Washington Revised Code Title 46. Motor Vehicles § 46.61.655. Dropping load, other materials--Covering

(1) No vehicle shall be driven or moved on any public highway unless such vehicle is so constructed or loaded as to prevent any of its load from dropping, sifting, leaking, or otherwise escaping therefrom, except that sand may be dropped for the purpose of securing traction.

(2) No person may operate on any public highway any vehicle with any load unless the load and such covering as required thereon by subsection (3) of this section is securely fastened to prevent the covering or load from becoming loose, detached, or in any manner a hazard to other users of the highway.

(3) Any vehicle operating on a paved public highway with a load of dirt, sand, or gravel susceptible to being dropped, spilled, leaked, or otherwise escaping therefrom shall be covered so as to prevent spillage. Covering of such loads is not required if six inches of freeboard is maintained within the bed.

(5) The state patrol may make necessary rules to carry into effect the provisions of this section, applying such provisions to specific conditions and loads and prescribing means, methods, and practices to effectuate such provisions.

(6) Nothing in this section may be construed to prohibit a public maintenance vehicle from dropping sand on a highway to enhance traction, or sprinkling water or other substances to clean or maintain a highway.