The IIHS released their findings Thursday.
A new report from the Insurance Institute for Highway Safety says auto crashes are up in states that have legalized recreational marijuana use, including here in Nevada. Missouri and Utah could expand medical marijuana, also in November. That makes it more hard to determine if the driver was under the influence while driving, said Highway Loss Data Institute President David Harkey.
Colorado and Washington were the first states to legalize recreational marijuana in 2012, with retail sales beginning in 2014.
"The bottom line of all of this is that we're seeing a consistently higher crash risk in those states that have legalized marijuana for recreational purposes", Harkey said.
Researchers say states considering legalization should consider the potential impact on highway safety.
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.More news: Left's claim O'Rourke beat Cruz in debate a fallacy?
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.
"We know a lot of states are considering making recreational marijuana available", Harkey said.
It points to a lack of driver drug-use stats in crash reports, in addition to the difficulty of actually testing to determine whether or not a driver was under the influence of cannabis in a collision.
Witnesses told authorities the driver had been driving erratically for more than 15 minutes before the crash.
The studies do, however, mention that the role of cannabis in these accidents isn't clear as drivers who test positive for drugs are often found with alcohol in their system as well.
Law enforcement says regardless if a person has consumed alcohol or drugs, they're all risky and can affect a person's ability to drive safely. "But the rising tide of drug-impaired driving did not begin with this driver, and it will not end with him".
Aside from the rise in collisions after approving the use of recreational marijuana, there are concerns about when marijuana is used.