The study included more than 3,000 drivers who were involved in crashes during a 20-month period in Virginia Beach, Virginia, plus 6,000 controls who drove in the same area during the same period but did not get into accidents. As usual, the study found that alcohol use was strongly correlated with crash risk. After adjustment for confounding, the crash risk for drivers with a blood-alcohol content (BAC) of 0.08 percent was twice the crash risk for sober drivers; it was six times as high for drivers with a BAC of 0.10 percent and 12 times as high at a BAC of 0.15 percent. But the picture for marijuana was quite different.
Over all, drivers who tested positive for active THC were 25 percent more likely to be involved in crashes. But once the researchers took sex, age, and race/ethnicity into account, the risk ratio shrank from 1.25 to 1.05 and was no longer statistically significant:
"This analysis shows that the significant increased risk of crash involvement associated with THC and illegal drugs...is not found after adjusting for these demographic variables. This finding suggests that these demographic variables may have co-varied with drug use and accounted for most of the increased crash risk. For example, if the THC-positive drivers were predominantly young males, their apparent crash risk may have been related to age and gender rather than use of THC."
