In the second study, researchers analyzed police-reported crashes in a few legalized marijuana states, (Colorado, Oregon and Washington) and found a 5.2-percent increase in the rate of crashes per million vehicle registrations compared with neighboring states. Also, more drivers are tested for alcohol after crashes. That makes it more hard to determine if the driver was under the influence while driving, said Highway Loss Data Institute President David Harkey.
A woman is detained by a U.S. Customs and Border Protection officer after marijuana was found in her vehicle while entering into the United States from Mexico in San Ysidro, California, on September 23, 2016.
'The new IIHS-HLDI research on marijuana and crashes indicates that legalizing marijuana for all uses is having a negative impact on the safety of our roads, ' IIHS-HLDI President David Harkey said.
A recent report by traffic safety officials in Washington State found a sharp rise in the mixing of drugs and alcohol since the state legalized recreational use of marijuana in 2014.
Colorado law specifies that drivers with five nanograms of active THC in a milliliter of their blood can be prosecuted for driving under the influence of marijuana.
"You shouldn't be behind the wheel regardless of what the substance is", Harkey said.
Researchers know a lot about the direct link between alcohol and auto crashes but are still working on defining the nature and scope of how marijuana contributes to crashes, the report noted.
There are no national standards or standardized tests for weed-impaired drivers like there are for alcohol.
Medical use of marijuana is legal in 30 states, and nine states and the District of Columbia have OK'd recreational use, NBC News noted.
Witnesses told authorities the driver had been driving erratically for more than 15 minutes before the crash.
The driver swerved his pick-up truck into the opposite lane and caused a head-on crash.
The research notes that determining impairment from marijuana is a tricky task and the drug's role in crashes isn't as clear as the link between alcohol and accidents. "We need it on the types of marijuana that people are actually using and we needed it 10 years ago, unfortunately".
While the two findings are similar, the IIHS noted the states have varying laws on marijuana and "these differences can influence how often consumers buy marijuana, where they buy it and where they consume it".