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. All portions of the pavement outside of such lines shall approximate the color of the shoulders or emergency standing areas.
Structural design of culverts.—All culverts shall be structurally designed as hereinbefore recommended for culverts on rural sections of the system.
III. Underpasses.
All underpasses on urban sections of the system shall be designed to the same standards as hereinbefore recommended for rural sections of the system, except that structures designed to carry crossing highways shall in no case be designed for less than the standard H20 loading specified by the American Association of State Highway Officials, as described in appendix VI, and the bridge width shall be sufficient to accommodate necessary sidewalks outside of the vehicular curbs. References to shoulders in the standards recommended for rural sections shall be interpreted as applying to either shoulders or emergency standing areas on urban sections of the system.
IV. Access facilities.
Urban sections of the system shall be designed for provision of access to the interregional highway only at the more important intersecting roads or streets.
Direct interchanges.—Where, at points of access on urban sections of the system, a large volume of interchanging traffic is expected, provision for direct interchange, by both left- and right-turning movements, shall be made, wherever feasible, in the same manner as recommended for rural sections of the system.
Arrangement of ramps at right-turning connections.—Where, on urban sections of the system, access is to be provided from grade-separated intersecting roads or streets, and the provision of direct connections for left- and right-turning movements is not feasible, ramps or connections between the intersecting roads or streets, providing for exit from and entrance to the interregional highway by right-turning movements only, may be provided in the same manner as hereinbefore recommended for rural sections of the system. Instead of connecting directly with an intersecting read or street, such ramps or connections from urban sections of the system may connect with a parallel service street or way and thence indirectly with the intersecting road or street; and similar connection may be made with a service street or way at any desired point apart from a grade-separated intersecting road or street. (See pl. X.)
All ramps and connections shall provide either within their own length or within such length in combination with a section of the parallel service street or way, sufficient storage space for traffic leaving the interregional highway so that such traffic, if temporarily blocked at the intersecting street, will not back up onto the interregional highway.
Alinement of ramps or connections.—All ramps connecting with urban sections of the interregional system shall preferably be designed as one-way roads separated for the whole length of ramp. Where two-way ramps are used, entrances and exits at the interregional
