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166 ''XI. Fences.''

Wherever necessary for protection against unauthorized entry by vehicles or pedestrians or for the exclusion of animals, fences of adequate design shall be erected on rural sections of the system, at one or both sides of the highway, on suitable lines, within or at the limits of the right-of-way.

BASIC STANDARDS FOR URBAN SECTIONS

I. Roads.

Condition of access.—All urban sections of the system shall be established as limited-access highways, and access to the highway shall be permitted only at designated points at which facilities for safe entrance, and exit shall be provided. On all such sections of the system there shall be no crossings of railways or other streets and highways at grade. All separations of grade at intersections shall be accomplished by means of adequate structures.

Location.—The location between control points shall be as direct as feasible, shall affect adjacent property as favorably as possible, and shall conform to the topography and property improvements in such manner as to avoid the appearance of forced alinement. Consideration should be given to providing two distinct one-way roads rather than a divided highway of fixed cross section.

Elevation and depression of interregional highways.—Wherever, on urban sections of the system, to avoid frequent intersection with other streets or highways, it is necessary to elevate or depress sections of the interregional highway of substantial continuous length, depression of the highway, if financially feasible, generally shall be preferred to elevation.

Elevation of the interregional highway shall be employed as a means of avoiding frequent grade intersection with other streets and highways mainly under conditions which make it difficult or excessively expensive to obtain sufficient right-of-way for adequate depression of the highway. Elevation of the highway, when employed, generally shall be accomplished by means of a structure of adequate and pleasing design.

Where, to avoid frequent grade intersections with other streets and highways the interregional highway is depressed, the sides of the excavation shall preferably have an upward slope not steeper than 1 foot measured vertically to 4 feet measured horizontally. In no case shall the sides of the excavation have a slope steeper than 1 foot measured vertically to 2 feet measured horizontally. All excavation slopes shall be rounded at the top and bottom to merge with adjacent ground slopes by curves of natural appearance. Where lateral space for a slope of 1 foot measured vertically to 2 feet measured horizontally, is not available, retaining walls shall be constructed, and the face of such walls shall preferably be at least 10 feet and in no case less than 8 feet from the edge of the outer lane of the through pavement of the interregional highway, and at least 4 feet from any additional lanes or ramps.

Service streets and barrier strips.—Wherever necessary for the service of property, local service streets or ways shall be provided at each side of urban sections of the interregional system. To facilitate exit from and entrance to the interregional highway such streets generally shall be designed as one-way streets, and shall be not less than 24 feet wide.