Miss. Code § 65-21-1
All bridges and culverts hereafter built, rebuilt, or placed in the traveled way of any public road in this state shall conform to the design standards in effect at the time of design and shall extend across the full width of the crown of the roadway. On roads having a design width greater than sixteen feet (16'), bridges shall be not less than sixteen feet (16') wide. All bridges hereafter built or rebuilt shall be built with railings or barriers and appropriate hazard marking signs on either side. This section shall not apply to public highways in this state within the area of, or contingent to, any national forest or reforestation project, which have been or may be taken over, constructed, or reconstructed by a department of the national government, may have bridges or culverts the width of said highways, bridges and culverts be less than twelve feet (12') wide.
Miss. Code § 65-21-1