For several months in 2013 and 2014, federal researchers administered free marijuana and alcohol to a group of people and set them loose in a driving simulator -- all in the name of science.
In what may be the most comprehensive study yet of cannabis' effects on drivers, around 20 volunteers, all between the ages of 21 and 55, got high on weed grown at the University of Mississippi, home of the federal government's only sanctioned marijuana farm. On some occasions, the volunteers were also given small amounts of alcohol. Once sufficiently high and/or buzzed, the subjects then performed a series of tests in the National Advanced Driving Simulator at the University of Iowa as researchers looked on.
"It's a very big, complex study and the very first ever of an illicit drug affecting driving in the world's most advanced driving simulator," Marilyn Huestis, chief of chemistry and drug metabolism at the National Institute on Drug Abuse, a research arm of the National Institutes of Health, told The Huffington Post.
Subjects in the simulator sat in a complete car and "drove" as a 360-degree realistic world was projected on the domed walls around them. Researchers at the NADS had previously developed programs to test the distractive effects of texting and cell phone use, and the sedative effects of alcohol, but Huestis and University of Iowa engineers had to specifically design environments for the cannabis driving test, since it was the first of its kind.
"There were six different driving sessions in this huge simulator," said Huestis. "We designed many different situations, including an urban portion with crowds and lights, a highway section and a rural section -- all with a lot of divided attention tasks. There are issues of deer coming out on the roadway, people coming out on the crosswalk, directions telling drivers to turn at certain locations, as well as cars approaching and passing."
Throughout the study, Huestis and her team collected blood and saliva from the drivers to determine their level of intoxication.
Because the University of Iowa is a smoke-free campus, researchers were forced to vaporize the cannabis for the test subjects to inhale. All of the participants were given the same amount of cannabis -- approximately one joint's worth -- and all of the cannabis was of the same mid-level potency. The subjects in the study were all "occasional" marijuana users who had used cannabis at least once a month, and no more than three days a week, during the three months prior to the beginning of the study.
States that have laws regulating the use of marijuana often administer THC-blood tests to determine a driver's level of impairment. Generally, drivers found to have THC in their bloodstream at a level of five nanograms of THC per milliliter of blood are considered "under the influence" and may be subject to penalty.
In Colorado, which permits the use of both medical and recreational marijuana, a driver is presumed to be too impaired to drive if his or her THC-blood level is higher than five nanograms of active THC per milliliter of blood, although the driver can rebut that charge and possibly overturn it in court. In Washington, where medical and adult-use marijuana are also legal, a driver found with a THC-blood level of five nanograms or more is automatically charged with being too impaired to drive.
But unlike blood-alcohol level, which is directly correlated with one's inability to drive, THC-blood levels don't necessarily indicate a clear level of impairment.
In one recent study, six of 25 participants who had consumed marijuana still tested positive for active levels of THC seven days later -- long after they had stopped feeling the effects of the cannabis.
In 2011, Denver marijuana reporter William Breathes memorably demonstrated THC's unpredictable effect on the body. After a night of sleep and not smoking pot for 15 hours, a sober Breathes still tested nearly three times higher than the proposed legal limit.
To add confusion to the matter, in 2013 the Washington state television station KIRO assembled a group of volunteers, had them smoke pot and set them loose on a driving test course to try and answer the question: How high is too high to drive?
Unfortunately, the less-than-scientific results, while entertaining, didn't yield much clarity. One regular smoker of marijuana tested above the legal limit to begin with, yet drove without much of a problem. Two other casual smokers also navigated the course without incident. However, after smoking more marijuana, some of the participants quickly grew less adept behind the wheel.
Huestis said it's important to take into account how often a person consumes marijuana, and that five nanograms per milliliter may actually be too permissive a standard.
"Many occasional users who are not chronic, frequent users can be very intoxicated at one nanogram per milliliter of blood and should not be driving," Huestis said. "Washington state chose five nanograms per milliliter because we showed in our work in chronic, frequent cannabis users -- people who use daily and multiple doses per day -- were less than five nanograms within 24 hours. In Sweden, they have published that if they had used five nanograms per milliliter they would have missed 90 percent of their impaired driving cases. So it's very difficult to say with certainty."
Huestis said that researchers have more than 200 parameters to evaluate from the study and will be examining their findings for more than a year before they announce any conclusions.