Roadside survey finds changes in pot use, attitudes after legalization

March 29, 2018

Drivers surveyed during the daytime in Washington were more likely to test positive for marijuana after the state legalized recreational sales of the drug than before, an IIHS analysis has found. The proportion of drivers surveyed at night who tested positive did not change.

In what could be a sign of changing attitudes after legalization, drivers who tested positive for marijuana a year after legalization were more likely to admit to researchers that they had used the drug recently than drivers who tested positive when sales were still illegal in the state. They were also less likely to say marijuana impairs driving.

With more states making marijuana legal, researchers are trying to get a handle on the drug's effect on crashes. Simulator and on-road studies have shown that marijuana can degrade some aspects of driving performance, but pinning down the relationship between marijuana use and real-world crashes has been more difficult.

An analysis by HLDI last year showed that states that have made marijuana sales legal have seen an increase in crashes relative to nearby control states. The study looked at collision claims in Washington and two other states that allow recreational marijuana sales — Colorado and Oregon — and found that legalization was associated with a 3 percent increase in collision claims rates (see "Legalizing recreational marijuana is linked to increased crashes," June 22, 2017).

In this new analysis of Washington roadside data, IIHS Senior Research Scientist Angela Eichelberger sought to learn how marijuana use and people's perception of its risks changed following legalization of recreational marijuana sales.

The Washington roadside survey was a collaboration of the National Highway Traffic Safety Administration, Pacific Institute for Research and Evaluation, Washington Traffic Safety Commission and IIHS. Researchers surveyed drivers three times — in June 2014, the month before retail marijuana sales began, in November-December 2014 and in June 2015. Information was collected on Fridays during the day and at night and on Saturday nights.

A total of 2,355 drivers completed a questionnaire about their past and current marijuana use and perceived risks of driving after using marijuana. Of those participants, 99 percent gave saliva or blood to test for THC, the primary psychoactive chemical found in marijuana and hydroxy-THC, a psychoactive metabolite. The presence of either generally indicates recent use of marijuana but doesn't necessarily indicate impairment because the chemicals can be detected in the body for hours or, in the case of some frequent users, days.

Among drivers surveyed in the daytime, the proportion testing positive for THC increased from 8 percent before retail sales began to 23 percent six months after. Among those surveyed at night, the proportion stayed constant at about one-fifth.

"This is very different from what we see with alcohol. Drinking and driving is much more prevalent at night than during the day," Eichelberger says.

THC-positive drivers were more willing to admit to marijuana use in the final survey wave. Seventy-two percent reported past-year marijuana use a year after retail sales began, while only about one-third did during the first two waves.

"Legalization may have made using marijuana more socially acceptable, so people more readily admit to it," Eichelberger says.

THC-positive drivers' perceptions about the effect of marijuana on driving also changed. Before sales began, 45 percent of those who tested positive said it impairs driving. A year into legalization, only 17 percent did. The percentage among THC-negative drivers went from 52 to 56 percent.

Media contact

Email Joe Young
Director of Media Relations
office +1 434 985 9244
mobile +1 504 641 0491

For more information, visit our press room.

Get news directly to your inbox