The King County Sheriff's Office sent out an advisory today warning that deputies will be conducting "impaired driver emphasis patrol" in southeast King County tomorrow, 4/20.
I don't usually like quoting press releases, but:
"In honor of '420 day' tomorrow (Friday), King County Sheriff’s Office Precinct 3 (SE King County) will be conducting an impaired driver emphasis patrol with the hopes of minimizing violators on the 'marijuana holiday' that many people celebrate on 4/20 each year."
The Sheriff's Office says that impaired driving because of alcohol has decreased, and impaired driving because of drugs (it's unclear how much impaired driving can be attributed to being high on weed alone) has increased.
Unlike driving drunk, it's notoriously difficult to physiologically test whether a person is indeed driving while high. Because THC is fat soluble, blood tests can show THC in the system long after a person last smoked. Alternately, someone who was high for the first time might not register any THC concentration in the blood at all. Thus, the limit makes little sense. Still, it's illegal in Washington State for people over 21 to be driving with more than five nanograms of THC per milliliter of blood, and for anyone under 21 to be driving with any concentration of THC.
Precinct 3 encompasses Covington, Maple Valley, and the Muckleshoot Tribal Nation reservation.