Legal Weed’s Effect on Stoned Driving Remains Hazy

Colorado and Washington struggle to work out whether legal marijuana leads to more impaired driving

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Ted S. Warren / AP

Marijuana is weighed and packaged for sale at the Northwest Patient Resource Center medical-marijuana dispensary in Seattle on Oct. 10, 2012.

Less than two weeks after recreational pot stores opened their doors in Colorado, proponents of legalization appeared to have their first black eye. A 23-year-old named Keith Kilbey, seemingly under the influence of drugs, crashed his car into a state patrol car—which then crashed into another state patrol car. A day after the incident, an official spokesperson said they suspected that the driver had been using marijuana.

A month later, it’s unclear whether Kilbey was actually stoned, and even less clear whether he purchased marijuana at a new pot shop. And a young man poised to be a harbinger of all the stoned drivers to come is now a reminder of how long it’s going to be before Colorado and Washington can really tell what impact legal weed is making on their states.

The potential for legal marijuana to yield more impaired drivers on the road has been a talking point throughout the political process in both states—as well as states like Alaska that may soon be voting on legalization—but the reality of that risk and how to assess it are pretty hazy. “It’s generally part of this conversation everywhere,” says Mason Tvert, communications director for the Marijuana Policy Project. “It’s not an easy question.”

While alcohol regulations have given officials a starting point for developing some marijuana-related rules, like how far vendors have to be from schools, there’s no simple parallel when it comes to driving. “It’s not like alcohol,” says Emily Wilfong, spokesperson for the Colorado Department of Transportation. “People metabolize it differently. There are different potencies. So there’s really no solution in terms of saying ‘you’re now at the limit.’”

Colorado will start spending $430,000 of federal grant money in March on an awareness campaign driving home the message that driving while high is illegal; the state will put up posters in pot dispensaries and broadcast ads with the tagline, “Drive high, get a DUI.” Wilfong says that the campaign won’t, however, include any advice like how long someone should wait to drive after smoking weed or eating a pot brownie. “I just don’t think there’s enough research that we can say, ‘Wait x amount of hours before getting on the road,’” says Wilfong. “I don’t know whether it’s five hours or 10 hours or the next day. We just don’t know.”

Both Colorado and Washington have, technically, settled on limits for how much THC a driver can have in his or her system while behind the wheel: a maximum of five nanograms per milliliter of blood. But there’s no handy chart that translates that scientific gobbledygook into as easy self-assessment. Various studies have tried to suss out how long marijuana remains in the system but reached no single conclusion that could be used as the basis for new DUI laws. Brian Vicente, a lawyer at a Denver firm that deals solely in marijuana-related issues, points out that research on high driving conducted in pre-legalization days often lacks the necessary context, like distinguishing between active THC (which might stay in someone’s blood for a few hours) and metabolized (which can remain for weeks).

Confusion about exactly how high is too high to drive—particularly for habitual users like medical marijuana patients, who may always have significant levels of active THC in their blood—also puts law enforcement in a precarious position, in part because administering a breathalyzer to determine a driver’s blood-alcohol content is a far cry from performing the blood test needed to measure THC levels. The last time Jason Hicks, a sergeant with the Washington State Patrol, sought a search warrant required to get such a test, he had to wait 45 minutes.

Hicks emphasizes that it is not illegal to smoke marijuana and drive; it is illegal to drive once you’ve smoked enough marijuana to be impaired. Still, police have little doubt that ingesting marijuana affects driver judgement, he says. “Operating a motor vehicle is a complex, attention-dividing task,” Hicks says. “Any time you introduce a substance into your body that hinders your ability to divide that attention, well, now you’re not your hundred-percent best behind that 4,000 pound weapon.” Studies have shown that marijuana use can cause drowsiness, lethargy and an altered sense of time, as well as inhibiting people’s ability to perform divided-attention tasks.

In Washington, state troopers like Hicks are generally more interested in the straightforward signs of impairment than measurements of THC. “I could give you study after study after study that shows that at [a blood-alcohol content of] .08, there are levels of impairment with just about everybody,” he says. “I couldn’t produce those same studies when it comes to the five nanogram limit for marijuana.”

Troopers have all been trained to spot the effects of various drugs on drivers, including cannabis. Many signs of highness are the same as drunkenness: poor balance, slurred speech, watery eyes. And troopers are often tipped off by the same behaviors, like slipping across dividing lines, failing to maintain a steady speed or just sitting there when a red light turns green.

