Driver-related risk factors of fatal road traffic crashes associated with alcohol or drug impairment

•Toxicology and crash data were coupled for fatally injured drivers/riders.•Associations between behaviours and drug/alcohol impairment were analyzed.•Speeding, no seatbelt, and no valid license were associated with impaired driving.•No helmet and no valid license were associated with impaired riding.•Associations were further tested across different substance groups. Fatal road traffic crashes are often related to speeding, non-use of a seatbelt, and alcohol/drug-impaired driving. The aim of this study was to examine associations between driving under the influence of drugs and/or alcohol and driver-related risk factors that have been reported as significantly contributing causes of fatal road traffic crashes. The data were extracted from Norwegian road traffic crash registries and forensic toxicology databases. Drug/alcohol investigated car and van drivers and motorcycle riders fatally injured in road traffic crashes in Norway during 2005–2015 were included in this study (n = 772). Drug and alcohol concentrations corresponding to 0.5 g/kg alcohol in blood were used as the lower limits for categorising drivers/riders as impaired; 0.2 g/kg was the upper limit for being categorised as sober. Associations between driver-related risk factors and impairment from specific substance groups were calculated using multivariable logistic regression, adjusted for other substance groups, age, and sex, and were reported when the confidence intervals did not contain the value 1 or lower. Substances found in concentrations above the impairment limits were mainly alcohol (20%), medicinal drugs (10%: benzodiazepines, opioids, z-hypnotics), stimulants (5%: amphetamines, methylphenidate, and cocaine), and cannabis (4%: THC). The drug/alcohol-impaired drivers had compared to the sober drivers more often been speeding (68% versus 32%), not used a seatbelt (69% versus 30%), and been driving without a valid driver license (26% versus 1%). Logistic regression analysis showed that impairment from alcohol or stimulants (mainly amphetamines) was associated with all three risk factors, medicinal drugs with all except speeding, and impairment from cannabis (THC) with not having a valid driver license. Among motorcycle riders, drug/alcohol impairment was associated with not having a valid driver license and non-use of a helmet. At least one of the risk factors speeding, non-use of a seatbelt/helmet, and driving without a valid license were present among the vast majority of the drug/al