New research released on June 23, 2015 by the National Institute of Drug Abuse (NIDA), proves smoking low levels of marijuana, 2.9% vs 6.7% THC, causes impaired driving similar to those with 0.08 breath alcohol.
The research was done at the University of Iowa on a sophisticated driving simulator that measured weaving between the lanes.
Additionally, drivers drinking alcohol and smoking marijuana exacerbated their impairment. Those using both substances weaved within lanes even if their blood THC and alcohol concentrations were below the impairment thresholds for each substance alone.
It was found that low amounts of alcohol significantly increased peak THC concentrations.
THC concentrations drop rapidly during the time required to collect a blood specimen in the U.S., generally within two to four hours. However, oral fluid THC showed a two to five fold greater variability than blood tests. This indicates that while oral fluid may be an effective screening tool for detecting recent marijuana use by a driver, it may not be a precise measure of the level of impairment.