There are other red flags, like a green tongue or inability to cross one’s eyes, that are potential reactions to pot that alcohol doesn’t share. Troopers sniff the air, perform the same field sobriety tests they do for suspected drunkenness, like asking drivers to stand on one leg, and make their best assessment. “It’s the totality of the circumstances,” Hicks says.

Statistics released by the Washington State Patrol last November suggested that the number of drivers who used weed may be on the rise; while about 1,000 drivers tested positive for marijuana in each of 2011 and 2012, 745 tested positive in just the first six months of 2013. Hicks says, however, that the total number of arrests made for driving under the influence of drugs decreased during the first year pot was legal in the Evergreen State, from 1,621 in 2012 to 1,357 in 2013.

The rub is that the Washington authorities don’t break down those statistics into how many drivers tested positive for cocaine or marijuana or speed—so the number of pot-related arrests is unknown. Colorado, similarly, conflates arrests for driving while high with arrests for driving under the influence of alcohol, meaning that hard answers to the question of whether legal marijuana leads to more impaired driving are going to be hard to come by for some time.

So far, Denver lawyer Vicente says, he hasn’t seen any uptick in clients flocking to his firm because they got arrested for driving while high. The state has been aggressive in getting out the message that it’s illegal to consume weed in a vehicle or drive while stoned, and marijuana users may be aware that their behavior could affect pot’s chances of being legalized elsewhere. “People realize that Colorado is really under the microscope,” Vicente says. “And it’s our duty as a state to be responsible.”


"There are other red flags, like a green tongue or inability to cross one’s eyes, that are potential reactions to pot that alcohol doesn’t share" WTF!? What have YOU been smoking!!

Don't forget that users also grow a second head so are easy to indentify :-D


Research at the University of Saskatchewan indicates that, unlike alcohol, cocaine, heroin, or Nancy (“Just say, ‘No!’”) Reagan’s beloved nicotine, marijuana actually encourages brain-cell growth. Research in Spain and other countries has discovered that it has tumor-shrinking, anti-carcinogenic properties. These were confirmed by the 30-year Tashkin population study at UCLA.

Drugs are man-made, cooked up in labs, for the sake of patents and the profits gained by them. Useful, but bearing lists of side effects as long as one's arm.

Marijuana is a medicinal herb, the most benign and versatile in history. “Cannabis” in Latin, and “kaneh bosm” in the old Hebrew scrolls, quite literally the Biblical Tree of Life, used by early Christians to treat everything from skin diseases to deep pain and despair. Kaneh bosm, ‘the fragrant cane’ of a dozen uses: ink, paper, rope, nutrition. . . . It was clothing on their backs and incense in their temples. Medicinal oil could calm one’s nerves, imparting a sense of benevolence and connection with all living things. No wonder that the ‘anointed one’ could gain a spark, an insight, a sense of the divine, and the confidence to share these feelings.

What gets to me are the politicians, prosecutors, and police who pose on church steps or kneeling in prayer on their campaign trails, but can’t face the science or the historical truth about cannabis, Medicinal Herb Number One, safe and effective for thousands of years, and celebrated by most of the world’s major religions.


HAHAHA!!! yeah green tongue and can't cross your eyes LOL!!! those are not real you know.
Cops usually say stuff like that to make a stoned person paranoid so they say something like "really is my tongue green" then they say "why? have you been smoking pot"
And its so so so so so stupid LOL!!!!

Like say the alphabet backwards "i cant even do that when I'm sober" so you are drunk.


There should be zero tolerance with any drug/ drinking and driving. I don't know anyone that gets smarter when they are impaired. It also shouldn't be a debate on choice btwn drink and driving and high driving, both should be eliminate. I don't want anyone impaired on the road. period. 


I'd much rather be on a road with stoned drivers than drunk drivers, or drivers tweaking on speed, meth, or steroids. Fact is, a lot of drivers could use a little toke, and just kick back and cruise. Drive friendly, y'all.


Marijuana and Driving: A Review of the Scientific Evidence
"Marijuana has a measurable yet relatively mild effect on psychomotor skills, yet it does not appear to play a significant role in vehicle crashes, particularly when compared to alcohol. Below is a summary of some of the existing data."

The incidence and role of drugs in fatally injured drivers
"There was no indication that cannabis by itself was a cause of fatal crashes.”
REFERENCE: Washington, DC: US Department of Transportation, National Highway Traffic Safety Administration,
Report No. DOT HS 808 065, K. Terhune. 1992.

