Many of us learn from a young age that drinking and driving is extremely dangerous. But as more states legalize marijuana, questions arise about how dangerous it is to drive under a different influence: cannabis. Like alcohol, marijuana intoxication comes with coordination and reflex issues, impaired cognitive skills, and a harder time making decisions. But unlike alcohol, intoxication levels are hard to quantify and can vary greatly based on factors such as mood, setting, dose, concentration of THC, ratio of cannabinoids, etc. So how can and should we determine the true effects cannabis has on driving safety?

Currently, driving under the influence of cannabis (DUIC) is determined simply by the presence of cannabis in the body. This is measured through a urine test. However, THC and its metabolites can last in the body anywhere from 3 days to a month after intoxication depending on regularity of use, so a positive cannabis test is not necessarily a good marker of whether a person was actually driving under the drug’s influence. Additionally, means of consumption affect how the body absorbs the drug, so even smoking marijuana cigarettes and using a vape pen have different time cycles of intoxication (Pearlson et al., 2021). Because there is no standard for cannabis use and intoxication, it also means that it is almost impossible to accurately measure how “under the influence” people are by simply using a blood or urine test on the side of the road or at a police station. This should be taken into consideration when looking at the existing statistics for the presence of cannabis in people who have gotten into motor vehicle accidents. 

Current data includes information from FARS in 2012, in which 2,083 people reported having car accidents while DUIC, which was the cause of 2,208 deaths. Additionally, in states where cannabis is now legal, an increasing percentage of drivers involved in fatal accidents have been cannabis-positive (Salomonsen-Sautel et al., 2014). Other studies that have applied current statistics to nationwide legalization argue that cannabis intoxication would be responsible for approximately double the traffic fatalities per year (Kamer et al., 2020). These trends indicate that driving under the influence of cannabis is extremely dangerous, but it is again very hard to measure how intoxicated people actually are simply due to the presence of cannabis in their bodies. Exploring how cannabis use can impact certain driving behaviors is important to determine if the effects of intoxication are fully responsible for the increase in motor vehicle accidents associated with the drug.

In behavioral studies, marijuana has been found to cause impairment on many tasks that are associated with good driving, including motor coordination, visual functions, tracking, and attention-requiring complex tasks (review by Berghaus et al.,1995). However, in actual driving simulations, results are mixed. According to some studies, people under the influence of marijuana do not have significant trouble driving during road tests, and in particular, experienced smokers exhibit only limited functional impairments (Sewell et al., 2009). On the other hand, cannabis-intoxicated drivers have demonstrated difficulties with staying in a lane, monitoring the speedometer, making decisions to pass, quickly braking or responding to a changing light, and avoiding sudden obstacles. All of these behaviors are important for road safety and defensive driving, so due to these associated impairments, cannabis can increase the risk for motor vehicle accidents. It is important to note that in most research on combined cannabis and alcohol use, even with alcohol levels below the legal driving limit, significant impairments occur (Sutton, 1983). Some of the reported instances of accidents involving cannabis-positive drivers may even be due to the combination of these substances (Pearlson et al., 2021). Based on all of this information and continuing research, it is possible that cannabis use does not have as many significant negative effects on driving as alcohol use, but it is probably safest to just not drive under the influence of any psychoactive substances.

References

Berghaus, G., & Guo, B. L. (1995). Medicines and driver fitness—findings from a meta-analysis of experimental studies as basic information to patients, physicians, and experts. Alcohol, drugs, and traffic safety, 295-300.

Kamer, R. S., Warshafsky, S., & Kamer, G. C. (2020). Change in traffic fatality rates in the first 4 states to legalize recreational marijuana. JAMA internal medicine, 180(8), 1119-1120. https://doi.org/10.1001/jamainternmed.2020.1769. 

NHTSA-FARS. Fatality Analysis Reporting System (FARS). National Center for Statistics and Analysis National Highway Traffic Safety Administration Washington DC: National Highway and Traffic Safety Administration (2012).

Pearlson, G. D., Stevens, M. C., & D’Souza, D. C. (2021). Cannabis and driving. Frontiers in psychiatry, 12, 689444. https://doi.org/10.3389/fpsyt.2021.689444. 

Salomonsen-Sautel, S., Min, S. J., Sakai, J. T., Thurstone, C., & Hopfer, C. (2014). Trends in fatal motor vehicle crashes before and after marijuana commercialization in Colorado. Drug and alcohol dependence, 140, 137-144. https://doi.org/10.1016/j.drugalcdep.2014.04.008. 

Sewell, R. A., Poling, J., & Sofuoglu, M. (2009). The effect of cannabis compared with alcohol on driving. American journal on addictions, 18(3), 185-193. https://doi.org/10.1080/10550490902786934. 

Sutton, L. R. (1983). The effects of alcohol, marihuana and their combination on driving ability. Journal of studies on alcohol, 44(3), 438-445.