The culpability of drivers killed in New Zealand road crashes and their use of alcohol and other drugs

• New Zealand crash fatalities have a high prevalence of alcohol and cannabis use, but few other drugs.
• Minimal increase in crash risk was observed with cannabis use.
• Combined alcohol and cannabis use shows an increased crash risk compared with cannabis use alone, but not with alcohol use alone.

Over a period of five years, blood samples were taken from 1046 drivers killed as a result of a motor vehicle crash on New Zealand roads. These were analysed for the presence of alcohol and a range of both illicit drugs and psychoactive medicinal drugs. Driver culpability was determined for all crashes. The control group of drug- and alcohol-free drivers comprised 52.2% of the study population. Drivers positive for psychoactive drugs were more likely to be culpable (odds ratio (OR) 3.5, confidence interval (CI) 95% 2.4–5.2) than the control group. Driver culpability exhibited the expected positive association with alcohol use (OR 13.7, 95% CI 4.3–44) and with combined alcohol and cannabis use (OR 6.9, 95% CI 3.0–16). There was only a weak positive association between cannabis use (with no other drug) and culpability (OR 1.3, CI 95% 0.8–2.3). Furthermore, the OR for drivers with blood tetrahydrocannabinol (THC) concentrations greater than 5 ng/mL was lower (OR 1.0, CI 95% 0.4–2.4) than drivers with blood THC concentrations less than 2 ng/mL (OR 3.1, CI 95% 0.9–10). This is inconsistent with results reported by other studies where a significant increase in crash risk was found with blood THC levels greater than 5 ng/mL. In this study, there were very few drivers who had used a single drug, other than cannabis or alcohol. Therefore, from this study, it is not possible to comment on any relationship between opioid, stimulant or sedative drug use and an increased risk of being killed in a crash for the drivers using these drugs. The results from a multivariate analysis indicate that driver gender, age group and licence status, (P = 0.022, P = 0.016, P = 0.026, respectively), the type of vehicle being driven (P = 0.013), the number of vehicles in the crash (P < 0.001), the blood alcohol concentration of the driver (P < 0.001) and the use of any drug other than alcohol and cannabis (P = 0.044), are all independently associated with culpability.