Californians are driving under the influence of drugs far more frequently than under the influence of alcohol, a state traffic-safety survey found.

A survey of drivers by the California Office of Traffic Safety found that 14% tested positive for impairing drugs (both illegal and prescription), while 7.3% tested positive for alcohol. Of the drugs, marijuana was the most prevalent at 7.4% of drivers.

"These results reinforce our belief that driving after consuming potentially impairing drugs is a serious and growing problem," said Christopher J. Murphy, Director of the Office of Traffic Safety.

Perhaps more troubling, the survey found that 23% of those who tested positive for alcohol also tested positive for at least one other drug. This combination can increase the effects of both substances. Also, 26.5% of drivers who tested positive for marijuana also tested positive for at least one other drug.

The impaired driver survey included more than 1,300 drivers who voluntarily agreed to provide breath and/or saliva samples at roadside locations set up in nine California cities.

The samples were collected between 10 p.m. and 3 a.m. on Friday and Saturday nights, the peak times of impaired driving. Breath samples were examined for alcohol, while saliva samples were tested for THC (the active ingredient in marijuana), major illegal drugs, plus prescription and over-the-counter medications that may adversely affect driving, according to a release.

The prevalence of "drugged" driving in motor vehicle fatalities has increased in California since 2006, the National Highway Traffic Safety Administration has reported. In 2010, 30% of those killed in crashes tested positive for legal or illegal drugs.

"Drugged driving poses a serious threat to public safety,” said Gil Kerlikowske, director of National Drug Control Policy and a former Seattle Police chief.