Results of this meta-analysis indicate that marijuana use by drivers is associated with a significantly increased risk of crash involvement. Specifically, drivers who test positive for marijuana or self-report using marijuana are more than twice as likely as other drivers to be involved in motor vehicle crashes. The increased risk of crash involvement associated with marijuana use is generally consistent across studies that were conducted in different geographic regions and driver populations, used different research design approaches, and were based on different methods for measuring marijuana use. Of the 9 studies included in the meta-analysis, all but one found a statistically significant association between marijuana use and crash risk. The only study that failed to detect a significant association between marijuana use and crash risk was a small case-control study conducted in Thailand (65
), where the prevalence of marijuana use appears to be far lower than reported elsewhere (63
). The validity of the association between marijuana use and crash risk is further strengthened by the empirical evidence that a dose-response relation exists between the dose and frequency of marijuana use by drivers and their risk of being involved in motor vehicle crashes (68
The crash risk associated with marijuana, if confirmed by further research, may have important implications for driving safety and public policy. Drug-impaired driving is a serious problem in the United States. Toxicologic testing data indicate that 28% of fatally injured drivers (71
) and more than 11% of the general driver population test positive for nonalcohol drugs (72
), with marijuana being the most commonly detected substance.
Although this meta-analysis provides compelling evidence for an association between marijuana use and crash risk, it is impossible to infer causality from these epidemiologic data alone. A more rigorous assessment of the nature of the relation between marijuana use and crash risk is complicated by several factors. Among them are bias from measured and unmeasured confounding and the difficulty posed by polydrug use. Although most of the studies included in this meta-analysis considered some confounding variables in evaluating the association between marijuana use and crash risk, adjustment was usually limited to a few measured covariates, such as age, sex, and alcohol use. Additional variables that may confound the relation between marijuana use and crash risk, such as exposure to driving and risk-taking propensity, are often difficult to measure and thus are not readily available. Other factors confounding the relation between marijuana use and crash risk are divergent definitions and assessments of marijuana use across studies.
Different methods of assessing marijuana use (e.g., self-report, urine tests, and blood tests) may have different levels of validity and reliability (39
). Most of these screenings determine whether marijuana was used within the past few weeks, whereas acute impairment in driving skills from marijuana use lasts only 3–4 hours (39
). Furthermore, because marijuana is an illicit drug in most countries, it is possible that drivers in the comparison groups might be less likely than those involved in crashes to submit to testing, which could lead to overestimation of the effect of marijuana use on crash risk (43
Polydrug use represents another challenge to determining the role of marijuana in motor vehicle crashes. Polydrug use by drivers is common, with up to a quarter of drivers injured in crashes testing positive for 2 or more drugs (including alcohol) (42
). Although it is necessary to understand the effect of individual drugs on driving performance, the high prevalence of polydrug use by drivers makes it difficult to do so. On the other hand, assessing interaction effects on driving safety of different drug combinations based on epidemiologic data would require very large study samples, comprehensive drug testing data, and tremendous financial and other resources (42
). One of the studies included in the meta-analysis evaluated the effect of marijuana in combination with alcohol on crash risk and found that the combination of marijuana and alcohol confers an exceptionally heightened risk to driving safety (68
). Experimental studies show that cannabis and alcohol affect different cognitive functions that may interact with driving performance (41
). Specifically, cannabis seems to impair automatic behaviors, such as tracking, at low doses and impair ability to perform more complex tasks at higher doses, whereas alcohol seems to more readily affect functions requiring cognitive control (41
). While both marijuana and alcohol can impair driving performance, there appears to be greater variations in effects of marijuana compared with alcohol (41
It is worth noting that the studies included in this meta-analysis did not directly assess medical marijuana use. Recreational marijuana is often administered differently than medical marijuana. The Institute of Medicine advises against smoking marijuana, the typical method of recreational consumption, and recommends using pills or a vaporizer (77
). The cannabinol and THC components in pills do not entirely correspond to the components in the plant form of the substance (17
). Medical marijuana, including cannabinoid medicines, may be administered in regulated doses, whereas recreational use is less quantifiable (78
). It is conceivable that differences in administration modes and dosages of marijuana between medical use and recreational use may have different effects on driving ability and crash risk. Although legislation permitting medical marijuana use and marijuana decriminalization has proliferated in the United States, many state medical marijuana laws include provisions prohibiting driving under the influence of marijuana. Quantifying the excess crash risk associated with marijuana use is essential for understanding the overall health consequences of legalizing medical marijuana and the effectiveness of policy interventions on drug-impaired driving in reducing injury morbidity and mortality from motor vehicle crashes.