Cannabis-impaired driving and Canadian youth

Abstract

Acute cannabis use results in inattention, delayed information processing, impaired coordination, and slowed reaction time. Driving simulator studies and epidemiologic analyses suggest that cannabis use increases motor vehicle crash risk. How much concern should we have regarding cannabis associated motor vehicle collision risks among younger drivers? This article summarizes why young, inexperienced drivers may be at a particularly high risk of crashing after using cannabis. We describe the epidemiology of cannabis use among younger drivers, why combining cannabis with alcohol causes significant impairment and why cannabis edibles may pose a heightened risk to traffic safety. We provide recommendations for clinicians counselling younger drivers about cannabis use and driving.

On October 17, 2018, the Canadian government passed the ‘Cannabis Act’ (Bill C-45), which legalizes and regulates the nonmedical use of cannabis (1). Legalization of cannabis has the following potential benefits: improved product safety, increased tax revenue, reduction in the size of the illicit market and related criminal activities, and reduced access to cannabis for minors (with harsher penalties for selling cannabis to minors). However, there are concerns that cannabis legalization will result in an increase in cannabis-related harms. Concerns about cannabis-related motor vehicle collisions (MVCs) are especially relevant to adolescent Canadians, since younger drivers may be at higher risk of driving after cannabis use and of crashing while impaired (2).

Nonmedical cannabis legalization has uncertain consequences for adolescent traffic safety. Medical marijuana legalization in several American states was associated with reductions in traffic fatalities, including crashes involving drivers aged 15 to 25 years (3,4). However, legalization of nonmedical cannabis for adults has an uncertain effect on cannabis use among adolescents (5–8). Moreover, nonmedical cannabis legalization in Washington and Colorado was associated with an increase in cannabis-related crashes (9–11). Nonmedical cannabis use during annual cannabis celebrations is also associated with an increase in MVCs, particularly among younger drivers (12,13). For these reasons, we believe paediatricians should consider the following points when counselling adolescent patients about cannabis use and driving.

Many young Canadians drive after using cannabis or ride in a vehicle driven by someone who has used cannabis. Among Canadian National Cannabis Survey respondents aged 15 to 24 years, about one in four reported using cannabis in the previous 3 months, 14% reported driving within 2 hours of using cannabis, and 12% reported being a passenger in a vehicle driven by someone who had used cannabis in the previous two hours (14). Among Canadian youth who use cannabis frequently, 80% of males and 75% of females reported that in the previous 30 days they had been in a car driven by someone (including themselves) who had recently used marijuana or other drugs and 64% of males and 33% of females reported being ‘intoxicated’ with marijuana while operating a vehicle (15).

Driving after cannabis use may be more common than driving after alcohol use among younger drivers. Roadside surveys in Ontario and British Columbia (BC) have measured drugs in a representative sample of evening and nighttime weekend drivers aged 16 to 18 years. In Ontario, 10% of these young drivers tested positive for drugs (5.8% for cannabis) (16). In BC, 4.1% of drivers aged 16 to 18 years tested positive for drugs (age group results for cannabis not reported) (17). In contrast, there were no drinking drivers under the age of 19 in the Ontario roadside survey, and only 1 drinking driver under the age of 19 (out of 118) in the BC survey (16,17).

Cannabis impairs the psychomotor skills required for safe driving. Cannabis contains over 60 cannabinoids but most impairing effects are caused by the main psychoactive compound, Δ-9-tetrahydrocannabinol (THC) (18). THC binds to presynaptic CB1 receptors in the brain and inhibits release of neurotransmitters, mediating its central nervous system effects (19,20). After smoking a ‘joint’, whole blood THC levels typically peak at >100 ng/mL within 10 to 15 minutes and then drop rapidly so that THC is usually <2 ng/mL within 4 hours after a single acute exposure in people who use cannabis infrequently (21–23). Psychotropic effects after smoking cannabis typically peak at 15 to 30 minutes and resolve by 4 to 6 hours (24). Oral ingestion of cannabis delays the onset and extends the duration of effect, with typical effects peaking at four hours and lasting for eight hours or longer. The onset and duration of effect depends on dose and on product composition (faster onset in THC infused drinks, slower onset in fatty cannabis edibles) (21). ‘Overdose’ with cannabis edibles is common because the THC dose can be high and, as a result of delayed onset, some people may take a second dose before the first has taken effect. As cannabis edibles often look like candy or food, unintentional ingestion is also a risk. Ingestion of a large dose of THC, particularly by a small child, results in prolonged effects including vomiting, panic, elevated pulse, irritability, agitation, and depressed level of consciousness (25,26).

