How Does Marijuana Affect Driving?

Chance of Crashing Increases If You Choose to Drive High

Car accident scene at night

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Using marijuana can impair your judgment, motor coordination, ability to concentrate, and slow your reaction time. Therefore, it 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. This may lead to poor speed control, drowsiness, distraction, and the inability to read road signs accurately.

The Chances of Crashing Increases

More than one research study has found a direct link between THC (the psychoactive chemical in marijuana) 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 significantly increases after using marijuana.﻿﻿ 

Another meta-analysis estimates that the risk of a crash that results in serious injury or death doubles after marijuana use.﻿﻿ In the 2015 "Traffic Safety Facts: Drug and Alcohol Crash Risk" report, the National Highway Traffic Safety Administration (NHTSA) notes that THC increased crash risk by 1 to 3 times more than sober drivers.﻿﻿

Some studies also note that high-risk groups for car accidents are those most likely to use marijuana.﻿﻿ Most notably, this is young men in their late teens and 20s. This could play a factor in some of the statistics as well.

Higher Levels, Higher Risks

When drivers are involved in auto crashes, the drivers with THC in their blood are more likely to be the driver responsible for the accidents. This is compared to drivers who were not using drugs or alcohol. It 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. With the two combined their effect on driving skills are not added, they are multiplied, research shows.

Driving While Stoned Common

Research from the NHTSA indicates that when drivers are killed in motor vehicle crashes, drugs and alcohol are involved about 11% of the time.﻿﻿ The NHTSA report also shows an increase in the number of drivers who have tested positive for marijuana. They state that one in four drivers tested had THC in their system.

Increases in driving while under the influence of marijuana is attributed to the recent legalization and popularization of medical and recreational cannabis in many U.S. states.

Some drivers who use marijuana claim that smoking weed actually improves their concentration and therefore, their driving skills. Researchers have concluded that this might be true for the first few minutes of driving. However, marijuana users can soon become weary, bored, or distracted and their attention can begin to drift.

Problems With These Studies

The NHTSA, CDC, and almost every researcher that studies this issue notes that there are concerns with the marijuana-impaired driving statistics. One glaring problem is the testing procedures because THC can be detected in a person's system for days or weeks at a time.

Unlike roadside tests for blood alcohol content, the tests currently used for marijuana can pick up traces even if the person is not impaired at the time of a crash. They may have smoked the day before or even earlier and the tests will still be positive.

The CDC notes that multiple drugs in a person's system make it difficult to determine which contributed to a crash.﻿﻿ Also, drivers are not always tested for drugs, particularly if they already have an illegal blood alcohol concentration level. All of these factors can throw off data used in scientific studies.

The Risks Do Remain

Despite these concerns, the research does show that marijuana impairs a person's ability to drive.

Also, though the laws vary by state, it is illegal to drive impaired in the United States. Using marijuana and driving while impaired can have serious legal consequences.

As a conclusion, the CDC says that "the safest option is not to have any alcohol or drugs in your system at all."﻿﻿

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5 Sources
  1. Rogeberg O, Elvik R. The effects of cannabis intoxication on motor vehicle collision revisited and revised. Addiction. 2016;111(8):1348-59. doi: 10.1111/add.13347


  3. Compton RP, Berning A. Drug and alcohol crash risk. (Traffic Safety Facts Research Note. DOT HS 812 117). Washington, DC: National Highway Traffic Safety Administration. February 2015.

  4. Li MC, Brady JE, Dimaggio CJ, Lusardi AR, Tzong KY, Li G. Marijuana use and motor vehicle crashes. Epidemiol Rev. 2012;34:65-72. doi:10.1093/epirev/mxr017

  5. Office of Noncommunicable Diseases, Injury, and Environmental Health. What You Need to Know About Marijuana Use and Driving. Centers for Disease Control and Prevention. 2017.

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