Whenever the department determines and orders that it is necessary for the convenience and safety of public travel and the use of (or construction, alteration, repair, improvement, or maintenance of) any state highway to have the full width of right-of-way of any such state highway or of any portion of the right-of-way of any such state highway free from any and all obstructions, encroachments, and occupancy, other than pole lines, pipe lines, or other structures maintained thereon for public or quasi-public utilities by virtue of a valid franchise, and causes due notice of the order to be given as provided by law, the obstructions, encroachments, and means of occupancy, and any structure, building, improvement, or other means of occupancy of any of the right-of-way of the state highway not removed within the time allowed by law shall become an unlawful property and may be confiscated, removed, and sold or destroyed by the state of Washington according to procedure as provided in this chapter, without any right in anyone to make any claim therefor, either by reason of the removal thereof or otherwise. It is unlawful for any person to keep, maintain, or occupy any such unlawful structure.