A study was conducted to gauge the possible role of cannabis use in motor vehicle collisions.38 Among 368 drivers of automobiles involved in a collision, a blood sample was obtained within six hours and tested for the presence of a metabolite of cannabis.

The proportion of the drivers who tested positive was compared to the proportion who claimed to have used cannabis within six hours prior to the time they had previously driven.

About one in five of the study subjects had evidence of the cannabis metabolite in their blood, a far higher proportion than would have been expected based on the proportion who claimed to have used cannabis in the six hours before the earlier driving episode (matched odds ratio = 12.0).