Drugs & Health Blog

Read This If You Know People Who Smoke Weed and Then Drive (Part 2 of 2)

The NIDA Blog Team

Our previous post looked at the National Advanced Driving Simulator that NIDA, the Office of National Drug Control Policy (ONDCP), and the National Highway Traffic Safety Administration (NHTSA) used for a three-year study on the effects of marijuana—with and without a low dose of alcohol—on people’s driving. What did the study discover?

Conducting the study

First, let’s look at how the study worked. Here are a few of the basics:

(1) Researchers selected 18 participants between the ages of 21 and 55 who met specific criteria. The participants:

  • reported drinking alcohol and using marijuana no more than three times a week;
  • had been a licensed driver for no less than 2 years, and had a valid unrestricted license; and
  • had driven at least 1,300 miles in the previous year.

The participants also:

  • had no past or current significant medical illness (i.e., a medical illness with a genuine, noticeable effect on daily life);
  • had no history of a significant negative experience with cannabis or alcohol intoxication, or with motion sickness;
  • were not pregnant or nursing; and
  • were not taking drugs that could cause harm if they were combined with cannabis or alcohol, or that are known to impact driving.

(2) The participants were given specific amounts of marijuana, alcohol, both, or a placebo (something that wouldn’t get them intoxicated) before each simulated drive. Since the University of Iowa is a non-smoking campus, participants were given vaporized marijuana instead of the kind you smoke.

(3) After spending the night at the University of Iowa Hospital to ensure that they were sober when the test began, the participants arrived at NADS, consumed the cannabis and/or alcohol or placebo, and then drove in the simulator for 45 minutes. They each did this six times, separated by at least a week between visits.

One lane per customer

As we mentioned in the previous post, the National Advanced Driving Simulator (NADS) measures many things about a driver’s behavior—eye movements, reaction times, steering—in lots of driving situations.

The first results of the study focus on three aspects:

  • how much someone weaved within the lane;
  • the number of times the car left its lane; and
  • how fast the weaving was.

Remember, the main research question was, What level of ∆9-tetrahydrocannabinol (THC) in the driver’s blood impaired their driving performance similarly to alcohol at the U.S. limit of 0.08%? THC is the main ingredient in marijuana that makes you high.

The answer: A blood concentration of 13.1 ug/L (micrograms per liter of blood) of THC increased weaving of the car within the lane to the same degree as drivers with a .08% breath alcohol concentration. A single marijuana cigarette can impair your driving skills.

Not quite “apples to apples”

It’s important to note that this was the “blood marijuana concentration” recorded during driving, not at the time a person’s blood would be collected after an accident or police stop. Why is that important? Because the concentration of THC in the blood starts to decrease as soon as a person stops using marijuana. By the time a driver gets a blood test, their THC level will be below 13.1 ug/L, but that doesn’t mean they were okay when they were driving.

Marijuana is often consumed in combination with alcohol. The study found that drivers who used both alcohol and marijuana weaved within lanes, even if their blood THC and alcohol concentrations were below the impairment concentrations of either one when used alone.

This basically means that when alcohol and marijuana are used together, you need less of each to impair your driving skills. If this sounds complicated, it is. It’s just too risky to drink and drive, or smoke weed and drive.

Did you miss Part 1? Read it here!

Categories: 
Marijuana
Comments posted to the Drugs & Health Blog are from the general public and may contain inaccurate information. They do not represent the views of NIDA or any other federal government entity.

Comments

weeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeed
those people are idiots why weeeeeeeeeeeeeeeeeee stupids people
hi
why do people smoke weed then drive is it there choice or does the weed make them drive

This study only looked at marijuana's effect on drivers, not whether it made people more likely to drive while high. Marijuana does impair judgment, so it can make people more likely to take risks and make bad decisions, but the focus of this study was on how it impairs coordination, which makes driving more dangerous. You can read more about how marijuana affects the brain here: http://teens.drugabuse.gov/drug-facts/marijuana

Untrue. Cannabis is known to decrease risk tolerance. Several other federal gov't driving impairment studies explained this fact, and the fact that after smoking, some subjects in the cannabis cohorts refused to get behind the wheel and had to be persuaded to continue the study. Over and over these studies explain that the cannabis-impaired subjects drove more cautiously and took fewer risks. Alcohol increases the likelihood of engaging in risky and regrettable behavior, cannabis decreases that risk. And that difference must be appreciated when attempting to quantify the actual, real-life effects of cannabis on driving.
it is bad for them to be driving and drinking weeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeed
hello fellow gangster, i have read most of your clever comments about this magnificent plant, but i think we are forgetting who is the reason we have this beautiful drug are lord and saver Jesus Christ. don't forget he loves you all
Oh God ... another Jesus Freak! Get a Life already!
weeeeeeeed
An 18 participants huh? Must be legit...
dont do it as simplele as that come on
weed be good for u make u feel good and its cheap
hello fellow gangsters, i have read your clever comments about this drug that we were blessed with from our lord and saver Jesus Christ.
Um... what? What does Jesus have to do with drugged driving?
hello fellow gangster, i have read most of your clever comments about this magnificent plant, but i think we are forgetting who is the reason we have this beautiful drug are lord and saver Jesus Christ. don't forget he loves you all
hello fellow gangster, i have read most of your clever comments about this magnificent plant, but i think we are forgetting who is the reason we have this beautiful drug are lord and saver Jesus Christ. don't forget he loves you all
How come you didn't report what the results were in terms of actually weaving OUTSIDE the lane (where the danger really is)? You tested for that, and yet all you mention is the slight weaving within the lane that is relatively normal. How did that compare to drivers who weren't stoned and to drivers on alcohol?
wow wow thats good to know.
wow wow wow i love it
this is stupid. most people I know hate alcohol and smoke weed exclusively because there's nothing to be gained from alcohol. and who cares if you weave a bit INSIDE the lines? I PURPOSELY weave to avoid potholes while high as well as straight because I know the roads. and honest to god everyone knows they're called joints, not "cannabis cigarettes." CIGARETTES have no medical value.
Please elaborate using your extensive professional medical background...
does weed make you drive

This study only looked at marijuana's effect on drivers, not whether it made people more likely to drive while high. Marijuana does impair judgment, so it can make people more likely to take risks and make bad decisions, but the focus of this study was on how it impairs coordination, which makes driving more dangerous. You can read more about how marijuana affects the brain here: http://teens.drugabuse.gov/drug-facts/marijuana

I'm sure every comment on here bashing this research and defending drugged driving was made from extensive professional medical experience, right?

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