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However, while the combined impact leads to greater behind-the-wheel impairment, it didn't double the effect.
The findings stem from a new investigation that looked at driver performance along a virtual roadway after drinking alcohol, smoking pot or both.
"What we saw was an additive effect, not a synergistic effect, when we put them together," study author Tim Brown, an associate research scientist who works with the University of Iowa's National Advanced Driving Simulator (NADS), said in a news release. "You get what you expect if you take alcohol and cannabis and merge them together."
Brown and his colleagues reported their findings recently in the journal Drug and Alcohol Dependence. Their work was sponsored by the U.S. National Highway Traffic Safety Administration (NHTSA), the U.S. National Institute of Drug Abuse and the U.S. Office of National Drug Control Policy.
The team noted that their work was prompted by recent statistics that illustrate that while drunk driving has plummeted by as much as a third since 2007, so-called "drugged driving" is on the rise.
For example, they pointed to a 2014 NHTSA survey that found that between 2007 and 2014 the number of drivers found to have pot in their system had grown by roughly 50 percent.
This finding might reflect the fact that marijuana has been made legal in more parts of the country. Medical marijuana is legal in 23 states and Washington D.C., and recreational marijuana is now legal in five states, the researchers said.
"Alcohol is the most common drug present in the system in roadside stops by police; cannabis is the next most common, and cannabis is often paired with alcohol below the legal limits," Brown noted. "So the questions are: 'Is alcohol an issue? Is cannabis an issue?' We know alcohol is an issue, but is cannabis an issue or is cannabis an issue when paired with alcohol? We tried to find out."
The team's simulated driving sessions involving 13 men and five women (between the ages of 21 and 37) that lasted between 35 and 45 minutes each.
In fact, drunk drivers were found to have impaired driving skills on all three principal measures: weaving within a lane, leaving the lane entirely and the speed of weaving, the researchers found.
By contrast, the study found that those solely under the influence of vaporized marijuana displayed impairment only in terms of increased weaving within a lane.
-- Alan Mozes
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