Does Cannabis Impair Driving?
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By Joan Eberhardt

Will using marijuana impair your driving? Cannabis users, who let’s be real can frequently be accused of paranoid behavior, might want to reconsider their own marijuana use before getting behind the wheel of a car. But determining if a marijuana user is impaired to the extent that a person should not be driving is not nearly as cut and dried.


Depending on personal anecdotes, people either become a hypercautious superdriver, or you get stuck waiting for the stop sign to turn green. Regardless about how any individual person feels their driving abilities are affected when using cannabis, the law in most states will treat impaired driving while high with the same severity as impaired driving while on alcohol.

Is it legal?

The thing is, though, determining what the legal limit for cannabis use while driving should be is an inexact science. Currently Colorado considers a driver impaired if they have more than 5 nanograms of THC in a milliliter of blood. However, because THC can remain in the body’s systems for much longer than its psychotropic effects, marijuana advocates argue that number will lead to a lot of false convictions. The state even released some pretty compelling ads that driving while under the influence of cannabis is illegal.

How much does cannabis really impair driving?

But really, how much does cannabis affect the ability to drive? That depends on a lot of things, including how frequently a person uses cannabis and the amount that person has used while behind the wheel of a moving vehicle. A study from the University of Iowa’s National Advanced Driving Simulator, showed that people who vaporized cannabis prior to a simulated driving test weaved less than the people who had consumed alcohol, or those who had consumed both alcohol and cannabis. Drivers who had a blood concentration of at least 13.1 nanograms of THC per milliliter of blood, did show increased weaving between the lane, but that it would be about consistent with the legal limit for alcohol consumption, .08% blood alcohol concentration. Remember, Colorado’s THC limit is a much lower 5 nanograms per milliliter.


That said, studies show repeatedly that cannabis impairs all the motor coordination, visual function and divided attention that are essential to safe driving, but most of those are fairly modest reductions to the average cannabis user. Basically, it seems most of the time cannabis only impairs your driving a moderate amount. Some scientists have argued that if a person were going to drink alcohol and drive, or smoke marijuana and drink alcohol and drive, the population as a whole might be safer if that person chose only to use marijuana.


But there are limits to how safe it is to drive while high and it is not difficult to reach that limit. As evidence, I submit this highly unscientific “study” CNN did with a driving school instructor, a “consultant with a marijuana smoking lab” and three very willing participants. Including 27 year old medical marijuana patient named Addy whose adventurous driving makes her a hero in my book.

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