After taking cannabis, most people experience the desired effects of euphoria and relaxation, although some develop anxiety, panic, or even paranoia. Unfortunately, cannabis also impairs attention, coordination, and reaction time. It may also cause an altered sense of time and slowed information processing (27). Driving simulator studies and on the road driving tests show that cannabis impairs driving performance (28–30). After taking cannabis, drivers in traffic ‘weave’ more, sometimes to the point of being unable to stay within the traffic lanes (28). They also have difficulty maintaining a constant speed or a uniform gap between themselves and a lead vehicle (28). In simulator studies, drivers tend to slow down after using cannabis, have delayed reactions when responding to a sudden hazard, and are more likely to crash (29–32). This evidence suggests that people who drive after using cannabis might have a large increase in collision risk. However, several factors potentially mitigate this risk. Some authors suggest that drivers impaired by cannabis are aware of their impairment and able to partially compensate by driving more slowly, leaving more headway, and avoiding risky manoeuvers (33). People who use cannabis regularly also develop partial tolerance so that the acute impairing effects of cannabis become less prominent in people who use it regularly than in those who only use it occasionally (27).

Epidemiological studies show a modest increase in crash risk after cannabis use. Several recent meta-analyses all concluded that cannabis use increases crash risk, with odds ratios (ORs) ranging from 1.36 to 2.66 (34,35). Most studies looking at cannabis and crash risk use either a case–control design which compares cannabis use in crash-involved drivers with noncrash-involved drivers (36–47), or a responsibility analysis design, which includes only crash-involved drivers and compares cannabis use in drivers deemed responsible for the crash versus in those deemed nonresponsible (48–55). Unfortunately, most studies have significant limitations. Cannabis exposure was often based on either presence of THC-COOH or any detectable level of THC, neither of which necessarily indicates acute use or impairment. In fact, a 2016 review found only five studies that calculated crash risk for drivers with blood THC >2 ng/mL (35).

In order to mitigate a possible increase in cannabis-related crashes following legalization, the Canadian parliament amended the criminal code related to impaired driving so that drivers with THC >2 ng/mL are subject to fines, and drivers THC >5 ng/mL are subject to harsher penalties including possible jail time (56). Our group recently published a responsibility analysis that prospectively studied 3,004 injured drivers treated in British Columbia trauma centres (57). We found no increased risk of crash responsibility in drivers with THC <5 ng/mL. In drivers with THC ≥5 ng/mL, the adjusted OR was 1.74 (95%CI:0.59 to 6.36; P=0.35). We note that failure to find an increased risk in drivers with THC ≥5 ng/mL may have been due to low statistical power as our analysis included only 20 drivers with THC ≥5 ng/mL. There was significantly increased risk of crash responsibility in drivers with blood alcohol concentration (BAC) ≥0.08% (OR=6.00; 95% confidence interval [CI]=3.87 to 9.75; P<0.01), and in drivers with other recreational drugs detected (OR=1.82; 95%CI=1.21 to 2.80; P<0.01), or sedating medications detected (OR=1.45; 95%CI=1.11 to 1.91; P<0.01).

At typical degrees of intoxication, driving after drinking is even more dangerous than driving after using cannabis (31). Large case–control studies clearly demonstrate a substantial increase in crash risk in drinking drivers (58). The Fort-Lauderdale-Long Beach study compared BACs in 4,919 drivers who were involved in 2,871 crashes with those in 10,066 matched control drivers and found that risk of crashing increased with increasing BACs. For example, in drivers over the age of 21 with BAC ≥0.08%, the OR of crashing was 6.63 (59), and the risk increased dramatically as BAC increased, with ORs over 100 at the highest BAC levels (60), for drivers under the age of 21, the crash risk versus BAC level increased even more steeply (59). Studies that investigated both cannabis and alcohol all found substantially higher crash risk with alcohol than with cannabis (43,47,51,53–55,61,62). When counselling youth not to drive after using cannabis, it is important to also mention the well-established risks of drinking and driving.

