Can Smoking Marijuana Impair Your Driving?

Marijuana use can distort the senses, slow reaction time, and impair judgment and coordination, making it highly dangerous to drive under the influence of the drug.

It is a mind-altering drug, and as such, it should not be combined with operating a motor vehicle.

A marijuana high generally lasts about two hours; however, the National Highway Traffic Safety Association (NHTSA) warns that marijuana can impair driving for up to three hours. Issues can include:

  • Poor car handling ability
  • Impaired reaction times
  • Difficulties estimating distance and time
  • Trouble maintaining headway and distance between cars
  • Swerving into other lanes
  • Balance, motor skills, and coordination issues
  • Sleepiness
  • Inability to remain alert and vigilant for long periods of time
  • In some cases, individuals who are aware of their impairment may overcompensate while driving. They may drive slow, keep a lot of distance between them and the car in front of them, and take more precautions, for example. They may be able to sustain this for short periods of time, but with increased stress, long drives, and unexpected situations, it’s hard for drivers to maintain this added level of caution.
  • Combining marijuana with other drugs or with alcohol will increase the overall level of impairment, making driving even more dangerous. About 20 percent of drivers surveyed in 2014 tested positive for drugs, NHTSA reports.
  • The National Institute on Drug Abuse (NIDA) publishes that marijuana is the most commonly found drug in the bloodstream of people who have been involved in a car crash, and it may increase the risk for a fatal crash by as much as 50 percent. It is possible that the demographic using marijuana may be at a higher risk of a car crash already, however. In 2009, in one out of every three fatal car crashes involving driver death, the driver tested positive for drugs, NIDA warns.

Laws Regarding Driving and Marijuana Use

Marijuana medical professionals discuss blood alcohol concentration of a patient is considered an illegal drug on the federal level; however, several states have passed laws legalizing its use for medical and recreational purposes. It is harder to measure the level of impairment on marijuana as opposed to impairment due to alcohol. Alcohol impairment is measured by a breathalyzer or blood test, which analyzes a person’s blood alcohol concentration, or BAC. All 50 states cite anything over a 0.08 g/dL BAC as being legally impaired. If a driver is found to have a 0.08 g/dL BAC or above, they are issued a driving under the influence (DUI) ticket and often arrested. Alcohol and marijuana are metabolized differently, however, and such a test does not exist for marijuana intoxication.

Laws are different regarding driving under the influence of drugs, depending on the state and local government. For example, some states have a zero-tolerance policy, NIDA reports, and if any amount of THC (the active psychoactive chemical in marijuana) is present then a DUI may be issued. Other states, like Colorado where recreational pot use is legal, have a limit of five nanograms of THC per milliliter of blood, NPR publishes. Anything above this is considered impaired, and drivers may be arrested and issued a DUI. Still other states may issue a DUI if impairment is even suspected.

This is not to say that someone who has less THC in their bloodstream is not impaired, and the opposite can also be true. THC can stay in the bloodstream for longer than alcohol even when it is not actively impairing a person. THC is stored in fat cells, and a person can test positive for the drug up to a week after using it, even longer after the negative effects have worn off. An infrequent user may be very impaired with five nanograms of THC per milliliter of blood, while a regular user may not be. In a regular user, THC is likely to stay in the bloodstream longer as well.

At this point, there are no consistent measures to test marijuana impairment as it relates to driving. New methods are being researched and tested for accuracy, but they are not ready yet.

  • Marijuana and Driving Do Not Mix
  • It is safe to assume that driving while high on any amount of marijuana is dangerous and can result in tragic consequences. As marijuana use is becoming more mainstream and publicly accepted, due to widespread legalization and declining perceptions of its dangers, more and more people are driving while impaired or getting into cars with people who are under the influence. From 2007 to 2014, the number of drivers testing positive for marijuana rose 50 percent, CBS News reports. In 2014, the year after recreational use was legalized in Washington, fatal car crashes involving marijuana more than doubled in the state, CNN publishes.
  • Just as drinking and driving is hazardous to the driver, passengers, and others on the road, so is driving while high on marijuana.
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