RCW 36.75.080: Highways used ten years are county roads.

All public highways in this state, outside incorporated cities and towns and not designated as state highways which have been used as public highways for a period of not less than ten years are county roads: PROVIDED, That no duty to maintain such public highway nor any liability for any injury or damage for failure to maintain such public highway or any road signs thereon shall attach to the county until the same shall have been adopted as a part of the county road system by resolution of the county commissioners.