Now playing
02:44
Studies link weed legalization, car crashes
KVVU
Now playing
00:59
Pot yoga: Highly relaxing and then some
Jonathan Reyes/CNN
Now playing
01:39
Your brain on marijuana
CNN
Now playing
01:22
The dangers of synthetic marijuana
An Israeli agricultural engineer inspects marijuana plants at the BOL (Breath Of Life) Pharma greenhouse in the country's second-largest medical cannabis plantation, near Kfar Pines in northern Israel, on March 9, 2016.
The recreational use of cannabis is illegal in the Jewish state, but for the past 10 years its therapeutic use has not only been permitted but also encouraged. Last year, doctors prescribed the herb to about 25,000 patients suffering from cancer, epilepsy, post-traumatic stress and degenerative diseases. The purpose is not to cure them but to alleviate their symptoms. Forbidden to export its cannabis plants, Israel is concentrating instead on marketing its agronomic, medical and technological expertise in the hope of becoming a world hub in the field.
 / AFP / JACK GUEZ        (Photo credit should read JACK GUEZ/AFP/Getty Images)
JACK GUEZ/AFP/Getty Images
An Israeli agricultural engineer inspects marijuana plants at the BOL (Breath Of Life) Pharma greenhouse in the country's second-largest medical cannabis plantation, near Kfar Pines in northern Israel, on March 9, 2016. The recreational use of cannabis is illegal in the Jewish state, but for the past 10 years its therapeutic use has not only been permitted but also encouraged. Last year, doctors prescribed the herb to about 25,000 patients suffering from cancer, epilepsy, post-traumatic stress and degenerative diseases. The purpose is not to cure them but to alleviate their symptoms. Forbidden to export its cannabis plants, Israel is concentrating instead on marketing its agronomic, medical and technological expertise in the hope of becoming a world hub in the field. / AFP / JACK GUEZ (Photo credit should read JACK GUEZ/AFP/Getty Images)
Now playing
02:28
Why weed is stuck in a legal limbo
John Lustig and his wife Anne learn about cannabis lotions
Jen Christensen, CNN
John Lustig and his wife Anne learn about cannabis lotions
Now playing
02:36
Seniors get on board the Cannabus
 GO WITH AFP STORY by Desiree Martin A picture taken on April 12, 2013 shows plants of marijuana at the plantation of the Sibaratas Med Can association in Mogan on the southwest coast of the island of Gran Canaria. The plants grow from cuttings for approximately two months and then blossom before being harvested, dried, stored in jars for a month and later processed to be consumed on site. Spanish law prohibits the possession of soft drugs like cannabis in public and its growth to be sold for profit is illegal. But the law does tolerate growing cannabis for personal use and its consumption in private. Dozens of private marijuana smoking clubs operate across Spain that take advantage of this legal loophole that serve cannabis users who do not want to get their drugs from the streets. AFP PHOTO / DESIREE MARTIN (Photo credit should read DESIREE MARTIN/AFP/Getty Images)
DESIREE MARTIN/AFP/Getty Images
GO WITH AFP STORY by Desiree Martin A picture taken on April 12, 2013 shows plants of marijuana at the plantation of the Sibaratas Med Can association in Mogan on the southwest coast of the island of Gran Canaria. The plants grow from cuttings for approximately two months and then blossom before being harvested, dried, stored in jars for a month and later processed to be consumed on site. Spanish law prohibits the possession of soft drugs like cannabis in public and its growth to be sold for profit is illegal. But the law does tolerate growing cannabis for personal use and its consumption in private. Dozens of private marijuana smoking clubs operate across Spain that take advantage of this legal loophole that serve cannabis users who do not want to get their drugs from the streets. AFP PHOTO / DESIREE MARTIN (Photo credit should read DESIREE MARTIN/AFP/Getty Images)
Now playing
02:11
Will recreational marijuana soon be legal nationwide?
CNN
Now playing
02:35
The weed distributor fighting 'big marijuana'
nfl marijuana use goodell fitzgerald bts_00001009.jpg
NFL Network
nfl marijuana use goodell fitzgerald bts_00001009.jpg
Now playing
00:48
Goodell addresses NFL ban on marijuana use
CNN Illustration/Getty Images
Now playing
02:00
California pushing back on Sessions' pot memo
AG Sessions, Rosenstein, Gore Civil Rights Ceremony
CNN
AG Sessions, Rosenstein, Gore Civil Rights Ceremony
Now playing
01:47
Sessions to rescind Obama-era marijuana policy
Jeff Sessions NAAG remarks medical marijuana_00004720.jpg
CNN
Jeff Sessions NAAG remarks medical marijuana_00004720.jpg
Now playing
01:12
Sessions: Marijuana won't fix opioid epidemic
california weed vs whiskey sidner pkg _00001109.jpg
CNN
california weed vs whiskey sidner pkg _00001109.jpg
Now playing
04:07
Will weed be bigger than whiskey?
CNN
Now playing
02:24
Designer marijuana boost THC potency
marijuana edibles high profits_00000130.jpg
marijuana edibles high profits_00000130.jpg
Now playing
01:09
Is eating marijuana riskier than smoking it?
history medical marijuana orig nws_00003424.jpg
history medical marijuana orig nws_00003424.jpg
Now playing
01:56
The quick hit history of medical marijuana

