Drivers who test positive for drugs are three times more likely than those who test negative to be involved in a deadly car accident, a new study finds.

Using drugs and alcohol together dramatically increases the risk of a fatal crash, according to researchers at Columbia University. People who test positive for both alcohol and drugs have a 23-fold increased risk, HealthDay reports.

The study analyzed the results of roadside surveys of drug and alcohol use by drivers. The researchers found almost 32 percent of drivers involved in fatal accidents, and about 14 percent of drivers not involved in such accidents, tested positive for at least one drug. Depressants were most likely to be associated with deadly accidents, followed by stimulants, narcotics and marijuana, the article notes.

About 9 percent of drivers overall, and 57 percent of drivers involved in fatal crashes, had elevated blood alcohol levels. Twenty percent of drivers involved in deadly accidents tested positive for alcohol and one or more drugs, compared with 2 percent of drivers overall.

The findings are published in Accident Analysis and Prevention.

“While alcohol-impaired driving remains the greatest threat to traffic safety, these findings about drugged driving are particularly salient in light of the increases in the availability of prescription stimulants and opioids over the past decade,” lead researcher Dr. Guohua Li said in a news release.