Government researchers are studying the effect of marijuana on drivers, USA Today reports. The findings will help regulators decide on guidelines for drugged driving.

The study is looking at the effect of marijuana alone, as well as the combination of marijuana and alcohol. The data has been collected and is now being analyzed. The study is similar to research that was conducted to determine the legal limit for drivers’ blood alcohol levels.

Study participants did not drive on real roads, the article notes. They used a simulator that mimics real driving conditions. Before using the simulator, participants consumed specific combinations of marijuana and alcohol, or a placebo. They used a vaporizer to consume their marijuana because the study took place at the University of Iowa, which is smoke-free.

The marijuana in the study was provided by a federal growing facility at the University of Mississippi. The study included 19 participants who gave blood and saliva tests so researchers could verify their intoxication levels. The investigators hope to have preliminary data available by October.

“In this country, there’s a huge controversy over whether there should be zero tolerance or there should be some level that’s acceptable. It’s a terribly difficult problem,” said Marilyn Huestis, Chief of Chemistry and Drug Metabolism at the Intramural Research Program at the National Institute on Drug Abuse. “We will be looking at what are the kinds of functions that are affected, and whether they are significantly different … whether alcohol is on board or not.”