Combined use of cannabis with alcohol is very dangerous. There is strong evidence that cannabis combined with alcohol causes more impairment than either substance alone. Bramness (2010) reported results from 5,042 Norwegian drivers apprehended by police for suspected impairment and subsequently examined by a police physician. Of these drivers, 589 tested positive for THC alone, 894 for both THC and alcohol, 3,480 for alcohol alone, and 79 were negative for both substances. Of the 589 drivers with only THC detected, half (49.6%) were considered impaired. Three quarters (77.4%) of the THC negative drivers with low BAC levels were considered impaired compared to 92% of THC-positive drivers with low BAC levels (63). A Dutch group gave drivers marijuana, alcohol, or placebo and compared driving performance in traffic. They found ‘dramatically impaired driving performance’, with increased weaving, increased time driving outside the lane markers, and markedly slowed reaction time in drivers given both alcohol and cannabis (28). Downey (2013) used a driving simulator to study driving performance in 80 drivers under placebo controlled, blinded experimental conditions and found that driving performance was significantly compromised when THC and alcohol were combined, especially in nighttime conditions (29). More recently, Hartman (2015) conducted a double-blind, placebo controlled study in a high-fidelity driving simulator and found that both THC and alcohol impaired driving performance with additive impairing effects on weaving when alcohol and cannabis were combined (30). Epidemiological evidence on the crash risk of marijuana combined with alcohol is limited but several studies suggest an increased risk when the two substances are combined (36,54,61).

Young drivers may be at a particularly high risk of crashing after using cannabis. Motor vehicle crashes are a leading cause of injury and death in children and adolescents in North America and worldwide (64–67). Even when sober, young drivers are at higher risk of crashing than older, more experienced drivers (68). The fatality rate per vehicle mile traveled among adolescents (16 to 19 years) is threefold that of adults (30 to 59 years) (69). Adolescents are prone to risk taking including experimentation with substance use and driving while impaired (70). Younger drivers that engage in substance use may also engage in other risky driving behaviours (15,71,72).

Collision risk after substance use increases more steeply among young drivers than for older drivers. Peck (2008) found that young drivers with BAC ≥ 0.08% were 39.2 times more likely to crash than young sober drivers. In comparison, drivers ≥ 21 years old with BAC ≥ 0.08% were 6.63 times more likely to crash (59). This is the reason for zero tolerance laws that prohibit novice drivers from having any detectable BAC. There is limited research on collision risk in young drivers after using cannabis, but inexperienced drivers would likely be less able to compensate for cannabis-related impairment. Adolescent drivers may be unaware of the risks of driving after using cannabis and might be under the impression that the likelihood of getting caught and penalized is low (73). Conversely, drivers subject to graduated licensing restrictions are less likely to drive after using cannabis (74), supporting the cautious approach, adopted by many Canadian provincial licensing authorities, that prohibits novice drivers from having any detectable amount of THC in their body.

RECOMMENDATIONS

Young drivers should be informed that cannabis increases the risk of motor vehicle crash. Adolescents should be advised not to drive after using cannabis and not to ride as a passenger with a driver who has used cannabis. While recommendations are evolving in response to new evidence, one recent Canadian guideline suggests abstaining from driving for at least 6 hours after cannabis use (75). Individuals should continue to abstain from driving if they feel any effects of the drug even after this time. Driving after co-ingestion of cannabis and alcohol should be discouraged since even low levels of alcohol cause significant impairment when combined with cannabis. Young drivers should be informed that most provinces have a zero tolerance policy for any cannabis use by novice drivers, putting them at risk of losing their license if they drive after using cannabis.

Funding: There are no funders to report for this submission.

Potential Conflicts of Interest: All authors: No reported conflicts of interest. All authors have submitted the ICMJE Form for Disclosure of Potential Conflicts of Interest. Conflicts that the editors consider relevant to the content of the manuscript have been disclosed.