A Timeline of Wash. State's Changing Drugged Driving Laws
Before December, 2012:
It's a crime to drive under the influence of marijuana if the intoxicant impairs your driving "to an appreciable degree." Prosecutors can use blood analysis, along with other evidence, to prove a person is impaired. But even though marijuana itself is illegal, there's no agreement on what level of THC in the blood constitutes a crime.
As of December, 2012:
With the passage of I 502, Washington establishes a "per se" limit for THC in the blood, meaning anyone with more than 5 nanograms of active THC per milliliter of blood is automatically considered guilty of DUI. Drivers are assumed to have given permission for police to have blood drawn, under the state's implied consent law. If you refuse, that refusal can be used against you in court.
The U.S. Supreme Court decides, in Missouri v. McNeely, that police must get a search warrant in most cases in order to take blood evidence without explicit consent.
The Washington legislature amends the implied consent law to exclude blood. Law enforcement begins taking blood samples only with a warrant from a judge.
The Washington Traffic Safety Commission posted this informational video dramatizing the process of getting a warrant for a blood draw, emphasizing the time it takes to do so. For most people, the body metabolizes THC relatively quickly, meaning any delay in obtaining the sample could greatly reduce the concentration of the drug found.