been reported up to 24 hours after use. These impairments are moderate in low doses and severe with high doses, chronic use, and in combination with alcohol.

Reported impairments include:

• Inability to concentrate and maintain attention
• Difficulty performing divided-attention tasks
• Reduced hand-eye coordination
• Distortion of time and distance
• Increased evaluation, decision, and reaction times
• Sleepiness

Even a moderate impairment can prevent drivers from responding quickly to unexpected hazards and maintaining the high level of attention needed for safe driving.

The National Organization for the Reform of Marijuana Laws (NORML), which promotes the legalization of the responsible use of marijuana, recognizes this danger in its Principles of Responsible Use, arguing that:

“The responsible cannabis consumer does not operate a motor vehicle or other dangerous machinery while impaired by cannabis, nor (like other responsible citizens) while impaired by any other substance or condition, including some medicines and fatigue.”

Limited Data Available on Marijuana-Impaired Crashes and DUIs

We already have a wealth of information on how alcohol, drowsiness, and even distraction affect our risk of being in a crash, so it may seem surprising that researchers do not yet have a clear picture of how many crashes or DUI convictions involve marijuana.

Law enforcement, court systems, and researchers have significant challenges to overcome in obtaining and interpreting this information:

• Standard field sobriety tests, which are 90% effective in identifying drunk drivers, only correctly identify 30% to 50% of marijuana-impaired drivers, according to the New York Times.
• Evaluations by specially trained Drug Recognition Experts are accurate but must be completed after an arrest, not at the scene of a traffic stop.
• Chemical tests of blood and urine are available but do not reliably predict how impaired a driver is. Drivers may test positive for marijuana days or even weeks after use.
• Many law enforcement and court record-keeping systems do not include whether an impaired driver was affected by alcohol, drugs, or both. The NHTSA has strongly recommended that these systems be updated to provide for more accurate monitoring of DUI cases.

As detection methods and reporting policies become more accurate, we can expect our understanding of this issue to become much clearer.

Los Angeles is already testing out a saliva swab drug test that can be used at the time of a traffic stop. Officials hope that having a quick and effective method to detect the presence of drugs will deter people from getting behind the wheel while under the influence of any substance.

Looking for More Information?

While the data is limited, researchers are still studying the issue! Take a look at the findings at the links below.

• A study by Columbia University released this past January found that the proportion of fatally injured drivers who tested positive for marijuana tripled from 1999 to 2010, from 4.2% to 12.2%. This may indicate that marijuana-impaired driving is playing an increased role in fatal crashes. The study is based on data from six states where toxicological testing is routinely performed on drivers involved in fatal car crashes.
• In Washington, 25% more drivers tested positive for marijuana in 2013, the first full year after the state legalized the drug than in 2012. However, there was no overall rise in DUI arrests and no significant increase in crashes.
• In Colorado, an increased proportion of drivers involved in fatal crashes tested positive for marijuana after 2009, when medical marijuana was legalized, than in the period before legalization. The study did not reveal whether the drivers were found to be impaired at the time of the crash, or whether they were at fault in the crash, so the results may only reflect an increase in use.

Curious about laws on marijuana and driving in your state? Check out NORML’s state-by-state list of drugged driving laws.