Frequently Asked Questions

How was the sample of young adults collected because I did not receive the survey?

The sample was collected statewide through a direct mail invitation and various social media sites, and includes a new sample/cohort at each year as well as annual follow-ups within cohorts. While it is not a random sample (and is instead a sample of convenience), data were weighted to match Washington Census data and improve generalizability. If you would like more information about the survey visit the Young Adult Health Survey site:

Why should I wait 6-8 hours before driving?

Driving under the influence of alcohol is easy to test and measure because of breathalyzers – unfortunately these are not available yet for marijuana.  While a marijuana DUI can be verified through a blood test, that is less helpful for people wondering what that means about their ability to drive if they’ve recently used marijuana.  The importance of a three hour window is this — one published study (Grotenhermen, et al., 2007) shows that it takes 3 hours to drop below 4.9 nanograms of THC per milliliter of blood, meaning any driving within 3 hours of use would necessarily result in a person being above this cutoff.  That study was with marijuana from 2007, and, as potency has increased nation-wide and in Washington, it could take even longer. More recent studies suggest impairment can last 6-8 hours, especially for potent weed, edibles, and medical marijuana. Despite some people’s perception, there is no evidence in any studies that people “drive better” after marijuana use.

What do studies really say about driving and marijuana?

This is tricky, since much research on marijuana and driving is, out of necessity, not truly “in real life.” Many of these contradictory studies tested Marijuana in low potencies nowhere near what we see today.

What we do know is that marijuana impairs reaction time and the ability to shift focus when there are multiple/competing stimuli surrounding the person (which, from a driving standpoint, would include stoplights, other cars, people, or other unanticipated issues).  

We also know that marijuana when combined with alcohol has an even greater impairing effect on driving.

The authors of I-502 included a per se limit for driving under the influence (DUI) with marijuana.  A per se limit means that independent of how people feel or think they’re doing, a value about the cutoff is considered in violation of the law (for example, for people over 21, we have a per se limit of DUI for alcohol of .08%).  The DUI limit for marijuana was set at 5 nanograms of THC per milliliter of blood for people over 21 years of age, and any positive amount for people under 21.  Why 5 nanograms of THC per milliliter of blood?  This cutoff was selected because research indicated this was the point at which impairment occurs.

Do people tell the truth on these surveys?

While there are always potential threats to the accuracy of self-report data, there are a number of things that can be done to boost the meaningfulness of data that are collected.  This includes keeping surveys confidential, allowing people to skip items they’re uncomfortable answering, assuring that there is no penalty/punishment related to any answers, using reliable/valid measures, and checking for inconsistencies in how people answered throughout the survey.  

Fortunately, studies that look at this very issue (i.e., whether people lie on surveys or not), show that the overwhelming majority of people are very honest and accurate.

Is 786 a valid sample size?

It’s always good to be critical of survey stats, especially surrounding sample size and collection method. This survey used multiple methods of recruiting to ensure the sample reflected the population of King County. The Washington Young Adult Health Survey also used well-established and validated questions from other national surveys and the King County results are consistent with statewide findings that included 2,101 participants so we can be pretty sure this is a good representative sample of young adults in King County.

Where'd you get these stats from?

These findings from 18-25 year olds in Washington were from the 2018 administration of the Washington Young Adult Health Survey and reflect the 786 participants from that year’s survey who reside in King County.