"It is unlawful for any public officer having charge of any vehicle owned or controlled by any county, city, town, or public body in this state other than the state of Washington and used in public business to operate the same upon the public highways of this state unless and until there shall be displayed upon such automobile or other motor vehicle in letters of contrasting color not less than one and one-quarter inches in height in a conspicuous place on the right and left sides thereof, the name of such county, city, town, or other public body, together with the name of the department or office upon the business of which the said vehicle is used."

