In just over a decade, the percentage of traffic deaths in which at least one driver tested positive for drugs has nearly doubled.
This has raised alarms as five states are set to vote on legalization of marijuana.
According to data released to USA Today, the National Highway Traffic Safety Administration has been tracking an increase in the percentage of drivers testing positive for illegal drugs and prescription medications.
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 is outlawed on the federal level yet it is legal in Colorado, Washington state, Oregon, Alaska and the District of Columbia. Five states including Arizona, California, Maine, Massachusetts and Nevada, are set to vote on legalization.
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.
Drugs are emerging as a more significant factor as a cause of crashes.
Auto-safety experts are particularly concerned about a spike in drugged driving in states that have legalized recreational marijuana, such as Colorado, where voters approved it in 2012. The nation's opioid epidemic could also be a contributing factor.