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New Insights into How Marijuana Affects Driving

A study reported this week offers new insights into how marijuana affects driving. These insights are increasingly important as both medical and recreational use of marijuana become legal in more states and more teens are finding access to sources of marijuana for their own use.

New Insights Into how Marijuana affects driving - Altizer Law PC

Marijuana Use in the United States

According to, “marijuana is the most commonly used psychotropic drug in the U.S., after alcohol. Here are some key facts about marijuana use, particularly as the facts are relevant to the new study.

  • In 2018, more than11.8 million young adults reported using marijuana in the past year.
  • Marijuana use is more prevalent among men than women.
  • Most measures of marijuana use by high school students “peaked in the mid-to-late 1990s.” It then declined gradually through the mid-2000s and then leveled out. In 2019, daily use in the younger grades increased significantly.”
  • In 2019, 11.8 percent of 8th graders reported marijuana use in the past year and 6.6 percent in the past month. For 10th graders, 28.8 percent in the past year and 18.4 percent in the past month. For 12th graders, 35.7 percent in the last year and 22.3 percent in the past month. Of these 12th graders, 6.4 percent said they used marijuana daily or near-daily.
  • As vaping devices had become popular, teens are vaping THC (the ingredient in marijuana that produces the high), with nearly 4 percent of 12th graders said they vape THC daily.

[Source: What is the scope of marijuana use in the United States?]

Effects of Marijuana on Driving

Tetrahydrocannabinol (THC) affects the areas of the brain governing body movements, balance, coordination, memory, and judgment. Marijuana affects several driving skills. It can:

  1. Slow your reaction time.
  2. Impede your ability to make decisions.
  3. Impair coordination.
  4. Distort perception.
  5. Lead to memory loss.
  6. Lead to difficulty with problem-solving.

When marijuana and alcohol are combined the risk of impaired driving appears to be greater than for either in isolation.

A number of studies conducted in Europe and in North America have returned conflicting results with regard to the risk of auto accidents when using marijuana. One European study concluded that marijuana use significantly increased the risk of causing or being involved in an auto accident. However, when the data were analyzed by the NHTSA the data did not indicate a significantly greater risk of a crash once age, speed, weather, driving experience, etc. were calculated.

Tracking and assessing the role of marijuana in car crashes is challenging. First, there is no immediate test that can be administered on the highway that can calculate drug levels in the body. Second, marijuana can remain in an individual’s system for days or weeks after their last use. Third, impaired drivers are not always tested for drugs. Fourth, marijuana and alcohol often are used in combination, making it difficult to identify the substance that caused a crash.

New Findings

This study was undertaken by McLean Hospital’s Mary Kathryn Dahlgren, PhD, Staci Gruber, PhD, and their team at the McLean Cognitive and Clinical Neuroimaging Core and the Marijuana Investigations for Neuroscientific Discovery (MIND) program. The study was published as “Recreational Cannabis Use Impairs Driving Performance in the Absence of Acute Intoxication” in the Drug and Alcohol Dependence Journal. The study was conducted with driving simulation.

The key points are these:

  1. Recreational cannabis impairs driving even when the driver is not acutely intoxicated.
  2. There is a link between earlier onset of marijuana use (before the age of 16) and worse driving performance.
  3. Marijuana users drove faster, had more accidents, and ran more red lights than non-users.

A critical point made in the study is this: “There’s been a lot of interest in how we can more readily and accurately identify cannabis intoxication at the roadside, but the truth of the matter is that it is critical to assess impairment, regardless of the source or cause. It’s important to be mindful that whether someone is acutely intoxicated, or a heavy recreational cannabis user who’s not intoxicated, there may be an impact on driving, but certainly, not everyone demonstrates impairment simply as a function of exposure to cannabis. This is especially important to keep in mind given increasing numbers of medical cannabis patients who differ from recreational users with regard to product choice and goal of use.”

We can certainly expect to see more studies of medical and recreational marijuana users and the particular effect of the drug on driving. We are likely to see more attention paid by municipalities and states in identifying the role of marijuana in auto accidents and driver behavior. In time, we are also likely to see the development of some means of assessing the level of impairment at the time of crashes. In the interim, these studies raise public awareness that marijuana is not an innocuous drug, but one that causes impairments that should be taken seriously.

Call Altizer Law, P.C.

If you or a loved one has been injured in an auto accident through no fault of your own, you may be entitled to recover financial compensation for damages you have sustained. Call Altizer Law, P.C., in Roanoke, VA when you need fierce legal representation and compassionate advocate. For more than 30 years, Bettina Altizer and her team of experts have been winning maximum financial compensation for those injured through no fault of their own. We understand that when rebuilding a life after tragedy, it’s about the money.