All culverts hereafter built, rebuilt, or placed in any public road in this state shall be not less than the full width of the crown of the roadway, and shall have guide or warning posts on either side. All bridges hereafter built, rebuilt, or placed in any public road of this state shall be built the full width of the crown of the roadway where the same is sixteen feet wide or less, and on roads having a greater width than sixteen feet the bridges shall be not less than sixteen feet wide; and all bridges hereafter built or rebuilt shall be built with banisters on either side. This section shall not apply to temporary bridges and culverts on detours; and all public highways in this state within the area of, or contingent to, any national reforestation project, which have been or may be taken over, constructed, or reconstructed by this department of the national government, may have bridges or culverts the width of said highways, but not less than twelve feet.