Bigmuddy your post had me search the Washington state RCW's and this is what I found for here -
RCW 46.37.380
Horns, warning devices, and theft alarms
"Every motor vehicle when operated upon a highway shall be equipped with a horn in good working order and capable of emitting sound audible under normal conditions from a distance of not less than two hundred feet, but no horn or other warning device may emit an unreasonably loud or harsh sound or a whistle"
Pretty vague if you ask me. Seems like one of those things that would depend on the Police Officers mood!