Cannabis and impaired driving

Cannabinoids present in the cannabis plant and its derived drugs are known to cause impaired driving in users, with effects on a driver similar to those of alcohol.[1][2] Meta-analysis of epidemiological studies shows that recent cannabis use elevates the risk of motor vehicle accident 1.5 to 2 times above control.[3] Many jurisdictions have laws forbidding cannabis-impaired driving, and some have per se impairment levels determined by metabolites detected in body fluids.

Effects on driving[edit]

While several studies have shown increased risk associated with cannabis use by drivers, other studies have not found increased risk.[4] Cannabis usage has been shown in some studies to have a negative effect on driving ability.[5]
The British Medical Journal indicated that “drivers who consume cannabis within three hours of driving are nearly twice as likely to cause a vehicle collision as those who are not under the influence of drugs or alcohol”.[6]

In Cannabis and driving: a review of the literature and commentary, the United Kingdom’s Department for Transport reviewed data on cannabis and driving, finding “Cannabis impairs driving behaviour. However, this impairment is mediated in that subjects under cannabis treatment appear to perceive that they are indeed impaired. Where they can compensate, they do, for example … effects of driving behaviour are present up to an hour after smoking but do not continue for extended periods”.[7] The report summarizes current knowledge about the effects of cannabis on driving and accident risk based on a review of available literature published since 1994 and the effects of cannabis on laboratory based tasks. The study identified young males, amongst whom cannabis consumption is frequent and increasing, and in whom alcohol consumption is also common, as a risk group for traffic accidents. The cause, according to the report, is driving inexperience and factors associated with youth relating to risk taking, delinquency and motivation. These demographic and psychosocial variables may relate to both drug use and accident risk, thereby presenting an artificial relationship between use of drugs and accident involvement.[8]

Kelly, Darke and Ross show similar results, with laboratory studies examining the effects of cannabis on skills utilised while driving showing impairments in tracking, attention, reaction time, short-term memory, hand-eye coordination, vigilance, time and distance perception, and decision making and concentration.[9] An EMCDDA review concluded that “the acute effect of moderate or higher doses of cannabis impairs the skills related to safe driving and injury risk”, specifically “attention, tracking and psychomotor skills”.[10]
In their review of driving simulator studies, Kelly et al. conclude that there is evidence of dose-dependent impairments in cannabis-affected drivers’ ability to control a vehicle in the areas of steering, headway control, speed variability, car following, reaction time and lane positioning. The researchers note that “even in those who learn to compensate for a drug’s impairing effects, substantial impairment in performance can still be observed under conditions of general task performance (i.e. when no contingencies are present to maintain compensated performance).”[9]

A 2012 meta-analysis found that acute cannabis use increased the risk of an automobile crash.[6]

An extensive 2013 review of 66 studies regarding crash risk and drug use found that cannabis was associated with minor, but not statistically significant increased odds of injury or fatal accident. The estimated fatal crash odds for cannabis (1.26) were lower than: opiates (1.68), antianxiety medications (2.30), zopiclone (sleep medicine) (2.60), cocaine (2.96), and amphetamines (5.17). The estimated injury odds for cannabis (1.10) were lower than: antihistamines (1.12), penicillin (1.12), antianxiety meds (1.17), antidepressants (1.35), antiasthmatics (1.31), zopiclone (sleep medicine) (1.42), cocaine (1.66), and opiates (1.91). The study concluded: “By and large, the increase in the risk of accident involvement associated with the use of drugs must be regarded as modest…Compared to the huge increase in accident risk associated with alcohol, as well as the high accident rate among young drivers, the increases in risk associated with the use of drugs are surprisingly small.[11]

A report from the University of Colorado, Montana State University, and the University of Oregon found that on average, states that have legalized medical cannabis had a decrease in traffic-related fatalities by 8–11%.[12] The researchers hypothesized “it’s just safer to drive under the influence of marijuana than it is drunk….Drunk drivers take more risk, they tend to go faster. They don’t realize how impaired they are. People who are under the influence of marijuana drive slower, they don’t take as many risks”. Another consideration, they added, was the fact that users of marijuana tend not to go out as much.[13]

