All movable bridges shall be provided with warning lights, signs, protection gates and other devices as shall be prescribed by the state highway commissioner and as may be required by the board of supervisors in its permit for construction, which said lights, signs, protection gates and other devices shall be so constructed, placed, maintained and operated as to provide reasonable safety to the public. Protection gates or devices shall at all times be closed before the draw or swing span is opened for any purpose and shall be kept closed until the draw or swing is closed, and the bridge is ready for public travel. Any person who shall wilfully violate any of the provisions hereof shall be deemed guilty of a misdemeanor and on conviction thereof shall be punished by a fine of not more than 50 dollars or by imprisonment in the county jail for not more than 30 days, or by both such fine and imprisonment in the discretion of the court.