In July 2014, Washington State allowed legal sales of recreational marijuana. Working with the Washington Traffic Safety Commission, NHTSA assisted the State in conducting a roadside study to examine the prevalence of marijuana use before and after sales began. The study also examined drivers’ use of alcohol and other drugs. National roadside studies have been conducted approximately every decade since the 1970s. This study used a similar research design as the national studies but included several enhancements. It included more than 70 over-the-counter, prescription (antidepressants, sleep medications, etc.), and illegal drugs (cocaine, opioids, etc.) that may impair driving-related skills. This project’s objective was to examine whether the percentage of drivers who tested positive for marijuana increased after sales of the drug became legal and what the effect was on alcohol-impaired driving. Data was collected in three waves: (1) immediately before implementation of legal sales, (2) six months after implementation, and (3) one year after implementation. * Across the three waves, 14.6%, 19.4%, and 21.4% of drivers tested positive for marijuana. The increases were not significant. * There was a statistically significant increase in daytime prevalence of marijuana-positive drivers between Wave 1 (7.8%) and Wave 3 (18.9%). * While there was also an increase in drivers positive for marijuana at night across each successive wave (17.5%, 19.8%, and 22.2%), these were not statistically significant. * Marijuana and alcohol were the most common substances for the drivers in the study. * There were no significant changes in alcohol use, either with or without marijuana. Alcohol use varied across the three waves with 6.0%, 3.9%, and 4.4% of participants testing positive for alcohol. This was not a statistically significant change. Use of alcohol and marijuana together was not common, varying from .04 percent to 1.3 percent of drivers across waves. * Although direct comparisons are not possible due to some differences in methodology, marijuana use in Washington was higher than in the 2013-2014 National Roadside Survey (using nighttime numbers, 12.6% of drivers in the national study were positive for THC, versus 17.5% to 22.2% of drivers participating in Washington depending on the wave). * As the presence of marijuana and some other drug types can be detected for days or even weeks after use, it is important to note that this study examined the prevalence of selected drugs and alcohol. We were not able to determine whether people were impaired at the time of participation. We considered a driver positive for marijuana if we found THC (delta- 9-tetrahydrocannabinol), the psychoactive substance in marijuana. ??Washington set 5 nanograms per millilitre (ng/ mL) or higher of delta-9-THC in the blood within two hours of driving as the per se level, at which the driver is guilty of driving while under the influence of THC. The study found a reduction in the percentage of THC-positive drivers who were over the per se limit. In Wave 1, 14.5% of drivers were over the 5 ng/mL limit, while in Wave 2, this decreased to 5.3%, a statistically significant reduction. In Wave 3, it increased to 9.2%. (Author/publisher)
Abstract