Marijuana and driving

Dear Alice,

I was wondering if you had anything about marijuana and driving.

Thanks. J

Dear J,

Many are familiar with the phrases, "Don't drink and drive," and "Friends don't let friends drive drunk." Not as many people realize that getting behind the wheel of a car (or bus, train, scooter, bike, or plane) while high on marijuana (also known as pot, cannabis, or weed among other names) can pose potential health risks as well. Studies indicate that marijuana is the drug most often found in car crashes (including fatal ones), and in fact, it's the most commonly detected substance among drivers. However, whether marijuana is actually responsible for the crashes is less well understood. A large case-control study conducted by the National Highway Traffic Safety Administration found no significant increased crash risk attributable to marijuana after controlling for factors such as drivers’ age, gender, race, and presence of alcohol. Contrarily, a retrospective study of over 4,000 drivers found that the risk of being responsible for causing a fatal crash is more than three times higher for those under the influence of cannabis. The contradictory findings make it difficult to say conclusively whether marijuana impairs driving ability, but some of its effects make it almost impossible to deny the potential connection.

First, it can be helpful to understand what pot does to the brain. Pot use impairs decision-making ability, and it slows reaction time. Motor skills and visual tracking skills are also diminished, which has shown to negatively affect a driver’s attentiveness and their perception of time and speed. Sometimes people report visual distortions, which may make it difficult to navigate a car, even in a driveway under the influence of this substance. Furthermore, driving while high may also pose risk because users often report feeling tired and having difficulty concentrating. Any one of these side effects, let alone in combination, would likely negatively impact a person’s ability to drive. 

Additionally, the impact of marijuana use on someone’s ability to drive safely is intensified when combined with alcohol use. Specifically, this combination has been shown to increase the risk of injury in car crashes by over four times. Interestingly, though, one study found a significant decline in the number of alcohol-involved fatal crashes correlating with the legalization of marijuana for medical purposes. The authors proposed the theory that legalizing marijuana increased the likelihood that pot would be substituted for alcohol, therefore decreasing the probability of drunk driving. Regardless, being both drunk and high can lead to increased risk of harm behind the wheel.

Now, you might ask, “What are the laws surrounding marijuana use and driving?” Currently, 18 states have zero tolerance or non-zero per se laws regarding marijuana and driving:

  • Zero tolerance laws state that it's illegal to drive with any measurable amount of marijuana in the body.
  • Non-zero per se laws state that it's illegal to drive with an amount of marijuana above a certain limit in the body.

Using Colorado as an example, where medical and recreational marijuana use is legal, there are per se laws stating that if someone has five or more nanograms per milliliter of blood of active tetrahydrocannabinal (THC), the main psychoactive chemical in marijuana, they're driving impaired and subject to legal punishment. Different states have different laws, so checking out the laws in your own state regarding marijuana and driving is recommended.

In summary, simply avoiding operating heavy machinery (cars, trucks, tractors, amusement park rides, etc.) while under the influence of any drug is the best way to prevent injury and legal issues. 

Last updated Feb 07, 2020
Originally published Feb 18, 2000

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