Driving, Drinking, and Drug Use

1. What the research shows about marijuana and driving

"Several meta-analyses of multiple studies found that the risk of being involved in a crash significantly increased after marijuana use13 -- in a few cases, the risk doubled or more than doubled.14-16 However, a large case-control study conducted by the National Highway Traffic Safety Administration found no significant increased crash risk attributable to cannabis after controlling for drivers’ age, gender, race, and presence of alcohol.17"

"Marijuana," National Institute on Drug Abuse, January 2017, last accessed March 5, 2017, p. 13.

2. Impairment Thresholds for Blood THC Level Compared With Blood Alcohol Content

"To combat drug-driving, most countries either operate a zero tolerance policy or take into account degree of impairment, sometimes in a two-tier system. Legal limits may be set low, at the limit of detection, or higher to take effects into consideration. For example, while the project set a detection limit of 1 ng/ml in whole blood for THC in the roadside surveys, it was found that 2 ng/ml THC in whole blood (3.8 ng/ml THC in serum) seems to cause impairment equivalent to 0.5 g/l BAC. Such equivalents could not be calculated for other drugs. It is not realistic to develop cut-off limits for all substances."

European Monitoring Centre for Drugs and Drug Addiction, "Driving Under the Influence of Drugs, Alcohol and Medicines in Europe — findings from the DRUID project" (Luxembourg: Publications Office of the European Union, 2012), doi: 10.2810/74023, p. 7.

3. Relationship Between Medical Marijuana Laws (MMLs) and Traffic Fatalities in the US

"Our study suggests that, on average, MMLs are associated with reductions in traffic fatalities, particularly pronounced among those aged 25 to 44 years, a group representing a great percentage of all registered patients for medical marijuana use,29 and with increased prevalence of marijuana use after the enactment of MMLs.30 Although increases in marijuana use following the establishment of marijuana dispensaries could reduce the occurrence of alcohol-related mortality by reducing the number of drivers driving under the influence of alcohol, other simultaneous factors at the state and local levels also may be responsible for these changes in traffic fatalities. Our findings show great heterogeneity of the MML–traffic fatalities associations across states, suggesting the presence of these other mechanisms. This is important for policy development and for the debate of the enactment or repealing of MMLs, given that alternative local strategies such as stronger police enforcement and programs aiming to reduce impaired driving involving any substance use could be local factors linked to reductions in traffic fatalities in MML states."

Julian Santaella-Tenorio, Christine M. Mauro, Melanie M. Wall, June H. Kim, Magdalena Cerdá, Katherine M. Keyes, Deborah S. Hasin, Sandro Galea, and Silvia S. Martins. US Traffic Fatalities, 1985–2014, and Their Relationship to Medical Marijuana Laws. American Journal of Public Health: February 2017, Vol. 107, No. 2, pp. 336-342. doi: 10.2105/AJPH.2016.303577

4. Increased Risk of Motor Vehicle Accident (MVA) From Various Drugs

"We identified cohorts of individuals hospitalized in California from 1990 to 2005 with ICD-9 diagnoses of methamphetamine- (n = 74,170), alcohol- (n = 592,406), opioids- (n = 68,066), cannabis- (n = 47,048), cocaine- (n = 48,949), or polydrug-related disorders (n = 411,175), and these groups were followed for up to 16 years. Age-, sex-, and race-adjusted standardized mortality rates (SMRs) for deaths due to MVAs were generated in relation to the California general population. Standardized MVA mortality ratios were elevated across all drug cohorts: alcohol (4.5, 95% CI, 4.1–4.9), cocaine (3.8, 95% CI, 2.3–5.3), opioids (2.8, 95% CI, 2.1–3.5), methamphetamine (2.6, 95% CI, 2–3.1), cannabis (2.3, 95% CI, 1.5–3.2) and polydrug (2.6, 95% CI, 2.4–2.9). Males and females had similar MVA SMRs."

Russell C. Callaghan, Jodi M. Gatley, Scott Veldhuizen, Shaul Lev-Ran, Robert Mann, and Mark Asbridge, "Alcohol- or Drug-Use Disorders and Motor Vehicle Accident Mortality: A Retrospective Cohort Study," Accident Analysis and Prevention, 53 (2013) 149–155, http://dx.doi.org/10.1016/j.aa....

5. Motor Vehicle Accident Risk From Cannabis Use and Estimated Wait-Time Before Driving

"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
"This time span could be recommended to users as a minimum wait period before driving. The required wait before driving would need to be longer for higher doses, and would also vary on the basis of individual variation."

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.