Skip to main content

Marijuana: Study Finds Minimal Changes in Driving Performance After Smoking

Submitted by Phillip Smith on (Issue #635)
Politics & Advocacy

The head of the Office of National Drug Control Policy, drug czar Gil Kerlikowske, is pushing a campaign targeting drugged driving and has singled out marijuana as a main problem. But if the latest research findings on stoned driving are any indication, the drug czar may want to shift his emphasis if he wants to (as he claims) let policy be driven by evidence.

According to clinical trial data published in the March issue of the Journal of Psychoactive Drugs, subjects tested both before and after smoking marijuana exhibited virtually identical driving skills in a battery of driving simulator tests. Researchers in the double-blind, placebo-controlled trial tested 85 subjects -- 50 men and 35 women -- on simulated driving performance. The subjects had to respond to simulations of various events associated with vehicle crash risk, such as deciding whether to stop or go through a changing traffic light, avoiding a driver entering an intersection illegally, and responding to the presence of emergency vehicles. Subjects were tested sober and again a half hour after having smoked a single medium-potency (2.9% THC) joint or a placebo.

The investigators found that the subjects' performance before and after getting stoned was virtually identical. "No differences were found during the baseline driving segment (and the) collision avoidance scenarios," the authors reported. Nor were there any differences between the way men and women responded.

Researchers did note one difference. "Participants receiving active marijuana decreased their speed more so than those receiving placebo cigarettes during (the) distracted section of the drive," they wrote. The authors speculated that the subjects may have slowed down to compensate for perceived impairment. "[N]o other changes in driving performance were found," researchers concluded.

Past research on marijuana use and driving has yielded similar results as well, including a 2008 driving simulator clinical trial conducted in Israel and published in Accident, Analysis, and Prevention. That trial compared the performance of drivers after they had ingested either alcohol or marijuana. "Average speed was the most sensitive driving performance variable affected by both THC and alcohol but with an opposite effect," the investigators reported. "Smoking THC cigarettes caused drivers to drive slower in a dose-dependent manner, while alcohol caused drivers to drive significantly faster than in 'control' conditions."

Something to keep in mind when lawmakers in your state start pushing for zero-tolerance "per se" Driving Under the Influence of Drugs laws that want to label people impaired drivers because of the presence of a few metabolites left over from last week.

Permission to Reprint: This content is licensed under a modified Creative Commons Attribution license. Content of a purely educational nature in Drug War Chronicle appear courtesy of DRCNet Foundation, unless otherwise noted.


Fireweed70 (not verified)

I've always known that pot did nothing negative to my driving ability. I've had three instances of driving after I smoked one where ANOTHER car crossed the center line into my lane and I saw it coming as if in slow motion and was able to sidestep the vehicle and avoid an accident.

One was when there was ice on the road, a car in the right hand oncoming lane swerved, pushing the oncoming car in the left lane into my lane, and I was able to look over my shoulder to see if my right lane was clear and swerve around easily.

Another time, I was on rt. 2 across N. Dakota, and it was like me and one other car on the road in the whole state, and I saw this cloud of dust coming from about a mile away and as it got closer, saw it was a car fishtailing out of control. It eventually spun out, bounced off the grassy median, and right into my lane. Again, I was able to move out if the way, and by the time both cars were stopped we were a good half mile apart. I backed up to check and see if they were okay, and it was these two ladies, the driver had fallen asleep at the wheel.

And the third one was a big muscle pickup that for no reason swerved over the double yellow line and again I moved around it just in time. So in those threee instances, tfell me again how was I impaired?

Fri, 06/04/2010 - 2:34pm Permalink
maxwood (not verified)

1. There should be research comparing the true (including indirect) impact on driving of cannabis and tobackgo. How many thousand times a year does "a $igarette or two" clear the head, as they say, and embolden a drunk or "buzzed" person to try driving anyway (got to get the car out of this neighborhood), resulting in fatalities. I would doubt very many drunks try this with cannabis. Maybe availability of cannabis without fear of persecution would have reduced the amount of alcohol consumption at that same party instead of enabling more as the expectation of a $igarette later does.