Marijuana and actual driving performance
“Drivers under the influence of marijuana retain insight in their performance and will compensate when they can, for example, by slowing down or increasing effort. As a consequence, THC’s adverse effects on driving performance appear relatively small.”
REFERENCE: U.S. Department of Transportation study, 1993

Marijuana’s effects on actual driving performance
“Evidence from the present and previous studies strongly suggests that alcohol encourages risky driving whereas THC encourages greater caution”
REFERENCE: University of Adelaide study, 1995

Role of cannabis in motor vehicle crashes
"There is no evidence that consumption of cannabis alone increases the risk of culpability for traffic crash fatalities or injuries for which hospitalization occurs, and may reduce those risks.. The more cautious behavior of subjects who have received marijuana decreases the impact of the drug on performance, whereas the opposite holds true for alcohol.”
REFERENCE: Marijuana: On-Road and Driving-Simulator Studies; Epidemiologic Reviews 21: 222-232, A. Smiley. 1999.

"Both simulation and road trials generally find that driving behaviour shortly after consumption of larger doses of cannabis results in (i) a more cautious driving style; (ii) increased variability in lane position (and headway); and (iii) longer decision times. Whereas these results indicate a 'change' from normal conditions, they do not necessarily reflect 'impairment' in terms of performance effectiveness since few studies report increased accident risk."  
REFERENCE: UK Department of Environment, Transport and the Regions (Road Safety Division). 2000.

Cannabis And Cannabinoids - Pharmacology, Toxicology And Therapy
“At the present time, the evidence to suggest an involvement of cannabis in road crashes is scientifically unproven”.
REFERENCE: G. Chesher and M. Longo. 2002.,Toxicology%20And%20Therapy.pdf

Cannabis: Our position for a Canadian Public Policy
“Cannabis alone, particularly in low doses, has little effect on the skills involved in automobile driving. Cannabis leads to a more cautious style of driving. However it has a negative impact on decision time and trajectory. This in itself does not mean that drivers under the influence of cannabis represent a traffic safety risk”
REFERENCE: Canadian Senate Special Committee on Illegal Drugs. 2002.

“The evidence to suggest an involvement of cannabis in road crashes is scientifically unproven.”
REFERENCE:  Cannabis and Cannabinoids: Pharmacology, Toxicology, and Therapeutic Potential, 2002
Cannabis and Cannabinoids: Pharmacology, Toxicology, and Therapeutic Potential, edited by Franjo Grotenhermen, MD and Ethan Russo, MD (Haworth Press 2002).,Toxicology%20And%20Therapy.pdf

The Prevalence of Drug Use in Drivers, and Characteristics of the Drug-Positive Group
"There was a clear relationship between alcohol and culpability. In contrast, there was no significant increase in culpability for cannabinoids alone."
REFERENCE: Accident Analysis and Prevention 32(5): 613-622. Longo, MC; Hunter, CE; Lokan, RJ; White, JM; and White, MA. (2000a).

The Effect Of Cannabis Compared With Alcohol On Driving
“Although cognitive studies suggest that cannabis use may lead to unsafe driving, experimental studies have suggested that it can have the opposite effect.” U.S. National Library of Medicine, 2009

The Effect Of Cannabis Compared With Alcohol On Driving
“Although cognitive studies suggest that cannabis use may lead to unsafe driving, experimental studies have suggested that it can have the opposite effect.”
REFERENCE: U.S. National Library of Medicine, 2009

Why Medical Marijuana Laws Reduce Traffic Deaths
“No differences were found during the baseline driving segment (and the) collision avoidance scenarios,”
REFERENCE: Research published in the Journal of Psychoactive Drugs, 2010

Top 10 Reasons Marijuana Users Are Safer Drivers
“20 years of study has concluded that marijuana smokers may actually have fewer accidents than other drivers.”

Risk of severe driver injury by driving with psychoactive substances
"The study found that those with a blood alcohol level of 0.12% were over 30 times more likely to get into a serious accident than someone who’s consumed any amount of cannabis. .. The least risky drug seemed to be cannabis and benzodiazepines and Z-drugs."
REFERENCE: Accident Analysis & Prevention; Volume 59, October 2013, Pages 346–356

Cannabis: Summary Report
“Cannabis alone, particularly in low doses, has little effect on the skills involved in automobile driving.”
REFERENCE: Canadian Senate Special Committee on Illegal Drugs And Cannabinoids - Pharmacology,Toxicology And Therapy.pdf

Acute cannabis consumption and motor vehicle collision risk
"There is no evidence that consumption of cannabis alone increases the risk of culpability for traffic crash fatalities or injuries for which hospitalization occurs, and may reduce those risks."
REFERENCE: British Medical Journal, 1999; M. Bates and T. Blakely


Let's face it.  How many fatal accidents are the stoners going to get into while driving at 20 mph and waiting at intersections for the stop signs to turn green?


