Large study of 5,000 injured drivers found THC modestly increased crash culpability while methamphetamine and alcohol were far riskier | Research

The takeaway

Among 5,000 injured drivers in Victoria, Australia, THC showed a modest increase in crash culpability (OR 1.9) compared to dramatic increases for methamphetamine (OR 19) and alcohol, with drug combinations substantially amplifying risk.

Traffic safety researchers, impaired driving policymakers, harm reduction advocates

THC: OR 1.9; methamphetamine: OR 19; drug combinations: substantially amplified risk

What the researchers found

THC-positive drivers had modestly increased culpability odds (OR 1.9, 95% CI: 1.2-3.1). Methamphetamine: OR 19 at all concentrations. Benzodiazepines: OR 3.2 (95% CI: 1.6-6.1). Antidepressants, antipsychotics, and opioids: no significant increase. Combinations of alcohol with THC or benzodiazepines, and THC with methamphetamine, showed large increases.

Why it matters

This large, well-designed culpability study puts THC's driving risk in context alongside other substances, showing it is real but modest compared to methamphetamine and alcohol, while drug combinations are particularly dangerous.

The numbers in context

5,000 drivers; THC OR 1.9 (1.2-3.1); methamphetamine OR 19; benzodiazepines OR 3.2 (1.6-6.1); alcohol showed large dose-dependent increases. Drug combinations: substantially amplified risk.

How the study worked

Culpability analysis of 5,000 injured drivers (1,000/year for 5 years) from Victoria, Australia, with comprehensive blood toxicology. Logistic regression with drug-free drivers as reference. Multivariable models adjusted for crash attributes.

What this study cannot tell us

Australian drivers may differ from other populations; blood THC levels do not perfectly correlate with impairment timing; injured drivers are a selected sample; some substances may be underdetected.

How to read the evidence

Large culpability study with comprehensive toxicology and appropriate statistical controls.

When this study was published

Published in 2020.

The bigger picture

The relative risk hierarchy (methamphetamine >> alcohol >> benzodiazepines > THC) and the dramatic amplification from drug combinations should inform both traffic safety policy and public messaging about impaired driving.

Questions still open

Should THC driving limits be set differently from alcohol given the lower individual risk? How can policy address the high risk of drug combinations?

Common questions

How much does THC increase crash risk?

THC-positive drivers had about 1.9 times the odds of being culpable for a crash compared to drug-free drivers. This is a modest but statistically significant increase, much smaller than the risk from methamphetamine (19x) or high blood alcohol levels.

Are drug combinations more dangerous?

Yes, substantially. Combinations of alcohol with THC, alcohol with benzodiazepines, and THC with methamphetamine all showed large increases in culpability that exceeded the risk of any single substance alone.

Read the original research

Odds of culpability associated with use of impairing drugs in injured drivers in Victoria, Australia.

Accident; analysis and prevention, 135, 105389

View on PubMed ↗Publisher’s paper ↗

Citation

Drummer, Olaf H; Gerostamoulos, Dimitri; Di Rago, Matthew; Woodford, Noel W; Morris, Carla; Frederiksen, Tania; Jachno, Kim; Wolfe, Rory. (2020). Odds of culpability associated with use of impairing drugs in injured drivers in Victoria, Australia.. Accident; analysis and prevention, 135, 105389. https://doi.org/10.1016/j.aap.2019.105389