In the largest and most precisely controlled study of its kind carried out by the U.S. Department of Transportation’s National Highway Traffic Safety Administration to research the risks of cannabis and driving,[14] it was found that other “studies that measure the presence of THC in the drivers’ blood or oral fluid, rather than relying on self-report tend to have much lower (or no) elevated crash risk estimates. Likewise better controlled studies have found lower (or no) elevated crash risk estimates”.[4] The study found that “after adjusting for age, gender, race and alcohol use, drivers who tested positive for marijuana were no more likely to crash than those who had not used any drugs or alcohol prior to driving”.[15] The study however cautions that “these results do not indicate that drug use by drivers is risk-free.”[4]

On the other hand, a recent study of Journal of Transport & Health indicated that the numbers of fatal crashes involving marijuana after the recreational marijuana legalization or decriminalization have significantly increased in Colorado, Washington, and Massachusetts.[16]

Detection of impairment[edit]

A major US insurance company states that “there are no widely accepted methods for detecting impairment from marijuana”.[17] The U.S. drug czar Gil Kerlikowske said in 2012, “I’ll be dead from old age, before we know the impairment levels” for cannabis.[18] A 2017 Canadian government report stated “science is unable to provide general guidance to drivers about how much cannabis can be consumed before it is unsafe to drive”.[19]

Some users seem to be able to perform risk compensation by driving slower or other behaviors, and some users appear to develop a physiological tolerance, making determining a standard impairment dose difficult.[2] A 2010 study found “cannabis and alcohol acutely impair several driving‐related skills in a dose‐related fashion, but the effects of cannabis vary more between individuals because of tolerance, differences in smoking technique, and different absorptions of THC”.[20] A 2014 study found “no correlation between degree of impairment and THCA-A blood concentration”.[21] Because of these findings, some safety organizations have advocated that police use behavioral impairment tests instead of metabolite testing.[22][23]

Despite these issues, some jurisdictions have taken to a urine or blood metabolite per se test standard.

False indications of driving impairment[edit]

Testing for metabolites of THC, versus the actual THC intoxicant, can result in DUID convictions of users who aren’t actually impaired.[24] According to National Institute on Drug Abuse, “the role played by marijuana in crashes is often unclear because it can be detected in body fluids for days or even weeks after intoxication”.[25]

False negatives[edit]

Fat solubility of THC also makes detection in body fluids, which are mostly water, problematic, leading to false indication of non-impairment for users.[24]

Legal standards[edit]


Germany’s blood serum (not whole blood like most standards) THC-A limit is 1 ng/mL.[26][27]


In Uruguay, the first nation to legalize cannabis, “any THC detectable in the body will deem a motorist impaired to drive”.[28] Reporting has indicated that blood tests would be utilized.[29] Canadian draft regulations proposed in 2017 (Bill C-46) had summary conviction offences starting at 2 nanograms per milliliter.[19][30]

United States[edit]

At the federal level, the United States Department of Transportation regulates public transportation (e.g. pilots, bus drivers, and train operators) and commercial truckers.[31][32] 49 CFR Part 199 and 49 CFR Part 40 set maximum urine concentration levels of THC-A as the threshold for impairment for regulated transportation occupations.[32] As of January 1, 2018, the THC-A testing cutoffs were 50 ng/mL for an initial immunoassay test and 15 ng/mL for a confirmatory gas chromatography–mass spectrometry test.[33]

In some U.S. jurisdictions that have legalized cannabis, such as Colorado (5 ng/ml), Nevada (2 ng/ml), Washington State (5 ng/ml), a maximum blood concentration level has been set by the state as a threshold for driving while intoxicated (DWI/DUI).[22][34]

Neither California nor Oregon, while having legalized cannabis, have a per se THC/THC-A limit.[35][36] Oral (saliva) test machines are used in San Diego.[36]

See also[edit]


