Weed Use

Driving While Under the Influence of Marijuana

We’ve all been told how important it is to avoid drinking and driving, but what about using marijuana and driving? Like alcohol, marijuana is a central nervous system depressant, meaning that it can blunt your reaction times and reduce your motor coordination in a manner similar to alcohol. What’s more, marijuana causes impaired judgment and alters your perceptions. Being under the influence can therefore place a significant handicap on your mental ability to complete tasks that require a high level of attention and coordination, especially driving. Here, we’ll describe some of the research that has been conducted to examine the impact marijuana can have on your ability to drive, and we’ll also mention some of the medicolegal aspects of marijuana use while driving. After all, it’s important to stay informed!

Marijuana Use and Driving Risks

Complex decision making, fast reaction time, and coordination are skills that are crucial to safe driving. If you find yourself in a situation where you anticipate needing to drive, think twice before you decide to use! Just like drinking and driving, driving while high is driving under the influence.

Several studies have demonstrated a direct relationship between the concentration of THC, marijuana’s active cannabinoid substance, in people’s blood and their degree of driving impairment (Lenné, Accid Anal Prev, 2010; Hartman, Clin Chem, 2013). One study examined the effects of inhaled marijuana on simulated driving performance: they found that a 5 mcg/L blood THC concentration combined with a 0.05 g/210L breath alcohol concentration (which is what a breathalyzer measures) produced the same impairment as a 0.08 g/210L breath alcohol concentration (Hartman, Drug Alcohol Depend, 2015). This is most likely because marijuana’s depressive impact on your central nervous system is synergistic with alcohol’s depressive effects, leading to an even greater risk of driving impairment when the two are combined.

While the impact that marijuana has on your ability to drive is hard to dispute, its actual impact on rates of car accidents is less straightforward. One study found that drivers with THC in their blood were about twice as likely to have been in a fatal crash than drivers who had not used drugs or alcohol, although it’s difficult to say what exact role marijuana played in those accidents (Biecheler, Traffic Inj Prev, 2008). This is because THC can often be detected in the blood long after use, and concurrent use of marijuana and alcohol is common. Several large analyses looking at multiple studies found that the risk of being involved in a car accident increased significantly after using marijuana, showing that marijuana may double your risk of being in an accident (Elvik, Accid Annal Prev, 2013). However, a different study conducted by the National Highway Traffic Safety Administration found that there was no association between marijuana use and car accidents after controlling for factors like drivers’ ages, genders, and levels of alcohol intoxication (Compton, Drug and Alcohol Crash Risk, 2015).

In summary, the jury is still out on the exact impact marijuana use has on rates of car accidents. We do know, though, that marijuana impacts your ability to drive. If you still need convincing, in our next section, we’ll dive into the possible neurobiological mechanisms for why this makes sense.

Cognitive Effects of Marijuana on Driving

Whenever you get behind the wheel, you’re activating multiple brain networks that help you to coordinate the sensory, motor, and cognitive neural circuits that you need to get safely from one place to the next. While driving may seem totally effortless you, it’s actually an incredibly complex task! Acute marijuana intoxication tampers with these brain circuits in a way that makes safe driving all the more difficult. Here’s how:

Reaction Time

When you use marijuana, cannabinoids bind to CB1 receptors in a section of your brain called the basal ganglia, which is responsible for several cognitive functions including your perception of time. Altered time perception secondary to marijuana intoxication makes it harder to react to your external circumstances as quickly as you would be able to while sober. Being able to respond fast is critical when, at any given moment, other cars or pedestrians may come into your path. When you’re under the influence, you’re increasing the chances that you won’t be able to swerve away in time, which makes for dangerous driving conditions.

Motor Coordination

When you use marijuana, cannabinoids also bind to CB1 receptors in an area of your brain called the cerebellum, which is responsible for coordinating smooth movements. This explains why using marijuana can cause you to feel wobbly or uncoordinated. Driving demands a lot of motor precision, which we normally don’t have to think about. Using your foot to press lightly on the gas pedal while simultaneously turning the steering wheel, though is no easy task for your brain. Marijuana throws a mental wrench into this motor coordination that otherwise comes as second nature.

