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NIDA. (2015, October 7). Read This If You Know People Who Smoke Weed and Then Drive (Part 2). Retrieved from

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The NIDA Blog Team
October 7 2015
Inside a driving simulator

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:

(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 percent? 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 percent 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!