Comment Re:Rate of use (Score 3, Insightful) 328
The data showed that 7.6% of crash-involved drivers tested positive for marijuana or THC, versus 6.1% of the control group. In raw terms, that would suggest that marijuana was associated with a 25% increased chance of crashing. But it's not that simple: the figures have to be adjusted for other factors possibly contributing to crash risk, including the driver's gender and age.
As it happens, males and young drivers have higher crash rates than females and older drivers; they're also more likely to be marijuana users. And once these factors are corrected for, "the significant increased risk of crash involvement associated with THC...is not found." The same outcome was determined for other drugs tested for, including sedatives, antidepressants, and stimulants.
Contributing to the doubts about marijuana's effect on auto safety was the inadequacy of the testing. Pot, like other drugs, can continue to show up in test samples for days or weeks after it's used, meaning that some subjects found to have THC in their systems may have been well past the period of impairment.