The Canadian Senate Committee on Illegal Drugs stated:
"Cannabis alone, particularly in low doses, has little effect on the skills involved in automobile driving.”

Road safety has become one of the biggest concerns surrounding marijuana legalization. Some people feel that “stoned drivers” will become a menace on our highways. To understand whether this is a valid issue, we have to consider the method of drug testing and the results from states that have already legalized recreational cannabis. Evidence shows that traffic fatalities have not increased in Colorado since legalization, that testing whether a driver is high can be difficult, and that increased availability of marijuana can actually decrease alcohol consumption and drunk driving. 

Marijuana impairment does not appear to play a significant role in on-road traffic accidents. A 2002 review of seven separate studies involving 7,934 drivers reported, “Crash culpability studies have failed to demonstrate that drivers with cannabinoids in the blood are significantly more likely than drug-free drivers to be culpable in road crashes.” (#1) This result is likely because subject under the influence of marijuana are aware of their impairment and compensate for it accordingly, such as by slowing down and by focusing their attention when they know a response will be required. 

 A big problem with accurately identifying whether drivers involved in accidents were under the influence of marijuana at the time or not is difficult because, unlike alcohol, marijuana remains detectable in one’s body often for days, long after any intoxicating effects have worn off. A driver involved in an accident on a Tuesday may test positive even though he last consumed the drug on Saturday.

In Colorado, the number of yearly traffic fatalities has remained relatively constant since 2005, with no detectable difference since marijuana was legalized in 2012.

Studies that indicate the presence of marijuana in drivers involved in traffic accidents often do not mention if alcohol was also involved. If the driver tested positive for alcohol then the alcohol had to have been consumed within several hours before the accident and caused the driver’s impairment. The presence of marijuana could simply indicate that the driver had consumed marijuana sometime within the past few weeks but does not indicate that he was under its influence at the time of the accident.

In Colorado, the number of yearly traffic fatalities has remained relatively constant since 2005, with no detectable difference since marijuana was legalized in 2012.(#2) There is also a case to be made that the availability of marijuana lowers alcohol use, and consequently lowers the rate of drunk driving. Jacob Sullum argued this point in Forbes magazine, stating, “A study published last year by the Journal of Law & Economics found that adoption of medical marijuana laws is associated with a decline in traffic fatalities, possibly because of such a substitution effect.” (#3) As we learn more about marijuana and develop intelligent systems for regulating it, we can continue to monitor and control drugged driving. But at this point, there is little reason to think that putting an end to marijuana prohibition would make our roads less safe.

To sum up, a report by the Canadian Senate Committee on Illegal Drugs stated: “Cannabis leads to a more cautious style of driving, [but] it has a negative impact on decision time and trajectory. [However,] this in itself does not mean that drivers under the influence of cannabis represent a traffic safety risk. … Cannabis alone, particularly in low doses, has little effect on the skills involved in automobile driving.” (#4)

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REFERENCE #1: G. Chesher and M. Longo. 2002. Cannabis and alcohol in motor vehicle accidents. In: F. Grotenhermen and E. Russo (Eds.) Cannabis and Cannabinoids: Pharmacology, Toxicology, and Therapeutic Potential. New York: Haworth Press. Pp. 313-323.

REFERENCE #2: https://www.codot.gov/safety/alcohol-and-impaired-driving/druggeddriving/drugged-driving-statistics.html

REFERENCE #3: http://www.forbes.com/sites/jacobsullum/2014/03/07/drunk-and-drugged-driving-arrests-fall-in-washington-as-anti-pot-group-warns-that-legalization-undermines-road-safety/#4bd80787768b

REFERENCE #4: Canadian Senate Special Committee on Illegal Drugs. 2002. Cannabis: Summary Report: Our Position for a Canadian Public Policy. Ottawa. Chapter 8: Driving Under the Influence of Cannabis.