Story highlights

A fatal accident in Texas is blamed on a driver high on marijuana and a sedative

Accidents overall are up in Colorado, Washington and Oregon, where recreational use is legal, new studies say

(CNN) —  

The National Transportation Safety Board said Tuesday that the probable cause of a crash that killed 12 people on a church bus near Concan, Texas, in March 2017 was that the 20-year-old man whose truck hit the bus was under the influence of marijuana and a sedative.

In its report, the board says that there has been an increase in the number of drug-impaired drivers across the country and that something must be done about it.

That call to action seems to match up with a report released Thursday from the Insurance Institute for Highway Safety’s Highway Loss Data Institute.

Though a 2017 study found that the legalization of recreational weed has not increased the number of accidents involving fatalities, states that have legalized recreational use are seeing more car crashes overall, according to the report, which includes two studies presented Thursday at the Combating Alcohol- and Drug-Impaired Driving summit at the insurance institute’s Vehicle Research Center.

The first study found that crashes are up as much as 6% in Washington, Oregon and Colorado, compared with neighboring states that haven’t legalized recreational use of weed. Researchers estimated the frequency of collision claims per insured vehicle year, controlling for differences in other factors that could contribute to an accident, including age, location, job status and weather, and still saw an increase.

The second study looked at the number of police-reported accidents before and after the legalization of recreational use of weed. The findings were similar: a 5.2% increase in crash rates after legalization than before weed was legal in those states.

Other research has found that in a small sample of drivers who used weed before driving, they had slower thinking and perceptual skills. Drivers under the influence of weed tended to weave more when tested in simulators, studies show, although scientists say more research needs to be done to better understand the correlation between blood or oral fluid concentrations and psychoactive effects of tetrahydrocannabinol or THC, the main psychoactive ingredient in cannabis.

Driving while impaired by any substance, including marijuana, is illegal in all states.

The National Transportation Safety Board said in its report on the fatal accident in Texas – where recreational use is illegal – that police across the country need better training and better tools to detect whether a driver is under the influence of drugs. It also encouraged the National Highway Traffic Safety Administration to come up with a system of best practices and model specifications for oral fluid drug screening devices that police can use when they pull someone over.

There are no national standards or standardized tests for weed-impaired drivers like there are for alcohol. The tests designed to determine whether someone is driving drunk are only moderately successful in determining that someone is impaired from using weed, studies have shown.

Get CNN Health's weekly newsletter

Sign up here to get The Results Are In with Dr. Sanjay Gupta every Tuesday from the CNN Health team.

About 6 in 10 Americans support marijuana legalization, an October poll found, and thus more states will probably legalize recreational use – meaning there will probably be more drivers on the road under the influence.

“The pick-up truck driver in this crash made terrible choices with tragic consequences,” National Transportation Safety Board Chairman Robert L. Sumwalt said in a statement. “But the rising tide of drug-impaired driving did not begin with this driver, and it will not end with him. Law enforcement needs additional tools and advanced training to detect impaired drivers before they crash, regardless of the impairing drug they’re using.”