Cannabis in America

Marijuana DUIs: How Much Weed is Too Much to Drive?

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When it comes to alcohol, the laws regulating when someone is “over the limit” are pretty easy to remember. For drivers over 21, the legal blood alcohol content (BAC) limit in all 50 states is .08. For most of us, that means roughly one drink per hour. But as some states legalize recreational use of marijuana, and others allow its use for medical purposes, defining driving under the influence is becoming increasingly difficult.

One of the big problems is that we don’t quite know how badly marijuana affects driving. Experts point out that it’s obviously bad to drive when any senses are impaired — but we still allow people to have a drink before they drive, because a safe threshold has been determined. That threshold hasn’t really been identified for marijuana use yet.

Marijuana users are definitely impaired. Reaction times for example, are usually slower. But unlike those who have had alcohol, people under the influence of marijuana are usually more aware of that impairment. They are more likely to be cautious and compensate for their dulled senses. Currently, the National Highway Traffic Safety Administration is working on a study to figure out how exactly smoking marijuana can affect driving. In states that do allow marijuana, whether or not the legalization has led to more fatal crashes is virtually impossible to determine. Some studies claim that it has, others say that there’s no correlation. With such a small sample population, as well as so many other contributing factors to automobile accidents, it’s just too soon to tell what effect marijuana use has on driving conditions as a whole.

The states that have legalized marijuana, or allow it for medical purposes, have attempted to institute some parameters. For example, Colorado has set the DUI limit for marijuana intoxication at 5 nanograms of THC per milliliter of blood. Some people worry, however, that it’s too early to appropriately determine such limits, and that until we can do so, a limit like Colorado’s is arbitrary. The Marijuana Policy Project stated:

The inability to accurately measure marijuana impairment is why both the National Highway Traffic Safety Administration and the National Institute on Drug Abuse have stated that marijuana impairment testing via blood sampling is unreliable.

The main complaint stems from the fact that there’s no good way to easily test marijuana intoxication. When someone is pulled over and suspected of driving while drunk, there are small breathalyzers that can be used to determine BAC. No comparative tool has been invented for marijuana intoxication at this point. There is apparently a very preliminary marijuana breathalyzer being created by a Canadian police officer; he has named it the “Cannabix.” It’s still in the very preliminary stages, and scientists aren’t sure about the efficiency or accuracy of a breath-based marijuana test.

As more states move toward the legalization of marijuana — currently there are serious pushes in Alaska, Massachusetts, Oregon, New York, and Washington D.C. — the question of marijuana DUIs needs an answer. Zero tolerance policies seem tough, especially with the now relatively common use of medical marijuana. But how much marijuana in your blood is too much? Scientists will have to tell us — hopefully the new National Highway Traffic Safety Administration study will provide us with some answers.


Anneliese Mahoney
Anneliese Mahoney is Managing Editor at Law Street and a Connecticut transplant to Washington D.C. She has a Bachelor’s degree in International Affairs from the George Washington University, and a passion for law, politics, and social issues. Contact Anneliese at



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