  1. ^ Brubacher 2011 ”There is clear evidence that cannabis, like alcohol, impairs the psychomotor skills required for safe driving.”
  2. ^ a b Small 2016.
  3. ^ Hall 2018.
  4. ^ a b c Compton & Berning 2015.
  5. ^ Li et al.
  6. ^ a b Ashbridge 2012.
  7. ^ Cannabis and driving: a review of the literature and commentary (No.12). Retrieved on 2016-12-19.
  8. ^ “Cannabis and driving: a review of the literature and commentary (No.12)”. The National Archives (UK). 8 February 2010. Archived from the original on 2010. Retrieved 1 February 2011.
  9. ^ a b Kelly, Darke & Ross 2004.
  10. ^ Sznitman, Olsson & Room 2008.
  11. ^ Elvik 2013.
  12. ^ Morrison, James (2013-01-01). “Separating fact vs. fear on medical marijuana”. The Herald News. Retrieved 2013-01-08.
  13. ^ “Driving stoned: safer than driving drunk?”. 2011-12-02. Retrieved 2013-01-08.
  14. ^ “NHTSA Releases Two New Studies on Impaired Driving on U.S. Roads”. Department of Transportation’s National Highway Traffic Safety Administration. 6 February 2015.
  15. ^ Ingraham, C (9 February 2015). “Stoned drivers are a lot safer than drunk ones, new federal data show”. Washington Post. Retrieved 13 February 2015.
  16. ^ Lee, Jaeyoung; Abdel-Aty, Ahmad; Park, Juneyoung (2018-05-26). “Investigation of associations between marijuana law changes and marijuana-involved fatal traffic crashes: A state-level analysis”. Journal of Transport & Health. 10: 194–202. doi:10.1016/j.jth.2018.05.017. ISSN 2214-1405.
  17. ^ DUID Is the New DUI: What California’s Marijuana DUI Laws Look Like, Los Angeles, California: Auto Insurance Specialists (Mercury Insurance Group), January 24, 2018
  18. ^ Trying to determine marijuana impairment level, Associated Press, March 18, 2012 – via Cleveland Plain Dealer
  19. ^ a b Peter Zimonjic; Catherine Cullen (October 13, 2017), Government releases legal limits for drugged driving but can’t say how much pot is too much – Federal government and experts say they expect the new offences to be challenged in court, CBC News
  20. ^ Sewell, Poling & Sofuoglu 2010.
  21. ^ Raikos et al. 2014.
  22. ^ a b Beth Mole (May 10, 2016), Blood THC levels after smoking pot are useless in defining “too high to drive”. Better metrics needed as study finds increase in fatal crashes involving weed., Ars Technica
  23. ^ Ashley Halsey III (May 10, 2016), “Unlike alcohol, it’s tough to set DUI limits for marijuana”, The Washington Post
  24. ^ a b Angus Chen (February 9, 2016), Blood Tests Can’t Tell Who’s Really Too Stoned To Drive, NPR
  25. ^ Does marijuana use affect driving?, Marijuana research report series, National Institute on Drug Abuse (NIDA), February 2018
  26. ^ Joint Efforts – efficient automated THC determination, Gerstel
  27. ^ Mirko Laudon, VERKEHRSRECHT: Entzug der Fahrerlaubnis bereits ab 1,0 ng/ml THC im Blut [Traffic law: Disqualification [to drive] due to 1.0 ng/ml THC in the blood] (in German), Hamburg
  28. ^ Comparison of Marijuana Laws/Regulations: Colorado, Washington, Uruguay, Oregon, Alaska and District of Columbia, Organization of American States Inter-American Drug Abuse Control Commission, November 11, 2014, retrieved 2018-03-28
  29. ^ Alfonso Serrano (May 3, 2014), Uruguay unveils marijuana regulation details – Law will be closely monitored throughout the hemisphere, as more countries mull drug policy alternatives, Al Jazeera America
  30. ^ Christopher Katsarov (January 23, 2018), “How will Canada’s new drugged-driving rules actually work?”, The Globe and Mail, Toronto
  31. ^ Overview (official website), United States Department of Transportation Office of Drug & Alcohol Policy & Compliance, 2012-03-02, retrieved 2018-03-28
  32. ^ a b Pipeline Regulatory Compliance: Drug and Alcohol (official website), DOT Pipeline and Hazardous Materials Safety Administration, June 28, 2017
  33. ^ What are the cutoff concentrations for drug tests?
    (DOT Rule 49 CFR Part 40 Section 40.87), DOT website, January 1, 2018
  34. ^ Marijuana Legalization: How Does It Affect DUI/DWI Law?, NOLO
  35. ^ Marijuana-related laws (PDF), Governors Highway Safety Association, January 2018
  36. ^ a b James Queally; Sarah Parvini (March 22, 2018), “For police, catching stoned drivers isn’t so easy”, Los Angeles Times


Further reading[edit]

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