Among 2,133 crash-involved THC-positive drivers in Norway, 84% also tested positive for other substances (61% sedatives, 38% stimulants, 33% alcohol), complicating the attribution of impairment to cannabis alone.
Read this if you are interested in cannabis-impaired driving research or drug driving policy.
84% polysubstanceProportion of crash-involved THC-positive drivers also testing positive for other impairing substances
What the researchers found
Of 10,520 apprehended crash-involved drivers (2013-2020), 20% had THC above the legal limit (1.3 ng/mL). Among THC-positive drivers, 84% also had other substances above legal limits. The most common combination was cannabis with sedatives and stimulants (22.9%). Polysubstance use was least common among drivers under 24. Drivers with high THC (>5 ng/mL) were less likely to have high concentrations of other substances, suggesting that when THC alone was present at high levels, cannabis may have been a more important contributor to impairment.
Why it matters
The attribution of crash involvement to cannabis is complicated by the fact that most THC-positive drivers are also using other impairing substances. This has major implications for cannabis-impaired driving policy and research.
The numbers in context
- 10,520 crash-involved drivers tested (2013-2020)
- 2,133 (20%) THC above 1.3 ng/mL
- 84% also positive for other substances
- 61% sedatives, 38% stimulants, 33% alcohol, 10% opioids
- Most common combo: cannabis + sedatives + stimulants (22.9%)
- Polysubstance least common in under-24 age group
How the study worked
Retrospective analysis of all non-fatally injured crash-involved drivers in Norway (2013-2020) with blood samples submitted for forensic toxicology testing. Comprehensive substance screening.
Who was studied
2,133 crash-involved THC-positive drivers in Norway (2013-2020)
What this study cannot tell us
Only includes apprehended drivers, not all crash-involved drivers. Norwegian patterns may not generalize globally. Blood collection timing varied, affecting THC concentration interpretation. Cannot determine which substance contributed most to impairment in polysubstance cases.
How to read the evidence
Large national forensic toxicology dataset over 8 years. Strong epidemiological data but cannot determine causation for individual crashes.
When this study was published
Published in 2023. Covers 8 years of comprehensive Norwegian crash toxicology data.
The bigger picture
The high rate of polysubstance use among crash-involved THC-positive drivers challenges simple narratives about cannabis-impaired driving. Most "cannabis crashes" may be primarily driven by sedatives, stimulants, or alcohol.
Questions still open
- What proportion of cannabis-only crashes exist, and are they different from polysubstance crashes?
- Should cannabis DUI enforcement focus on polysubstance detection rather than THC alone?
- Does the lower polysubstance rate in young drivers suggest different cannabis use patterns by age?
Read the original research
Crash-involved THC-positive drivers in Norway have a high frequency of polysubstance use.
Drug and alcohol dependence, 244, 109800
Citation
Gjerde, Hallvard; Bogstrand, Stig Tore; Jamt, Ragnhild Elén Gjulem; Vindenes, Vigdis. (2023). Crash-involved THC-positive drivers in Norway have a high frequency of polysubstance use.. Drug and alcohol dependence, 244, 109800. https://doi.org/10.1016/j.drugalcdep.2023.109800