The influence of cannabis on traffic crashes is a growing concern. Experimental studies provide ample evidences of cannabis influence on psychomotor and cognitive performances. Epidemiological works describe the excess crash risk that this substance causes. And yet, this psychotropic drug influence in causing crashes is still at the centre of many discussions. The present analysis consists in exploiting 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 occurring in France were studied: the Cannabis group (n=174) and a Control group (n=174) involving neither cannabis nor alcohol. The results of this analysis notably show that cannabis consumption significantly increases 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.