A new study—partly funded by the federal government—has found that alcohol affects a person's ability drive way more than marijuana. 
According to researchers, alcohol "significantly increased lane departures/minimum and maximum lateral acceleration; these measures were not sensitive to cannabis." In addition, they concluded that stoned drivers "may attempt to drive more cautiously to compensate for impairing effects, whereas alcohol-influenced drivers often underestimate their impairment and take more risk."
CNN reported that the study occurred with a test group of 19 adults and participants either drank alcohol, inhaled vaporized marijuana or had a placebo before embarking on a 45 minute driving session inside one of the "most sophisticated driving simulator of its kind to mirror real-life situations." 
"Alcohol, but not marijuana, increased the number of times the car actually left the lane and the speed of the weaving," researchers said. 
They did find that marijuana reduced drivers' peripheral vision and higher levels of THC showed increased weaving from drivers, but the impact was much less dramatic than those under the influence of alcohol. 
The study, which was funded by the National Institute on Drug Abuse, the Office on National Drug Control Policy, and federal safety regulator, the National Highway Traffic Safety Administration, also noted the many challenges of testing drivers for marijuana and determining a threshold of what's too high to drive. 
"The concern is that implementing concentration-based cannabis-driving legislation 'will unfairly target individuals not acutely intoxicated, because residual THC can be detected in blood for up to a month of sustained abstinence in chronic frequent smokers,'" CNN reported. "The study concedes testing marijuana levels that reflect 'driving impairment remain elusive.'"
(Photo Courtesy of Komorn Law)
http://www.hightimes.com/read/federal-study-alcohol-impacts-driving-more-marijuana