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Rh the approach highway. The color of portions of bridge pavements outside of such Fines shall approximate the color of the shoulders of the approach highways.

Structural design of bridges.—All bridges constructed on rural sections of the interregional highway system shall be of steel or reinforced concrete, and shall be designed for the standard H20-S16 design loadings specified by the American Association of State Highway Officials, as defined in appendix VI. All short bridges shall be of deck construction, and long bridges shall preferably be of that type of construction.

Width of culverts.—The over-all clear width of all culverts shall be equal to the sum of the widths of the surfaces or pavements, median strip, and shoulders of the section of the interregional highway in which they are installed.

On divided highways the two roadways shall be separated, over culverts, by a median strip of the width provided on the highway approaching the culvert. In the case of culverts supercharged with an earth embankment, shoulders and slopes shall be carried over the culvert, identical in design with the shoulders and slopes of the approach highway.

In all other cases the entire clear width shall be paved; and, between lines joining the edges of the pavements on the approach highways at the culvert, the character of such paved areas shall be identical with that of the pavement on the approach highway. ll portions of the pavement over culverts outside of such lines shall approximate the color of the shoulders.

Structural design of culverts.—All culverts on rural sections of the interregional system shall be constructed of steel, reinforced concrete, or stone masonry, or pipes composed of material of a probable durability of not less than 50 years, and all culverts, regardless of the material employed, shall be designed to carry with complete safety the recommended H20-S16 design loadings and the weight of any supercharged embankment, as such loads and weight are transmitted to the supporting structure.

''III. Underpasses.''

Clear width of underpasses.—All underpasses on rural sections of the system shall provide a width for passage of the interregional highway equal to the sum of the widths of the surfaces or pavements, median strip, and shoulders of the interregional highway approaching the underpass, such width to be clear of all osbtructionobstruction [sic] except that a central pier of the underpass structure may be founded within the median strip of a divided highway, but the sides{of such pier, parallel to the direction of the interregional highway, shall be not less than 18 inches from the faces of nonmountable curbs which shall be not less than 3 feet from the edges of the pavements of the interregional highway.

Where provision is to he made for walks, the space may be provided in the clear width provided for the highway or may be provided beyond the supports adjacent thereto.

Auxiliary lanes at underpasses.—Where auxillary lanes or designs for other purposes provide a pavement at undercrossings that extends beyond the normal edge of through traffic lanes, the face of abutment walls or other support shall be at least 4 feet outside the edge of the widened pavement.