Cannabis was detected in 52.8% of suspected drug-impaired drivers in Ontario from 2008-2019, followed by cocaine (44.3%) and methamphetamine (24.8%), with 80% of cases involving multiple substances.
Traffic safety researchers, law enforcement, cannabis policy makers, impaired driving educators
52.8% of impaired drivers tested positive for cannabis
What the researchers found
Over 12 years, 5,388 samples from suspected drug-impaired drivers were analyzed. Cannabis was most frequently detected (52.8%), followed by cocaine (44.3%) and methamphetamine (24.8%). Eighty percent of cases involved more than one substance. Sample submissions increased after cannabis legalization in 2018.
Why it matters
Understanding which substances impaired drivers use, and how often they combine them, helps shape enforcement strategies and public safety messaging. Cannabis as the top substance has implications for post-legalization road safety.
The numbers in context
5,388 samples over 12 years. Cannabis: 52.8%. Cocaine: 44.3%. Methamphetamine: 24.8%. Multiple substances: 80%. Samples increased after Drug Evaluation and Classification Program implementation and cannabis legalization.
How the study worked
Retrospective analysis of blood and urine samples submitted to Ontario's Centre of Forensic Sciences from suspected drug-impaired drivers (2008-2019). Standardized comprehensive toxicological analysis tested for a wide variety of impairing drugs.
What this study cannot tell us
Only suspected impaired drivers (not a random sample). Detection does not equal impairment (cannabis can be detected days after use). Sample collection increased over time due to more trained officers, not necessarily more impaired driving. Ontario-specific data.
How to read the evidence
Large forensic toxicology dataset provides comprehensive detection data, but samples are from suspected drivers (selection bias) and detection does not equal impairment.
When this study was published
2024 analysis of Ontario forensic toxicology data from 2008-2019
The bigger picture
The 80% polysubstance rate means cannabis-impaired driving rarely occurs in isolation, complicating efforts to attribute impairment to any single substance and highlighting the challenge of setting cannabis-specific driving limits.
Questions still open
- Did cannabis legalization actually increase impaired driving, or just detection? How should enforcement handle the 80% polysubstance rate when attributing impairment? Are current THC per se limits appropriate given detection windows?
Common questions
Is cannabis the most common drug in impaired drivers?
Does finding cannabis mean the driver was impaired by cannabis?
Read the original research
Toxicology findings from drivers suspected of drug-impaired driving in Ontario (2008-2019).
Traffic injury prevention, 25(7), 894-901
Citation
Beirness, Douglas J; Rajotte, James W; Peaire, Amy E. (2024). Toxicology findings from drivers suspected of drug-impaired driving in Ontario (2008-2019).. Traffic injury prevention, 25(7), 894-901. https://doi.org/10.1080/15389588.2024.2355593