The National Highway Traffic Safety Administration found that when looking at California drivers killed in motor-vehicle accidents in 2010, 30 percent tested positive for drugs.
A new Office of Traffic Safety survey of nine cities during Friday and Saturday nights recently concluded more Californians are driving under the influence of drugs than alcohol: more than double, at 14 percent.
"Nearly half had marijuana. That was the drug. And sometimes it was marijuana plus other drugs, or marijuana plus alcohol," said Chris Cochran, Office of Traffic Safety.
Besides illegal drugs, the study also found people had prescription and over-the-counter drugs, which can impair driving.
The sleep aid Ambien has been linked to some high-profile accidents. Low levels of the drug were found in the bloodstream of former U.S. Commerce Secretary John Bryson after a string of collisions in Southern California this summer.
A few weeks later, Kerry Kennedy, daughter of Robert F. Kennedy, crashed north of New York City with the generic for Ambien in her blood.
Neither of those two cases involved alcohol.
Mothers Against Drunk Driving recently added drugged driving to its campaign because of its rise.
"Driving under the influence no longer means just alcohol," said Silas Miers, a program manager with Mother Against Drunk Driving California. "It includes alcohol and drugs and a combination of both. The result and the consequences are really the same. People get arrested or they end up dead."
Those advocating the recreational use of marijuana are not deterred by the report.
The Drug Policy Alliance opposes driving while impaired by pot, but says California's numbers shouldn't prevent efforts to loosen up laws.
"No one should drive under the impairment of anything," said Stephen Gutwillig, deputy executive director of programs, Drug Policy Alliance. "I'm just saying that the risks associated with marijuana and driving are substantially lower than the risks associated with alcohol and with some prescription drugs."
The state will have more concrete numbers in a few years because beginning in 2014, law enforcement will have to start indicating on DUI reports whether the case is related to alcohol, drugs or both.
The Office of Traffic Safety surveyed nine cities, including Anaheim, Chula Vista, Eureka, Fresno, Gardena, Modesto, Ontario, Redding and San Rafael, during Friday and Saturday nights.