2. Research should extend to comparing the effect of a 25-mg. single toke out of a one-hitter or vaporizer with the effect of a hot-burning overdose "joint" (overdose means all that heat shock and carbon monoxide). The tobackgo companies fight cannabis legalization through their "conservative" paid politicians to prevent just precisely this Downdosage Revolution destroying their 700-mg. "traditional" cash cow $igarette format. The driving disasters can be blamed on the alcohol. after all, heh heh.

Fri, 06/04/2010 - 3:44pm Permalink
Baked, not fried (not verified)

This is a weak high. "Subjects were tested sober and again a half hour after having smoked a single medium-potency (2.9% THC) joint or a placebo."

The good thing about cannabis is that one has to make an effort to have the equivalent impairment of alcohol at 0.08% blood alcohol. And the impairment is most noticeable during the first half-hour to hour after ingestion.

The first half-hour after strong cannabis is when one is most impaired. Ask pizza drivers. Having been one, I can say that most pizza drivers that smoke cannabis avoid getting too high when driving. This info is from people who drive for a living and not wannabe experts in cannabis driving.

Just because you have managed to drive around while baked, and survived does not mean you should believe that driving within the first half-hour after strong cannabis is OK. Or after quickly smoking a lot of average cannabis. You are legally impaired during that first half-hour or hour. Driving tests have shown this.

Many of us know heavy alcohol users who managed to drive legally impaired for years without an accident. It's just luck. Don't be an idiot. Don't drive after ingesting a lot of THC, etc., especially during that first half-hour or hour.

Also, if you mix ANY alcohol with cannabis you are definitely legally impaired (equivalent impairment to 0.08% blood alcohol or more).

For more info:

Fri, 06/04/2010 - 4:28pm Permalink
Smoke and driv… (not verified)

I have been driving sense the age of 12, I am now 50. I have smoked and drove sense the age of 16. I have never had one accident or wreck"what ever you prefer" in my complete time I have been driving.
I have been rear ended by a 97 year old woman, when setting waiting to make a turn with my blinker on. She said I never saw you...DA
Any way Pot, Weed, what ever you want to call it does not effect your driving.
If one of my kids, who are all grown now, broke a arm and I had been drinking a 12 pack of beer, there would be NO WAY I could drive anyone anywhere. But give the same senario but put pot in there and you could smoke all day and still drive.
I have never heard of anyone nor has anyone ever ODed on POT, NEVER. You can OverDose on asprin. I know of no other drug you can not OD on. You can OD on all drugs but not POT, WEED whatever you call it, you can NOT O.D. ON POT, it does not effect your driving. PERIOD....
Or goverment started all this REFER MADNESS crap to get the Mexican's out of this country. Its time we as American's stop this stupid racial crap, its the 21 centery, not the 18 or 19th centery.
Now our goverments keeps this Reffer Madness Crap going for the money the Goverment makes from busting its people. LETS STOP THIS NOW, IT IS TIME FOR CHANGE.

Fri, 06/04/2010 - 6:07pm Permalink
I'm With Ya, But... (not verified)

In reply to by Smoke and driv… (not verified)

Unfortunately I think all of your credibility is thrown out the window when I have a very hard time trying to even read your comment. That said, I smoke when I drive. That said, I'm pro legalization. But for the benefit of all 420 friendly folks, please be intelligible when you convey your message.  

Wed, 10/06/2010 - 1:46am Permalink
GrowerGabe (not verified)

"Subjects were tested sober and again a half hour after having smoked a single medium-potency (2.9% THC) joint or a placebo."

If 2.9% passes for medium potency where you are at, you need to find a better dealer. 20% or higher is not uncommon with several strains and anything less than 10% is ditch weed. This isn't 1968 anymore people.

Sat, 06/05/2010 - 4:11am Permalink
Anonymouss (not verified)

2.9% is low, but a whole joint is a lot of smoke. When people smoke 20% thc buds, they usually do so in pipes. An entire joint of 2.9% is a medium high. I do agree that waiting half an hour is stupid, though. They should drive two minutes after smoking. Either way, it is way overdue to have a comprehensive study where they test several different amounts of several different potencies (and they do so two minutes after smoking).

