THC—the intoxicating component of marijuana—impairs driving. Does CBD (the non-intoxicating component) counteract THC’s psychoactive effects, as some claim? Scientists had 26 young adults vape (inhale) THC, CBD, THC + CBD, or a placebo on different days. Each dose of THC or CBD was 13.75 milligrams. After 40 to 100 minutes, and again after four to five hours, the volunteers drove for an hour at 60 mph. (Relax. The car had a second set of pedals for the research team.)