Drivers test how a legal marijuana high effects their driving A look at the driving experiment which involved volunteers who smoked small amounts of marijuana before getting behind the wheel. DOES having a joint effect your driving? An experiment captured on video in the US has tested the driving competence of people who smoked a small amount of marijuana. After some US states legalised the recreational use of marijuana, law enforcement officials in America had to set specific limits on what counts as driving under the influence for marijuana smokers. Though several states had legalised medical marijuana in the past, none had established an acceptable THC blood content level for drivers. Basically, any motorist found driving with THC in their system was guilty of driving under the influence - even if that THC came from legally sanctioned medicinal marijuana. Washington state has now set an official threshold of 5 nanograms of THC per millilitre of blood. Though the bill's authors insist that level is based on significant scientific evidence, some claim that it's completely arbitrary. How does 5 nanograms of THC affect drivers? News network CNN wanted to find out, so it located an open test course in Washington state and three drivers willing to devote their lungs to an afternoon of scientific research. How did they go? You can judge for yourself by watching the video clip at Cars Guide.