Judgment

Marijuana affects an area of the brain called the prefrontal cortex, which is responsible for high-order cognitive functions such as inhibition and decision-making. Whether you realize it or not, you’re constantly making decisions when you’re behind the wheel. Think about it: if you’re approaching a traffic light and it suddenly turns yellow, you have to make a split-second decision on how safe it is to proceed. If you’re disinhibited due to marijuana, you may not be able to make a decision about stopping before the light in time. That’s just one of the many possible examples of how marijuana and driving make for a poor combination.

Testing for Marijuana Intoxication (Marijuana Field Sobriety Test)

In states where marijuana is illegal, detection of any marijuana in your vehicle is considered a crime. In states where marijuana is legal, however, assessing the driver’s degree of mental impairment is critical to deciding whether or not to arrest an individual who is suspected of driving under the influence.

Roadside tests designed to determine whether you’re driving under the influence of marijuana will vary state to state, so it’s worth reading up on the regulations in your state so you are aware of how officers may assess suspected drivers. In California, for example, officers conduct field sobriety tests, which are a series of physical and mental exercises that determine a driver’s level of impairment. These exercises may include the following:

  • The “horizontal gaze nystagmus test” involves an officer moving their finger from side to side while you follow their finger with your gaze. When you’re high, these eye movements may provoke an involuntary jerking, or quivering, of your eyes, which may signal that you’re under the influence.
  • The “walk and turn test” involves you walking in a straight line in a heel-to-toe fashion. Any loss of balance, inability to stay on the line, breaks in walking, or beginning before instructed, raises some flags, because these mistakes hint at a potential lack of motor coordination and lack of inhibition secondary to substance use.
  • The “one leg stand test” involves an officer instructing you to raise your foot, hold still, count, and then look down. Things like swaying, hopping, or putting your foot down prematurely raises the suspicion for marijuana intoxication. This is because the test is designed to split your attention between the various tasks, which can be demonstrably more difficult while under the influence of marijuana.

Officers may also assess for physical signs of marijuana intoxication, including dilated pupils, rapid breathing, rapid heart rate, red eyes, or an apparent odor of marijuana.

Other forms of detection for marijuana use include blood, breath, urine, or saliva tests; these tend to be less common in the context of DUI assessments because they take longer to perform. They may, however, be administered if the officer has probable cause to believe you are driving under the influence of marijuana or another substance. There is no consensus on how much marijuana can be detected on these tests to indicate whether you are impaired while driving, and precise guidelines will vary by state.

Getting Help if You Have a DUI

It shouldn’t be a surprise that there are harsh legal penalties for driving under the influence of alcohol. What many people don’t realize, though, is that you can also receive a driving under the influence (DUI) charge for marijuana. Being found guilty of a marijuana DUI may result in a misdemeanor charge.

Mistakes happen. If you have been arrested and charged with driving under the influence of marijuana, you have options for ways to get help. It’s always wise to obtain legal counsel, who can provide defense on your behalf in court. If you’re unable to pay for your own attorney, you will be given the option of being appointed an attorney by the court. Depending on your specific context, your lawyer may invoke a number of possible legal defense strategies.

If you’re ultimately charged with a marijuana DUI, you still have the opportunity to seek post-conviction relief by obtaining an expungement, or clearing, of your charge from your record. This requires petitioning the court to clean your records and fulfilling particular eligibility criteria that vary by state. Example eligibility criteria include requirements like 1) not serving any time in prison in the past, 2) completing all terms and conditions of your probation, 3) not committing any subsequent felonies, and 4) not having any other pending criminal charges. By expunging your conviction, you can state under oath that you were not convicted of a DUI, and the conviction will not appear on any public record databases. Though there are many hoops to jump through for this option, it offers many people who have made an honest mistake a clean slate.

Again, mistakes are human nature, so seeking professional help to navigate the intricacies of the legal process is the best strategy to maximizes your chances of getting a second chance.