More evidence is emerging that crash rates go up when states legalize recreational use and retail sales of marijuana. This should be no surprise.

A recent webinar that I attended covered the most recent Insurance Institute for Highway Safety and Highway Loss Data Institute research showing that crash rates go up in states that legalize recreational use and retail sales of marijuana. The research also indicates that marijuana is adding to the substantial ongoing problem of alcohol impairment due to the combined use of both substances.

However, the preliminary results of a separate IIHS study of injured drivers who visited emergency rooms in California, Colorado and Oregon showed that drivers who used marijuana alone were no more likely to be involved in crashes than drivers who hadn’t used the drug. That is consistent with a 2015 study by the National Highway Traffic Safety Administration that found that a positive test for marijuana was not associated with increased risk of being involved in a police-reported crash.

More than a third of U.S. states have legalized recreational marijuana for adults 21 and older. The hefty tax revenues those states are earning have others exploring similar legislation, and recent polls indicate that 68% of American adults favor legalization, according to the IIHS.

That’s a potential concern for those who care about road safety. Driving simulator tests have shown that drivers who are high on marijuana react more slowly, find it harder to pay attention, have more difficulty maintaining their car’s position in the lane and make more errors when something goes wrong than they do when they’re sober. But such tests have also shown marijuana-impaired drivers are likely to drive at slower speeds, make fewer attempts to overtake and keep more distance from the car ahead of them.

The most recent of these studies from IIHS shows that injury and fatal crash rates in California, Colorado, Nevada, Oregon and Washington jumped in the months following the relaxation of marijuana laws in each state. Combined, the impact of legalization and, subsequently, retail sales in the five states resulted in a 6% increase in injury crash rates and a 4% increase in fatal crash rates compared with other Western states where recreational marijuana use was illegal during the study period. Only the increase in injury crash rates was statistically significant.

Despite those increases in crash rates, studies of whether marijuana itself makes drivers more likely to crash have been inconsistent. The latest one from IIHS — which used data collected from injured drivers in three emergency rooms in Denver, Colorado; Portland, Oregon; and Sacramento, California — showed no increased crash risk associated with the drug, except when combined with alcohol. Those combined-use numbers could help explain why crash rates have increased. Legalization may be encouraging more people to drink and use marijuana together.

Studies comparing the simultaneous use of alcohol and marijuana in states where marijuana is legal with states where it is still against the law will be needed to test this hypothesis. Nevertheless, traffic safety people, enforcement, and the public in general will need to be aware of the effects that marijuana may have on the ability to drive a vehicle safely.