A new study by the National Highway Traffic Safety Administration examines the crash risk associated with alcohol and drug use by drivers. The study was conducted in Virginia Beach, Virginia, over a 20-month period ending in 2012. More than 3,000 crash-involved drivers were included as case subjects. The study used a case-control methodology.
The study found that alcohol use by drivers was clearly associated with elevated risk of crash involvement:
Drivers who had been drinking at the .08 breath alcohol concentration (BrAC) had about 4 times the risk of crashing as sober drivers.
Drivers with alcohol levels at .15 BrAC had 12 times the risk.
Drivers participating in the study were tested for a large number of potentially impairing drugs using both oral fluid (saliva) and blood samples. Marijuana (THC) was the only single category of drug for which study findings reached statistical significance.
Drivers testing positive for THC were overrepresented in the crash-involved (case) population. However, when demographic factors (age and gender) and alcohol use were controlled, the study did not find an increase in population-based crash risk associated with THC use.