A new law that sets legal limits on marijuana levels in the bloodstream took effect in Colorado on May 28. Under the new law, drivers are assumed to be impaired if their blood test shows a level of THC—the active ingredient in marijuana—of 5 or more nanograms per milliliter.
Josh Lewis, a spokesman for the Colorado State Patrol, said the new law will not significantly change how police handle stops for driving under the influence. More officers will be trained in drug recognition, he said.
There is no broadly accepted consensus on how much marijuana a person must consume before they are impaired, The New York Times reports. Some critics of the law say the 5-nanogram threshold is too low because medical marijuana users always have some THC in their blood. Others argue the blood test used to measure THC levels is too intrusive. Washington state has also imposed a 5-nanogram limit for THC for drivers.
The National Highway Traffic Safety Administration is studying the effects of marijuana on drivers, the article notes. Results are expected in late 2014.