How Does Marijuana Affect Driving?

Chances of Having a Auto Crash Can Double for Marijuana Users

Smoking Weed/Driving
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Question: How Does Marijuana Affect Driving?

Answer: Using marijuana can impair your judgment, motor coordination, ability to concentrate and slow your reaction time, and therefore can impair your driving skills. Anytime the skills needed to drive safely are impaired, even slightly, the chances of having an auto crash increase.

Specifically, studies have found that marijuana use affects the driver's concentration and ability to perceive time and distance.

These impairments can lead to poor judgment, slower reaction times, poor speed control, drowsiness, distraction, and the inability to read road signs accurately.

Chances of a Crash Doubles

More than one research study has found a direct link between THC concentration in the blood and impaired driving skills. An analysis of several studies has found that the risk of being involved in a motor vehicle crash doubles after using marijuana.

In highway crashes, including those that include fatalities, the most frequently found illegal drug found in the blood of the drivers is marijuana, according to the National Comorbidity Survey.

Higher Levels, Higher Risks

When drivers are involved in auto crashes, the drivers with THC in their blood are three to seven times more likely to be the driver responsible for the accidents than drivers who were not using drugs or alcohol. This is particularly true when THC is found at higher levels.

When marijuana use is combined with alcohol, the risk of having a highway mishap is significantly greater - much greater than with either drug used by itself. When marijuana and alcohol are combined, their effect on driving skills are not added, they are multiplied, research shows.

Driving While Stoned Becoming Common

Research from the National Highway Traffic Safety Administration indicates that when drivers are killed in motor vehicle crashes, drugs other than alcohol are involved about 18% of the time.

That means that one in six drivers killed in traffic mishaps are under the influence of drugs.

Another study found that of all drivers involved in auto crashes, 6.8% tested positive for THC, the active ingredient in marijuana. The same survey found that 21% of those drivers were above the legal level for blood alcohol content.

Some drivers who use marijuana claim that smoking weed actually improves their concentration and therefore their driving skills. One research study concluded that this might be true for the first few minutes of driving, but marijuana users soon become weary or bored with maintaining intense concentration and their attentions begins to drift. 

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National Institute on Drug Abuse. "Marijuana." DrugFacts Updated January 2014

National Institute on Drug Abuse. "Want to Know More?- Some FAQs about Marijuana." Marijuana: Facts for Teens Updated October 2013

National Institute on Drug Abuse. "Marijuana." Research Report Series Updated July 2012

The Partnership at "Marijuana." Drug Guide. Accessed April 2014.

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