Among over 1,000 crash-injured drivers, alcohol was involved far more often than cannabis, and more than a third of cannabis-positive drivers were also drunk.
Read this if you're interested in how cannabis compares to alcohol in real-world driving crashes.
1 in 5 crash drivers had illegal BAC; 1 in 10 tested positive for drugs
What the researchers found
Researchers analyzed hospital and forensic data from 1,074 crash-involved drivers and motorcyclists admitted to hospitals in South Australia over three years. About 1 in 5 had blood alcohol above the legal limit of 0.05. Routine drug testing found about 1 in 10 tested positive for drugs, with cannabis being the most common.
A key finding was the overlap: more than a third of cannabis-positive cases also involved alcohol, and when alcohol was present alongside cannabis, blood alcohol levels tended to be very high (above 0.15 g/100 mL). The coroners' reports from fatal crashes showed similar patterns.
The authors concluded that while drug driving is a genuine problem, alcohol remains the primary substance driving road trauma, and roadside drug testing should not come at the expense of random breath testing programs.
Why it matters
Understanding the relative contribution of different substances to road crashes helps allocate enforcement and prevention resources effectively. The frequent co-occurrence of cannabis and high-level alcohol use suggests that polydrug impairment, not cannabis alone, may be the more relevant safety concern.
The numbers in context
1,074 crash participants studied over 3 years. 1 in 5 had BAC above 0.05. 1 in 10 tested positive for drugs. Over half of drug-positive cases involved cannabis. More than a third of cannabis cases also involved alcohol. BAC in combined cases typically exceeded 0.15 g/100 mL.
How the study worked
Cross-sectional study linking hospital admission data, police crash reports, and forensic blood test results for 1,074 crash participants over three years. A supplementary sample of 135 coroners' reports from fatal crashes was also analyzed.
What this study cannot tell us
Cross-sectional design cannot establish whether cannabis directly contributed to crashes. THC detection in blood does not necessarily indicate impairment at the time of the crash. The study was limited to South Australia.
How to read the evidence
Cross-sectional analysis of hospital, police, and forensic data from over 1,000 crash participants.
When this study was published
Published in 2015 using data from 2008-2014 in South Australia.
The bigger picture
Debates about cannabis-impaired driving often focus on cannabis in isolation, but this study highlights how frequently cannabis and alcohol co-occur in real-world crashes. Policies addressing impaired driving need to account for polydrug use rather than treating each substance independently.
Questions still open
- How much of the crash risk attributed to cannabis is actually driven by concurrent high-level alcohol use? Would the crash involvement patterns differ if cannabis-only cases were analyzed separately from polydrug cases?
Common questions
Is cannabis or alcohol more dangerous for driving?
Does testing positive for cannabis mean you caused a crash?
Read the original research
Examination of the role of the combination of alcohol and cannabis in South Australian road crashes.
Traffic injury prevention, 16(5), 443-9
Citation
Baldock, M R J; Lindsay, V L. (2015). Examination of the role of the combination of alcohol and cannabis in South Australian road crashes.. Traffic injury prevention, 16(5), 443-9. https://doi.org/10.1080/15389588.2014.969804
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