The legalization of marijuana and retail sales in five states combined resulted in a 6% increase in injury crash rates and a 4% increase in fatal crash rates as compared with other Western states where recreational marijuana use was illegal, according to a recent study from the Insurance Institute for Highway Safety. 

Specifically, IIHS found that injury and fatal collision rates in California, Colorado, Nevada, Oregon and Washington jumped in the months following the relaxation of marijuana laws in each state.

That’s consistent with a 2018 IIHS study of police-reported crashes — most of which did not involve injuries or fatalities — that found that legalization of retail sales in Colorado, Oregon and Washington was associated with a 5% higher crash rate compared with the neighboring control states.

While at first glance marijuana users appear to be the problem in the rise of collisions, an additional study from IIHS paints a broader picture and indicates the issue runs deeper. 

The preliminary results of a separate IIHS study of injured drivers who visited emergency rooms in California, Colorado and Oregon showed drivers who used marijuana alone were no more likely to be involved in crashes than drivers who hadn’t used the drug. Instead, increased crash risk was associated with drivers who used the drug combined with alcohol.

Dual users of marijuana and alcohol could help explain why crash rates have increased, notes IIHS. Legalization may be encouraging more people to drink and use marijuana together and that, in turn, is leading to higher crash rates.