Sat, 06/05/2010 - 2:05pm Permalink
newageblues (not verified)

"Something to keep in mind when lawmakers in your state start pushing for zero-tolerance "per se" Driving Under the Influence of Drugs laws that want to label people impaired drivers because of the presence of a few metabolites left over from last week."

How many states have these utter crap laws? Anyone keeping track? It was 13 a couple of years back, as I remember. This is so crazy it would be funny if it isn't ruining people's lives under the delusion (or pretend delusion) that smoking marijuana yesterday has anything to do with your ability to drive today, or sooner in most cases.
Anyone worried about people driving UI cannabis should be frantic with worry about the problem of people driving UI alcohol. That's where the bodies are buried, as prohibitionists are well aware.

Sat, 06/05/2010 - 7:07pm Permalink
David Schempf (not verified)

Back in the days when it was a Felony here, I used to score some Kickass Columbian (It's Good Shit, Man), and drive. Try driving a Colorado Mountain Pass when you're somoking Pot. Very Gratifying! No wrecks or other incidents.

"It isn't the destination, it's the ride"
Stephen Stills, "Thoroughfare Gap", 1976

Sat, 06/05/2010 - 7:19pm Permalink
gary o rourke (not verified)

In reply to by David Schempf (not verified)

He knows not where he's going
for the ocean will decide
its not the distination
but the glory of the ride.

Tue, 06/08/2010 - 8:10pm Permalink
aloner stroner (not verified)

in a world run by extroverted politicians, introverted logicians and scientists are often brushed aside, even when they are the ones who know the truth. i have driven stoned off more than a few spliffs and never once have a caused, or been in, an accident, neither a fender bender nor car wrecker. marijuana is innocuous and politicians fail to realize that, as they are more focused on their careers and good names than the truth

Tue, 06/08/2010 - 12:55pm Permalink

I have to say that I think I drive more safely when I'm stoned because I'm so paranoid that I'm going to get stopped by the police. Going for a drive to get baked is sometimes the safest place to smoke. So I agree that waiting 1/2 hour is too long... how about 5 minutes? Also, how about driving while taking a hit? How does that effect safety?

Tue, 06/08/2010 - 11:13pm Permalink
OZZIEREP (not verified)

I am a sales rep traveling about 80-90,000ks a year and find that if I have a quick pipe I slow down and tend not to rush as much. I find this extremely helpful when traveling down steep windy roads in my travels and when I am keen to get home after 3 or 4 days on the road.They have started swabbing for pot in NSW now so I have to be careful

Wed, 06/09/2010 - 7:19am Permalink
[email protected] (not verified)

In reply to by yccit (not verified)

Typically 10% is considered rather low with 18-20% being rather close to good quality hydro/aero-ponic grade marijuana.  But they are using a placebo in the test as well, does not hurt to make it somewhat hard to distinguish when judging psychosomatic effects.  Also they are smoking an entire joint, something that I have not felt the need to do since the 80's lol.

However they do not list the mass of the joint.  So it is moot what the % was to begin with- although this is certainly not the first study i've seen that is similar in result

Sat, 12/18/2010 - 8:50pm Permalink
Anonymous2 (not verified)

I'm a pizza delivery driver, and while we can all agree that the pizza industry is already shrouded by a purple haze, smoking is the best part of work. I drive all night, every night, baked out of my mind, and so do all my fellow drivers. it makes the job tolerable, sure doesn't make us incapable behind the wheel.

Thu, 06/17/2010 - 1:31pm Permalink
smokeweedeveryday (not verified)


Did you read the actual journal article?

In conclusion, based on our results, smoking the active ingredient in marijuana, THC, at dosages of 13 mg and 17 mg, may impair driving ability in recreational users of marijuana and can lead to produce unsafe driving. These effects are dose dependent and are reflected in all three types of measurement techniques used: performance, subjective sensations, and physiological. The higher dose of THC (17 mg) was found to be more potent than the lower dose (13 mg) causing higher increase in heart rate and a greater feeling of discomfort and physical effort while driving after smoking. In terms of performance, the effects of 0.05% BAC seem to be similar to those of the low-level (13 mg) THC cigarettes although subjects drove faster after alcohol compared to driving sessions after THC smoking. No THC-related effects were measurable 24 h after smoking the high (17 mg) level of THC.

