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158 Widening of underpasses at curves.—Underpasses on curves shall be widened on the inside as necessary to maintain, free from obstruction by any abutment walls or other supports, at least the minimum sight distance used in the design of the section of the interregional highway on which the underpasses are located.

Length of underpasses.—The length of underpasses shall be such as to provide ample bridge width for the accommodation of the crossing railway or public highway. The bridge width to be provided for railways shall be determined by agreement with the railway companies concerned. The bridge width, to be provided for public highways shall be determined in the same manner as previously prescribed for bridges of like length on rural sections of the interregional system.

Clear height of underpasses.— underpasses on rural sections of the system shall provide a clear height above the entire paved width of the interregional highway of not less than 14 feet, and not less than 12½ feet above the outer edges of the shoulders of the highway.

Structural design.—All structures designed to carry railways over rural sections of the interregional system shal] have a load-supporting capacity to be agreed upon with the railway companies concerned. All structures designed to carry crossing highways shall be designed in agreement with the authorities in charge of such highways, but in no case for less than the standard H15 design loading specified by the American Association of State Highway Officials and described in appendix VI.

''IV. Access facilities.''

Rural sections of the system shall be designed, wherever feasible, for provision of access to the interregional highway only at the more important intersecting roads.

Direct interchanges.—Where, at points of acccss on rural sections of the system, a large volume of interchanging traffic is expected, the lanes for traffic in each direction on the interregional highway shall be treated as separate one-way roads, and each shall be connected, for both left- and right-turning movements, with the appropriate lanes of the intersecting highway by means of direct, connecting roadways, separated in grade at all points of crossing with other roadways.

Right-turning access at grade-separated intersections.—Where, on rural sections of the system designed with four or more through-traffic lanes, access is to be provided from grade-separated intersecting roads, and the provision of direct connections for left- and right-turning movements is not feasible, ramps or connections between the intersecting highways shall be constructed in such manner as to permit exit from and entrance to the interregional highway by right-turning movements only. Wherever feasible, such ramps or connections shall be provided in the four quadrants of the intersection, each to accommodate traffic turning from or to the interregional highway to or from the lanes for traffic in one direction on the intersecting highway. If such provision is not feasible, ramps or connections for right-turning movements only shall be provided in at least two of the four quadrants of the intersection, one on each side of the interregional highway, and, wherever site topography will permit, in the nearer quadrant for traffic approaching the intersection on the interregional highway.