I am from Washington also. There are specific rules in the law for DUI of pot. It's very very small. Something like .01 micrograms or something. Essentially if you smoke and drive you are DUI as I understand the law. It's akin or more stringent than alcohol. What we don't yet understand is the limit at which you are impaired under pot as no significant studies have yet to be done, so the DUI limit will be tested in court.
Now the rub.... It's a blood test and can't be done on site. But if they are DUI on pot they are clearly liable.
We shall see how this all works out in the state and for me the jury is still out but its the law for now. So I will be a more vigilant driver going forward.
Ok all done.