And from the other study he cited:

This study enhances the current literature by identifying distracted driving and the integration of prior experience as particularly problematic under the influence of marijuana.


This study supports the existing literature that marijuana does affect simulated driving performance, particularly on complex tasks such as divided attention. It is anticipated that many teenagers and young adults driving under the influence of marijuana are doing so while conversing with friends in the car, listening to music, talking on the cell phone and/or text messaging others. These behaviors divide the driver's attention and are particularly dangerous under the influence of marijuana.

I dont know what the author of the web article read, but the journal articles he cited do not back up his conclusion at all.

Guys, we do not need to be pushing incorrect ideas, this does not help public perception. Lets be rational.

Sat, 02/12/2011 - 6:11pm Permalink

I find that a bit hard to believe. I find I can drive the same if not better when smoking or just after but only while I can keep concentration which usually is not long. I once spent 5 minutes waiting for a stop sign to turn green. Now I am not saying that would happen if I had a guy testing me sitting in the next seat but it definitely wouldn't have happened if I had been sober.

Tue, 03/29/2011 - 11:36am Permalink
Ted Narcotic (not verified)

In reply to by Karl (not verified)

I agree with your comment,and here is a thought about the "guilty of being out of control aspect". (because you sat at a stop sign for 5 minutes all spaced out, and did not remember  to go)  Your attention was adequate to the task of driving up to the stop sign, but then you "failed" to make the most of your time and be an efficient driver. Your mind went on a little journey at that time. But that journey had nothing to do with hurrying along to meet a schedule.  You were not in an emergency requiring that you hurry. So you daydreamed away 5 minutes at the stop sign for no practical reason and that could be embarrassing if one of your passengers called you on it. But the question is, were you safe while sitting in a time warp at the stop sign for 5 minutes. I say yes. Your mind did not ceded control of the realization that you are in a car, it just gives up on worrying about control because you are now safe from any type of moving accident. You could actually take your eyes off of the road for awhile and the worst thing that might happen is that someone would honk or a policeman might walk up to your door. You might be reading a map, dialing your cell phone, or looking for a newspaper to swat a yellow jacket that can't find its way out the window. If a car came swerving towards you, or even if a cop had driven by you, you would have snapped out of your unconcerned 2000 yard stare and been that guy that is now driving a car and that means you are looking at the road and aware of everything just like all drivers who are not crashing. Inattention to driving is something we all do when we are behind the wheel, but only when we are stopped in a safe spot and our foot is on the brake or we are in park. So even though you were not paying any attention to the time, you did know that you weren't driving a moving vehicle at the time. I guess you might say you were driving a motionless vehicle, but no one has ever crashed while driving a motionless car. 

I saw a television show where the FBI  was looking for a kidnap victim and when they busted down the door yelling "FBI,put your hands up" the actor playing the stoner is asked if this bag of weed is his and he says yes and the  actor playing the FBI agent says "Are you stoned right now?"( in a really mean and snarling voice) and the stoner says "I was". The point being that you can be out there in la la land because you decided it would be safe for a moment, but when the crap hits the fan, or when you spill your soda on your plush leather seats you will "snap to" instantly .I  saw an interview with Michael Schumacher on 60 minutes and he said that when he is going over 200 miles an hour down the straight away he thinks about what he is going to do with his family on his day off. But when he reaches a turn and it is time for him to brake, turn and go through it as fast as possible without crashing, he only thinks about that task and not anything else at all. We all daydream but that doesn't mean that the mind has abandoned reality enough to stop being responsible for what the next point of focus needs to be.

In other words I don't think that you were "out of control" in any real sense that could be indicative of your potential for just losing your good sense, stop knowing what you were doing, and causing an accident born of neglect or loss cognition that you are at wheel of a car.

Sun, 07/10/2011 - 11:45am Permalink

Add new comment

The content of this field is kept private and will not be shown publicly.
This site is protected by reCAPTCHA and the Google Privacy Policy and Terms of Service apply.