Blood samples from 2,500 injured drivers were analysed for alcohol, cannabinoids (measured by the presence of THC), benzodiazepines and stimulants. The relationship between the prevalence and concentration of drugs and the culpability of the driver was examined using an objective method for assessing culpability. There were no significant differences between males and females with respect to culpability. However, there was a relationship between age and culpability: drivers under 26 years and over 60 years were more likely to be culpable. Drivers who tested positive for alcohol only, benzodiazepines only and the combinations of alcohol and THC and alcohol and benzodiazepines were significantly more likely to be culpable for the crash compared with the drug-free group. Conversely, a lower percentage of drivers who only tested positive for THC were culpable for the crash compared with drug-free drivers. This difference was not statistically significant. For car drivers in single-vehicle crashes, the majority of drivers were judged culpable irrespective of drug use. In multiple-vehicle crashes, car drivers testing positive for alcohol only or benzodiazepines only were more likely to be culpable for the crash compared with drug-free drivers. For motorcycle riders in both single- and multiple-vehicle crashes, there were no significant differences between the drug-positive and drug-free groups. A higher percentage of drug-free riders in multiple-vehicle crashes were culpable compared with riders who only tested positive for THC, but this difference was not statistically significant. There was a significant concentration-dependent relationship between alcohol and culpability: as blood alcohol concentration increased, so did the percentage of culpable drivers. When THC was used alone, there was no significant increase in culpability. For those drivers with benzodiazepines at therapeutic concentrations and above, there was a significant increase in culpability. The relationship between stimulants and culpability was not significant, although a higher proportion of stimulant-positive drivers were culpable compared with drug-free drivers. The combinations of alcohol and THC, and alcohol and benzodiazepines also produced a significant increase in culpability, but this increase was not significantly greater than that produced by alcohol alone.