Officials in Colorado and Washington are concerned about an increase in car crashes related to marijuana, now that recreational use of the drug by adults has been approved in both states.

Colorado’s new marijuana law does not make any changes to the state’s driving-under-the-influence laws, according to the Associated Press. Washington’s law sets a new blood-test limit for marijuana. Police are training to enforce the new limit, and some lawyers are preparing to challenge it, the article notes.

“We’ve had decades of studies and experience with alcohol,” Washington State Patrol spokesman Dan Coon told the AP. “Marijuana is new, so it’s going to take some time to figure out how the courts and prosecutors are going to handle it. But the key is impairment: We will arrest drivers who drive impaired, whether it be drugs or alcohol.”

Betty Aldworth, Outreach Director for Colorado’s Campaign to Regulate Marijuana Like Alcohol, which advocated for the state’s new law, said there is disagreement about how much THC—the active chemical in marijuana—causes impairment while driving.

An analysis of studies published earlier this year concluded driving under the influence of marijuana is associated with an increased risk of a motor vehicle crash, especially for fatal collisions. The analysis found driving under the influence of marijuana was associated with almost twice the risk of a motor vehicle crash compared with unimpaired driving. The studies in the analysis included nearly 50,000 people.

A 2009 report from the National Highway Traffic Safety Administration based on blood, breath and saliva tests collected on weekends from drivers in 300 locations nationally, found that 16.3 percent of drivers at night were impaired from legal or illegal drugs, including 9 percent of drivers who had detectable traces of marijuana in their system.