While drunk driving fatalities are on the decline, a state agency this week warned that the number of deadly crashes involving drugged California motorists has been steadily increasing over at least four years.

Officials with the California Office of Traffic Safety said 30% of drivers killed in car accidents in 2010 tested positive for legal and/or illegal drugs.

"You can be as deadly behind the wheel with marijuana or prescription drugs as you can with over-the-limit alcohol," said Christopher J. Murphy of the Office of Traffic Safety.

The agency said drug-impaired driving often goes unreported or undetected, partly because of costly toxicology tests, according to a news release.

The Office of Traffic Safety has teamed up with the California Highway Patrol to give training on how to better spot drugged drivers and apprehend them. Police departments across the state are also being urged to send officers to get training on the issue and become Drug Recognition Experts.

The training "for law enforcement is the key to successfully removing drug-impaired drivers from the road," said CHP Commissioner Joe Farrow.

Chris Cochran, spokesman for the Office of Traffic Safety, said the agency has not yet determined why there has been such an increase in drugged driving fatalities, but he speculated that increases in legal marijuana use and in prescription drug impairment could be contributing factors.

"We're sort of where we were with drunk driving in 1950," Cochran said, "we're just getting a handle on it."