Number of American deaths per year that result directly or primarily from the following selected causes nationwide, according to World Almanacs, Life Insurance Actuarial (death) Rates, and the last 20 years of U.S. Surgeon Generals' reports.

TOBACCO - 340,000 to 450,000

ALCOHOL  (Not including 50% of all highway deaths and 65% of all murders) - 150,000+

ASPIRIN  (Including deliberate overdose) - 180 to 1,000+

CAFFEINE  (From stress, ulcers, and triggering irregular heartbeats, etc.) - 1,000 to 10,000
"LEGAL" DRUG OVERDOSE  (Deliberate or accidental) from legal, prescribed or patent medicines and/or mixing with alcohol - e.g. Valium/alcohol - 14,000 to 27,000
ILLICIT DRUG OVERDOSE - (Deliberate or accidental) from all illegal drugs - 3,800 to 5,200

(Marijuana users also have the same or lower incidence of murders and highway deaths and accidents than the general non-marijuana using population as a whole. Cancer Study, UCLA; U.S. Funded ($6 million), First & Second Jamaican Studies, 1968 to 1974; Costa Rican Studies, 1980 to 1982; et al. LOWEST TOXICITY 100% of the studies done at dozens of American universities and research facilities show pot toxicity does not exist. Medical history does not record anyone dying from an overdose of marijuana (UCLA, Harvard, Temple, etc.)


Colorado Historical Fatal Crash Trends - Updated 2/8/2014

Colorado fatal crashes 2002: 677

Colorado fatal crashes 2004: 596

(dispensaries open in 2009)

Colorado fatal crashes 2009: 438

Colorado fatal crashes 2012: 433

(marijuana legalized 2012)

Colorado fatal Crashes 2013: 422

* Source of Data: Colorado DOT & "As Reported" to NHTSA by FARS


the greatest plant in the universe is almost free, LET FREEDOM RING!!!13

been in 1 accident in 20 years cause i was sober and aggressive, had just gotten off work, still wish i had been high, lucky i'm alive

from 0 states to half the country, from low 20% approval to almost 70%, cali runs this planet by 2 decades,time to tie marijuana to the 2014, and 2016 elections

20 years behind us southern states, sad and scary....nobody denies freedoms like the south, nobody...even if marijuana reforms did pass the republiCANTS in charge would deny you all your freedoms, centuries of matter though, we never planned on getting your backwards brethren from day one, half the country already but not one southern state, lol...

love and freedom forever



 I just feel i need to explain this, humanity has been using trial and error forever to figure  thingz out the truth is we dont know how it effectz driving, they just take statistics and we use that info to decipher wat it meanz by producing and eliminating variables and the  at this moment weve hav no info. truth is ppl die everyday, and the world is to big, when we try and make things better we potentially make thingz worse, every demographic has idiots. Truth is legalizing pot  was just a decision itz not right nor wrong, to make ppl happy u hav to make some ppl unhappy and vice versa and so now we r on the road to trial and error how do we clean up this situation to the best of our abilities, one idiot does not speak for the rest of us or for any other group thatz why we r called individuals think about that next time u see protestors for anything. We try and do with wat weve got using the resources we have. Nothing is  ever perfect, and  love one another u would b suprised how the world would b no matter wat race, places of origin, sexual orientation, how much money u make, we all hav our idiotz sadly mine lives next to me but i still care about them. Now if u dont mind im going to get back to looking at parts for water cooling my pc. Sorry for grammar im not great t it not to mention im using a tablet 


No one should drive a car or operate machinery while intoxicated on anything ever.  That being said people in the United States have been driving under the influence of alcohol, prescription drugs and cannabis in increasing numbers ever since the mass produced automobile was invented.  Alcohol and prescription drug intoxication are commonplace as the direct cause in a very large percentage of fatal auto accidents.  Both alcohol and prescription drugs are well known statistical serial killers both in driving and in life in general. 

