Wear a blue tooth headset, or a regular cell phoneear bud with mic.
Have a home telephone recorder that records to tape AND one that you can set to record messages up to 30+ minutes. (you could also set up a computer to do that)
When cops approach you just speed dial your pre-arranged message recorder. Keep your microphone visable, just as if you were 'normally using your cell phone.'
That will enable you to record the entire conversation with the cops AND since you are using a cell phone with a VISABLE microphone it should be legal in WA (no presumption of privacy, since mic is visable).
I'm no lawyer so you should check the legality of this for yourself.