The percentage of traffic deaths in which at least one driver tested positive for drugs has nearly doubled over a decade, raising alarms as five states are set to vote on legalization of marijuana, reports USA Today.

Amid a disquieting increase in overall U.S. traffic fatalities, the National Highway Traffic Safety Administration has tracked an upswing in the percentage of drivers testing positive for illegal drugs and prescription medications, according to federal data released to USA TODAY and interviews with leaders in the field.

The increase corresponds with a movement to legalize marijuana, troubling experts who readily acknowledge that the effects of pot use on drivers remain poorly understood. Recreational marijuana use is now legal in Colorado, Washington state, Oregon, Alaska, and the District of Columbia, even as it remains outlawed on a federal level. Five states — Arizona, California, Maine, Massachusetts, and Nevada — are set to vote on legalization.

It's "very probable" that Colorado's move to legalize recreational marijuana has caused an increase in fatal crashes, said Glenn Davis, the state's highway safety manager.

In 2015, 21% of the 31,166 fatal crashes in the U.S. involved at least one driver who tested positive for drugs after the incident — up from 12% in 2005, according to NHTSA. The rate rose in 14 of the last 15 years, falling for the first time last year. It was down less than one percentage point compared with 2014.