There is however no real statistically significant data in the crime logs year to year about cannabis and auto accidents.   And no one has ever died of cannabis overdose in all medical history.  If cannabis use did cause  driving incidents we would see the news and the statistical data and we do not.  I have never seen a single fatal accident report in which cannabis intoxication was listed as the sole cause of the accident.  I have seen ever increasing numbers  alcohol was sited.  I am against anyone driving intoxicated.  Bottom line is the silly "OMG cannabis users are going to start driving and cause a rash of accidents" argument is just more reefer madness nonsense and has absolutely no basis in actual crime statistics! Stop the lies and Legalize! 


Video tape the field sobriety test and show it to the jury.  If no problems show up on those skills that are need to drive then maybe that individual is not impaired.  I don't think that any of us would fail to convict someone who is obviously impaired.  We drive on those roads too.  Despite the predictions that the sky will fall I think that many will be surprised at additional reductions in traffic deaths if more people substitute marijuana for alcohol.    


All that is needed is an impairment test not a chemical test. 

If you can perform a specified task you are fit to drive.

Studies indicate cannabis substitution for alcohol in tolerant states and a significant reduction in road fatalities.


It is telling that the states that have legalized medical marijuana has experienced, on average, a 9% drop in traffic fatalities in the first year that law was in operation.


these people have never smoked weed before. we all know that weed does not make you have crashes. they keep on and on about it.  i have never heard of anyone every having a crash because of weed. have you? 


The test today can detect usage over a month ago, not actual intoxication. Make comparable test before you say there are more stoned drivers. Very easy for the Prohibitionist to throw out numbers, trying to confuse the gullible. Time will tell.


Weed being illegal didn't stop people from driving stoned. There is no change now that it's legal, except that you won't be busted for possession of it.

If one subscribes to the scare tactics like, "if marijuana is legalized, then there will be stoned drivers everywhere," and, "if marijuana is legalized, then kids will have easy access to it," then one deserves to be confused.

Pot smokers can now buy from legitimate sources instead of having to buy it on the black market. Pot smokers are NOT suddenly appearing on our roads. They've always been there.


@kcbblondie so should we also take away driving from older people. After around age 40 there are noticeable signs of slower reaction times impaired vision.
What about people who drive poorly from the start are they can drive even though their abilities at 100% is only 20% of a normal persons ability?
Or are we playing this like everyone starts equal and it is the effort we put into driving. as if someone kills me for driving poorly sober is any better than someone driving poorly stoned.

Use scientific evidence and start testing people on actual reaction time and visual comprehension not just some test and a guideline written by legislators picking numbers that sound good.

We need to stop all impaired driving not just substance impairment.


@kcbblondie People who use Cannabis rarely attain a level of impairment comparable to that of .08% BAC.  We put down the joint/pipe long before that uncomfortable level of inebriation occurs.  Novice users ingesting strong edibles might get too high to drive, but likely would not even attempt it with such a buzz.  Alcohol, OTOH, creates lowered inhibitions and reckless behaviour!


@420aquaman  I actually have. And the problem is you can't test on site for it. Unlike alcohol, where you can test for it on the spot. 


@420aquaman  I think that many don't drive when they are stoned because they are aware that they are impaired and let's face it, it's just a real hassle to provide the sustained attention needed for driving if you are high.  They usually wait to drive until they are not impaired while those who drink alcohol are less aware of impairment and drive anyway with disastrous consequences.  The drinker blasts through the stop sign speeding and narrowly misses the lady with a baby carriage.  The stoner, on the other hand, stops at the stop sign, brushes the Cheetos from his t shirt, and ... waits for the stop sign to turn green.  LOL, Don't worry.  I am convinced that our roads will be safer when the additional data is in.  To address your last question I am sure that some do get into accidents when they are stoned.  Even people who are not impaired get into accidents.  I don't think that people driving stoned will contribute much to traffic deaths or that it is any way as dangerous as texting and driving.


@AlanHall I've always said that if a person thinks that making pot legal will increase kids ease of access has no idea the best way to find a dealer in a new town is to ask a high school kid. My daughter is 19 and over the last few years  I've asked her and dozens of her friends what's easier to get, pot or beer. Every one said "Pot". Because beer is legal and controlled, and pot is illegal and uncontrolled. 

Like almost everything about the war on people who happen to use unapproved drugs, the reality is 180 degrees different from the common (carefully propagandized) belief.

Edward Bernays would be proud of his invention, "public relations", a.k.a. propaganda.


@AlanHall Besides, anyone who is just now trying pot because it's legal isn't likely to be on the road. They're more likely to be sitting at home wondering why they never noticed before how amazing their ceiling is. ;)


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