About one in seven drivers on California roadways may be under the influence of drugs, according to a new survey by the state’s Office of Traffic Safety.

The survey released Monday tested more than a thousand drivers on weekend nights in nine California cities.

Roughly 14 percent of those drivers tested positive for drugs that might impair driving. Half as many of the drivers surveyed by the OTS tested positive for alcohol.

Compared to national statistics, the number of drug-impaired drivers has increased throughout the years. It reinforces officials’ belief that driving under the influence of drugs – in addition to alcohol – is a serious and growing problem, said Christopher J. Murphy with the OTS.

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The results highlight the need for more officers who are trained to detect drug-impaired driving. Without blood tests, it’s harder for officers to prove in court that a driver was under the influence of drugs.

“But these folks that have been through the drug enforcement expert training -- if one of them can evaluate a driver accused of being under the influence of drugs, that testimony will normally hold up in court,” said OTS spokesperson Chris Cochran.

The office also plans to increase the numbers of District Attorneys dedicated to drug-impaired driving cases, and purchase better lab equipment.

Of the drugs found in those tested, marijuana was the most prevalent. More than 7 percent of drivers tested positive for the substance.

The survey also found a significant number of drivers under the influence of both drugs and alcohol. About 23 percent of the drivers who were found to have alcohol in their systems also tested positive for some kind of drug, be it prescription or otherwise.

About 1,300 drivers volunteered to provide breath or saliva samples set up at nine different locations in the state. Those who were judged to be too impaired were advised that they should have someone else drive them home, Cochran said.