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How Marijuana Can Impair Your Driving and Your Judgment

Recreational marijuana became legal in the state of California on January 1, 2018—but “recreational use” does not include smoking it in the car. As the Mercury News points out,  a new state law makes it officially illegal for drivers and passengers alike to smoke weed in vehicles—just one attempt to curb what officials fear will be an upsurge in drug-related DUI in the wake of legalized pot.

Some users might be tempted to view marijuana as a “grey area” drug when it comes to driving. After all, THC, the substance in weed that makes you high, is difficult to measure in the body, and in fact it can remain detectible in your blood stream for days after you use it, making it even more difficult to measure. However, make no mistake: Multiple studies have shown that marijuana use has a significant impact on people’s ability to drive safely, and that pot in your system increases your risk of getting into an accident. Furthermore, no matter how difficult it is to measure how much THC is in your system or when it got there, if an officer suspects you’re impaired while driving, he can arrest you based on that suspicion. In fact, as the law mentioned above states, you can be arrested for smoking it in a car even if you’re a passenger.


The drug THC found in marijuana can affect your driving in a number of different ways. For our purposes, we’ve boiled it down to two basic effects: Coordination and judgment. Let’s explore both.

Effects on Coordination

Numerous studies over the years confirm that marijuana use can reduce a person’s coordination and motor skills—that is, their ability to react physically to stimuli in a timely manner. For example, if you’re behind the wheel and a pedestrian darts out in front of you, your brain normally sends a quick signal to step on the brakes to avoid hitting the pedestrian. Under the influence of marijuana, this signal gets transmitted more slowly, so you are more likely to hit the pedestrian because you were physically unable to apply the brakes in time—even if you saw the pedestrian in time.

The effects don’t end here, though. Studies have revealed at least two other very disturbing trends about the link between pot use and coordination:

1. More experienced users are more likely to show reduced coordination than novices, according to one study. In other words, the more you use the drug recreationally, the more of an effect it can have on your motor skills.

2. The effects on coordination may last well past the drug use itself. As LiveScience reports, a collective of studies shows that using weed can affect your long-term motor skills even when you’re no longer high. The reason may be because THC doesn’t just disrupt certain centers of the brain—it may actually change the way the brain functions.

The takeaway from these studies suggests that smoking pot could have both immediate and lasting effects on your motor skills, making you more likely to get into an accident behind the wheel. If you become a chronic user, this coordination problem could also become chronic, even at times when you’re not high.

Effects on Judgment

Driving a vehicle involves making hundreds of decisions every minute. Making good decisions has everything to do with perception, or judgment—that is, being able to interpret data accurately and act upon it. According to a study reviewed by, marijuana use may have a profound impact on someone’s ability to make decisions by clouding their perception of the facts. In short—it increases the likelihood that you’ll make a wrong decision behind the wheel. Going back to the example of a pedestrian jumping in front of the car—if you have trouble perceiving the location of the pedestrian in relation to your vehicle, you may have more trouble discerning what to do to avoid hitting the pedestrian.

Again, these effects may last beyond the presence of the drug itself. PBS Frontline quotes one study that indicates that marijuana’s effects on judgment can last up to 24 hours afterward: “Heavy marijuana use is associated with residual neuropsychological effects even after a day of supervised abstinence from the drug,” it says.

When we tie these two effects together—the effects on motor skills and the effects on judgment—we can see how dangerous a combination marijuana use and driving can become. If you’re driving under the influence and a pedestrian jumps in front of you, not only are you less likely to respond in time to avoid hitting the pedestrian, but you’re less likely even to notice that you’re about to hit the pedestrian in the first place!

Likelihood of Arrest

Our ability to measure the effective amounts of THC in the bloodstream may be limited for the time being, but the laws about driving under the influence are clear enough. If an officer suspects you’re impaired, you may be arrested for drug DUI, regardless of how exactly how much THC is in your system. Since law enforcement is already skittish on this issue, they’re likely to err on the side of caution. And remember, THC can be present in your blood stream up to a week or more after your last hit—so even if you weren’t actually using the drug while driving, the ambiguity doesn’t necessarily work in your favor.

In short—using recreational pot and driving don’t mix. The further you can keep those two activities away from each other, the better off you’ll be. If you’re facing drug DUI charges and need legal representation, please call our offices today.

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