Accident causes were analysed in an Australian study with 3398 fatally-injured drivers. While drivers with low THC concentrations in their blood had a lower probability of causing a traffic accident than drug free drivers, higher THC concentrations were associated with a considerable higher culpability ratio.

For all drivers with only THC in their blood the odds ratio (OR) for causing an accident compared to drug free drivers was 2.7 (which means 2.7-fold). For drivers with more than 5 ng/ml THC in the blood the OR increased to 6.6. However, the culpability ratio for drivers with 5 ng/ml THC or less in their blood was lower than drug free drivers. Drug free means that no legal (alcohol, medical drugs) or illegal drugs were found.

The culpability ratio of drivers with a blood alcohol concentration above 0.05 per cent was about three times higher than that for the THC only group. The OR for drivers with THC and alcohol compared to the THC only group was 2.9, suggesting an additive effect of THC and alcohol on impairment of driver performance.

Drivers above the age of 60 and below the age of 25 had a higher culpability risk than drivers 30-59 years of age, the first probably due to a decreased psychomotor performance, the latter probably due to inexperience and higher risk-taking. The OR of drivers 18- 25 years of age compared to drivers of 30-39 years of age was 1.7, the OR of drivers above the age of 60 compared to drivers of 30-39 years of age was 2.2.
(Sources: Drummer O, et al. The involvement of drugs in drivers of motor vehicles killed in Australian road traffic crashes. Accid Anal Prev 2004;36(2):239-48; personal communication by Olaf Drummer)