Effects of Marijuana on Driving
April 20, 2024 - State Cannabis Laws and Cannabis Positivity among Fatally Injured Drivers. This research publication (Leavitt et al., 2024) focused on cannabis-related fatal automobile accidents. The authors analyzed data of a 2-year period (2019-2020) from the Fatality Analysis Reporting System (FARP) to examine the association between state level legalization status and fatal cannabis positive traffic accidents.
FARP is sponsored and maintained by the National Highway Traffic Safety; it’s a census of all crashes that occur on a public road and that results in a fatality to any person within 30 days of the crash in the US.
Toxicological testing results for 14,079 fatally injured drivers were available for the present analysis. Excluded from consideration were the following: non-drivers, drivers who were under 15 years, and drivers who died after surviving longer than 1 hour following the crash (to prevent false negative toxicological reports).
Overall, about one-third of fatally injured drivers tested positive for cannabis use, and 40% of them tested positive for an alcohol level >0.08%.
The analysis of legalization status and cannabis positivity controlled for these confounds: age, sex, race/ethnicity, year, previous DWI convictions, restraint use and alcohol level.
These two key findings were observed:
1. States with laws permitting recreational use of cannabis (RML), fatally injured drivers were significantly more likely to test positive for cannabis use compared to non-legal states.
2. States with only medical cannabis laws had little impact on the odds of cannabis positivity among fatally injured drivers.
November 6, 2023 - Are oral fluid testing devices effective for the roadside detection of recent cannabis use? A systematic review. This systematic review of the research literature (Dobri et al. 2019) looks at on-site oral fluid drug screening devices for detecting cannabis use. Fifteen articles that used an on-site testing device to detect cannabis use were selected for review. The authors conclude there is a lack of standardized test protocols with respect to testing oral fluids for all substances. The accuracy of current devices for detecting THC is inconsistent, although it’s relatively better with lower THC concentration levels compared to higher concentration levels. The authors conclude that no existing oral fluid device meets acceptable standards for detecting roadside THC levels. Also, this problem remains: What level of THC is a reliable marker for impaired driving?
November 6, 2023 - How research and policy can shape driving under the influence of cannabis. The authors (Metrik & McCarthy, 2023) discuss the challenges and opportunities of testing for cannabis impaired driving. Part of the article focuses on a promising oral fluid test (OF). The authors note: “Oral fluid (OF) tests are likely the best candidates for detecting recent use. OF screening is non-invasive, carries minimal risk of adulteration, can be conducted in proximity to the time of driving and has reduced inter-individual variability and reduced variability between THC doses compared to blood. At very low thresholds (e.g. ≤1 ng/mL), OF testing detects recent (past 3 h) use of smoked THC with very high sensitivity, but has modest specificity at longer detection windows, which may lead to positive tests outside of the typical time course of impairment.” But these testing problems still remain: testing for edibles, and determining the association of driving impairment based on OF test results.
July 25, 2022 - Changes in Traffic Crash Rates After Legalization of Marijuana: Results by Crash Severity. The objective of this study conducted by researchers at the Insurance Institute for Highway Safety was to estimate the effects of marijuana legalization and the subsequent onset of retail sales on injury and fatal traffic crash rates in the United States during the period 2009–2019.
A state-by-state quarterly crash rates per mile of travel were analyzed as a function of numerous factors known to be linked to injury/fatal traffic crash rates (i.e., time, unemployment rate, maximum posted speed limit, seat belt use rate, alcohol use rate, percent of miles driven on rural roads), as well as indicators of legalized recreational marijuana use and sales.
Legalization of the recreational use of marijuana was associated with a 6.5% increase in injury crash rates and a 2.3% increase in fatal crash rates, but the subsequent onset of retail marijuana sales did not elicit additional substantial changes.
When the combined effect of when state legalization was passed and when retail sales were permitted, there was a 5.8% increase in injury crash rates and a 4.1% increase in fatal crash rates. States varied considerably. Across states, the effects on injury crash rates ranged from a 7% decrease to an 18% increase. The effects on fatal crash rates ranged from a 10% decrease to a 4% increase.
The study authors conclude that “The estimated increases in injury and fatal crash rates after recreational marijuana legalization are consistent with earlier studies, but the effects varied across states.”
Additional note from this writer (K.C. Winters): These data show provocative associations with the onset of commercial sales of cannabis and an increase in injury cash and fatal crash rates. Yet the authors did not have data from states that would address the question of possible rate changes regarding THC-positive injury/fatal crashes, that is, incidences when the vehicle driver had detectable THC in his or her blood.
February 14, 2022 - Simultaneous Alcohol/Cannabis Use and Driving Under the Influence. The simultaneous use of alcohol and cannabis is commonly involved in motor vehicle crashes and fatalities. Data among drivers aged 16 or older from the 2016−2019 National Survey on Drug Use and Health (N=34,514) were analyzed pertaining to self-reported any past-year driving under the influence of alcohol-only, cannabis-only, both alcohol/cannabis, or not driving under the influence. The main finding was that 2 in 5 drivers with history of alcohol and cannabis use reported driving under the influence of alcohol and/or cannabis, and drivers reporting a history of simultaneous alcohol/cannabis use were more likely to report cannabis-related driving under the influence. (KCW note: Self-reported “driving under the influence” does not indicate extent of driving impairment.)
February 14, 2022 - Driving Performance and Cannabis Users’ Perception of Safety. This driving impairment study by Marcotte and colleagues (2022) examined the magnitude and time course of driving impairment (based on driving simulator performance) and perceived impairment produced by smoked cannabis of varying THC levels. The double-blind, randomized controlled study had three “smoking” groups: placebo, 6% THC, and 13% THC. Compared to the placebo group, both THC groups showed driving simulator-derived composite performance scores that were equally significantly lower at both the 30 minute and the 1 hour 30 minute post-smoking time points, and both THC groups trended lower at the 3 hours 30 minute time point. There were no group differences at 4 hours 30 minutes. Also, nearly 70% in the THC groups indicated a “readiness to drive” at the 1hr 30min point, which presents a “false confidence” in driving ability soon after THC exposure.
June 17th, 2021 - Studies Find Crash Rates Rise When States Legalize Marijuana; Suggest Link to Alcohol. This article discusses two studies of the crash rates which spiked with the legalization of recreational marijuana use and retail sales.
January 10, 2019: Use of Alcohol and Cannabis Among Adults Driving Children in Washington State. According to a roadside survey conducted in Washington State, 14.1% of drivers with children in the car–nearly one in seven–tested positive for THC, the principal psychoactive compound in marijuana.
January 9, 2019: Driving under the influence of cannabis among medical cannabis patients with chronic pain. Driving under the influence of cannabis (DUIC) is a public health concern among those using medical cannabis. Understanding behaviors contributing to DUIC can inform prevention efforts.
December 31st, 2019: Two recent studies from the JAMA of show the negative effects of marijuana on driving. The first studied teens driving after using marijuana (DAUM). Of teens that reported current use 48.8% reported DAUM which was twice as high as those that drank and drove. The second study of driving showed significant impairment up to 100 minutes after using.