A new study finds medical marijuana laws are associated with a reduction in traffic deaths. The most likely reason for the decrease is that some people in states with the laws use marijuana instead of alcohol, the researchers say. They note that alcohol is more deadly than marijuana when combined with driving, according to The Wall Street Journal.

The researchers examined data from the National Highway Traffic Safety Administration before and after passage of medical marijuana laws in 16 states. They found a nearly 9 percent decrease in overall traffic deaths. The decline was caused almost entirely by a decrease in alcohol-related traffic deaths.

In most states with medical marijuana laws, the researchers saw an increase in marijuana consumption in addition to prescription uses among people over 18, but not among minors. Those states also experienced a small drop in alcohol consumption. The researchers conclude that marijuana is being used as a partial replacement for alcohol.

The researchers point out that people who use marijuana may be more likely to do so in private, while alcohol is often consumed in restaurants and bars, which often leads to drinking and driving. They also say people who are drunk tend to be less aware of their intoxication than people who are high on marijuana.

The researchers’ findings appear in a working paper for the Institute for the Study of Labor.