How Does Marijuana Affect Driving?

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.

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 1.25 times. This figure took into account contributing factors such as alcohol use and testing procedures for intoxication, so it is actually lower than some contradictory reports.

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.

Research from the NHTSA indicates that when drivers are killed in motor vehicle crashes, drugs and alcohol are involved about 11 percent 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.

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.

The NHTSA, CDC, and almost every researcher that studies this issue do note 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.

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

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