The influence of cannabis on traffic crashes is a growing concern. Experimental studies provide ample evidence of cannabis influence on psychomotor and cognitive performances. Epidemiological works describe the excess crash risk that this substance causes. Yet this psychotropic drug's influence in causing crashes is still at the center of many discussions. The present analysis exploits crash data in detail to obtain a more precise understanding of the failures that drivers are subject to when they have consumed cannabis, depending on the level of tetrahydrocannabinol (THC) measured in blood samples. Two groups representative of fatal crashes in France were studied: a cannabis group (n = 174) and a control group (n = 174) involving use of neither cannabis nor alcohol. Results of this analysis notably showed that cannabis consumption significantly increased the rate of generalized failures by the driver, reflecting an alteration of all sensorial, motor, and cognitive functions, specifically for high blood levels of THC (>5 ng/mL). At lower levels of intoxication, cannabis leads to a poor diagnosis of driving difficulties.
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