Driving, Drinking, and Drug Use
- Overview & Basic Data
- Estimates of Impairment & Risk
- Estimated DUID Prevalence
- Roadside Drug Testing Devices
- Other Specific Research Findings on Marijuana and Driving
- Laws & Policies
Page last updated June 10, 2020 by Doug McVay, Editor/Senior Policy Analyst.
36. Cannabis Use, Alcohol Use, Smartphone Use, and Accident Risk
"Although for the mobile phone conversation and cannabis studies the reaction times were slightly different, they were still comparable. The same visual stimulus was used and was presented in the same visual scene. When reaction times under each condition were compared with the baseline reaction times measured, alcohol gave a 12.5% increase in reaction times, cannabis a 21% increase, a hands-free mobile phone conversation increased reaction times by 26.5%, texting by 37.4%, using a smartphone for social networking by 37.6% and using a mobile phone for a hand-held mobile phone conversation increased reaction times by 45.9% compared to the baseline condition. Thus, using a smartphone for social networking resulted in a greater impairment to reaction times than alcohol, cannabis, hand held mobile phone conversations and texting, but less than a hand held mobile conversation."
Basacik, D.; Reed N. & Robbins, R., "Smartphone use while driving: A simulator study," Institute of Advanced Motorists (London, United Kingdom: Transport Research Laboratory, 2011), pp. 37-38.
37. Cannabis Use and Motor Vehicle Accident Risk
"We found only limited evidence to support the claim that cannabis use increases accident risk. Participants who had driven under the influence of cannabis in the previous year appeared to be no more likely than drug-free drivers to report that they had had an accident in the previous 12 months. Prima facie, this would seem to suggest that cannabis-intoxicated driving is not a risk factor for non-fatal accidents. In this sense, the results would support those of Longo et al. (2000b) who found no relationship between recent cannabis use and driver culpability for non-fatal accidents."
Jones, Craig; Donnelly, Neil; Swift, Wendy; Weatherburn, Don, "Driving under the influence of cannabis: The problem and potential countermeasures," Crime and Justice Bulletin, NSW Bureau of Crime Statistics and Research (Syndey, Australia: September 2005). p. 11.
38. Times for THC Absorption, Bioavailability, and Excretion
"Absorption is slower following the oral route of administration with lower, more delayed peak THC levels. Bioavailability is reduced following oral ingestion due to extensive first pass metabolism. Smoking marijuana results in rapid absorption with peak THC plasma concentrations occurring prior to the end of smoking. Concentrations vary depending on the potency of marijuana and the manner in which the drug is smoked, however, peak plasma concentrations of 100-200 ng/mL are routinely encountered. Plasma THC concentrations generally fall below 5 ng/mL less than 3 hours after smoking. THC is highly lipid soluble, and plasma and urinary elimination half-lives are best estimated at 3-4 days, where the rate-limiting step is the slow redistribution to plasma of THC sequestered in the tissues. Shorter half-lives are generally reported due to limited collection intervals and less sensitive analytical methods. Plasma THC concentrations in occasional users rapidly fall below limits of quantitation within 8 to 12 h. THC is rapidly and extensively metabolized with very little THC being excreted unchanged from the body. THC is primarily metabolized to 11-hydroxy-THC which has equipotent psychoactivity. The 11-hydroxy-THC is then rapidly metabolized to the 11-nor-9-carboxy-THC (THC-COOH) which is not psychoactive. A majority of THC is excreted via the feces (~65%) with approximately 30% of the THC being eliminated in the urine as conjugated glucuronic acids and free THC hydroxylated metabolites."
Couper, Fiona J., Logan, Barry K., et al., "Drugs and Human Performance Fact Sheets," (Washington, DC: National Highway Traffic Safety Administration, April 2004), p. 8.
39. THC and Cannabis Dosages
"THC is the major psychoactive constituent of cannabis. Potency is dependent on THC concentration and is usually expressed as %THC per dry weight of material. Average THC concentration in marijuana is 1-5%, hashish 5-15%, and hashish oil ³ 20%. The form of marijuana known as sinsemilla is derived from the unpollinated female cannabis plant and is preferred for its high THC content (up to 17% THC). Recreational doses are highly variable and users often titer their own dose. A single intake of smoke from a pipe or joint is called a hit (approximately 1/20th of a gram). The lower the potency or THC content the more hits are needed to achieve the desired effects; 1-3 hits of high potency sinsemilla is typically enough to produce the desired effects. In terms of its psychoactive effect, a drop or two of hash oil on a cigarette is equal to a single “joint” of marijuana. Medicinally, the initial starting dose of Marinol® is 2.5 mg, twice daily."
Couper, Fiona J., Logan, Barry K., et al., "Drugs and Human Performance Fact Sheets," (Washington, DC: National Highway Traffic Safety Administration, April 2004), p. 7.
40. Cannabis Use and Motor Vehicle Accident Risk
"Cannabis use impairs cognitive, memory and psycho-motor performance in ways that may impair driving.10 Recent data suggest that approximately 5% of Canadian drivers/adults report driving after cannabis use in the past year.39 Large-scale epidemiological studies using different methodologies (e.g., retrospective epidemiological and case control studies) have found that cannabis use acutely increases the risk of motor vehicle accident (MVA) involvement and fatal crashes among drivers.40,41 Recent reviews have found the increase in risk to be approximately 1.5-3.0, an increase which is substantially lower, however, than that in alcohol-impaired drivers. The impairment from concurrent alcohol and cannabis use may be multiplicative, so individuals who drive under the influence of both drugs may be at higher risk for MVAs.42 An expert consensus view was that a THC concentration of 7-10 nanograms per millilitre in serum would produce impairment equivalent to that of 0.05% blood alcohol content (BAC). It was suggested that this level could serve as a 'per se' limit to define cannabis-impaired driving.43 Current research suggests that acute impairment from cannabis typically clears 3-4 hours after use.44
Fischer, Benedikt; Jeffries, Victoria; Hall, Wayne; Room, Robin; Goldner, Elliot; Rehm, Jürgen, "Lower Risk Cannabis Use Guidelines for Canada (LRCUG): A Narrative Review of Evidence and Recommendations," Canadian Journal of Public Health (Ottawa, Ontario: Canadian Public Health Association, September/October 2011) Vol. 102, No. 5, p. 325.