Washington and Colorado, two states that recently legalized the recreational use of marijuana, have set a legal limit for how much THC can be in a driver's blood.
That cutoff is 5 nanograms per milliliter of blood for both states.
In Colorado, the state presumes that a driver with 5 nanograms of THC or higher is impaired, but drivers can rebut the finding in court if they can prove they weren't acting impaired. In Washington, a person over the 5 nanogram cutoff is considered impaired regardless of their actions.
Colorado's limit went into effect in May 2013, before marijuana was legalized for recreational use, said Emily Wilfong, a spokeswoman for the Colorado Department of Transportation.
"It certainly helps in being able to convict. It provides a standard," she said.
Colorado law does not differentiate between alcohol and drug DUIs, so Wilfong could not say how the limit had affected marijuana DUI arrests and convictions.
Washington has tracked DUI arrests for marijuana and found a significant uptick between 2012 and 2013, when the drug became legal and the limit of 5 nanograms was enacted. Of 36,661 DUI arrests in 2012, 2.6 percent involved THC, Washington State Patrol spokesman Bob Calkins said. In 2013, the percent of marijuana-involved arrests grew to 4.1 percent, despite overall DUI arrests declining.
"It's a shockingly large percent increase in a number that wasn't that big to begin with," Calkins said.
Officials from both Washington and Colorado said an officer's observation of a driver's behavior is still the key to proving impairment. Colorado has a goal of increasing its current stable of 212 drug-recognition experts to 300 by 2013, Wilfong said. The state trained almost twice as many such experts in 2013 as it did in 2012.
Colorado has also unveiled an aggressive public education campaign with the slogan, "Drive high, get a DUI."
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