Daytime drivers in Washington State more likely to drive under the influence of marijuana after the drug was legalized for recreational use, according to an analysis recently published by the Insurance Institute for Highway Safety.

The roadside survey found that drivers in the state were more likely to test positive for marijuana during the daytime. There was no noticeable effect on the proportion of nighttime drivers who tested positive for marijuana.

IIHS says the legalization may have also loosened drivers' attitudes toward the drug. Drivers who participated in the roadside survey were more likely to admit that they had recently used marijuana, and also less likely to say that they thought the drug impaired their ability to drive a vehicle.

Researchers are currently working to pin down how marijuana use affects crash rates, since some studies suggest it has little effect and others say it can more than double the risk of a crash. Last year, the Highway Data Loss Institute, an organization associated with IIHS, found that collision claim rates in Colorado, Washington, and Oregon were 3 percent higher than they would have been if the states had not legalized marijuana. However, claim frequencies varied considerably from state to state, and there was no significant change in fatal crash rates.

IIHS says studies of both simulator driving and on-road travel have concluded that marijuana use can inhibit driving performance in some areas, such as slowing reaction time and diminishing one's ability to make decisions. In a July report to Congress, the National Highway Traffic Safety Administration said tetrahydrocannabinol—the primary psychoactive ingredient in marijuana—has a varying effect on marijuana users and can be detected in the blood for several hours or even days after use, making it more difficult to determine whether a driver is impaired.

The most recent report by IIHS conducted roadside surveys in Washington in a collaboration with the NHTSA, Pacific Institute for Research and Evaluation, and Washington Traffic Safety Commission. Researchers surveyed drivers in June 2014—a  month before retail sales of marijuana began in the state—as well as November and December 2014 and June 2015.

The surveys, which took place on Friday and Saturday nights as well as during the day on Fridays, questioned a total of 2,355 drivers. Nearly all of these drivers—99 percent—consented to take a blood or saliva test for THC

Prior to the legalization of recreational marijuana use in Washington, 8 percent of the drivers surveyed in the day tested positive for THC. This share increased to 23 percent six months after legalization.

There was no significant change in the share of nighttime drivers who tested positive for marijuana. Both before and after the legalization of recreational marijuana use, about 20 percent of drivers in the surveys tested positive for THC in their blood or saliva.

"This is very different from what we see with alcohol," said Angela Eichelberger, senior research scientist at IIHS. "Drinking and driving is much more prevalent at night than during the day."

Legalization seemed to make drivers more willing to admit that they had used marijuana. Only about one in three drivers who tested positive for THC in the first two surveys admitted to using marijuana within the past year, compared with 72 percent who did so in the last survey.

Drivers were also more likely to believe that marijuana had little effect on their driving ability after the drug was legalized for recreational use. Forty-five percent of those who tested positive for THC before retail sales began said they thought marijuana impaired their driving ability, but this share shrank to 17 percent in the last survey, which took place one year after legalization.

At the same time, drivers who did not test positive for THC use had a slightly more negative view of marijuana's effect on driving ability. Fifty-two percent of drivers said they thought marijuana inhibited driving ability in the survey that took place before retail sales began, but this share increased to 56 percent one year into legalization.