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• The use of drugs by drivers is frequent in Spain, involving 1 in every 13 drivers. • THC was the most frequently detected substance, followed by cocaine. • Almost 80 % of drug-impaired drivers were found positive for one substance only. • Positive drug tests occurred more frequently in young males during daytime hours. • Positive drug drivers profile differs from positive alcohol drivers. The study assesses the prevalence rates of alcohol- and drug-involved driving in Catalonia (Spain). Drivers were randomly selected for roadside testing using a stratified random sampling procedure representative of all vehicles circulating on non-urban roads. Mandatory alcohol and drug tests were performed during autumn 2017. A sample of 6860 drivers were tested for alcohol use, of these 671 were also tested for drugs. Standard procedures were employed by traffic officers to detect alcohol and drug use. Alcohol breath tests were performed with breathalyser devices and on-site drug screening systems were used to test for drugs. The prevalence of alcohol use above the legal limit and drug use were 1.2 % (95 % CI: 0.9–1.5 %) and 8.3 % (95 % CI: 5.8–11.2 %), respectively. The most frequent drugs detected were THC (5.6 %, 95 % CI: 3.7–8.0 %), cocaine (3.5 %, 95 % CI: 2.0–5.5 %) and amphetamines (1.6 %, 95 % CI: 0.6–3.4 %). Alcohol use was detected more frequently on conventional roads, at weekends and during night-time hours. Drug use was detected more frequently in young males during daytime hours. Driver risk profiles associated with alcohol use and drug use differ. Positive alcohol use is not a predictor of drug use when controlling for all other factors. [ABSTRACT FROM AUTHOR]