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Rh Service streets or ways shall be separated from the slopes or border areas of the interregional highway by means of nonmountable curbs, and the distance from the face of such curb to the edge of the pavement of the interregional highway shall preferably be not less than 15 feet and in no case less than 10 feet.

Alinement.—Alinement of urban sections of the system shall be of as high a standard as feasible; and the speed assumed for design purposes for a section of road shall be as high as practicable, consistent with the topography, proximity of urban improvements, and expected traffic volume. Under urban conditions, the assumption of a design speed higher than 50 miles per hour will usually be impracticable. The design speeds assumed for adjacent sections of the highway shall not differ widely.

Horizontal curvature.—Horizontal curvature on urban sections of the system shall be under all conditions of the lowest practicable degree, and at no point sharper than hereinbefore recommended for rural sections of the same assumed design speed.

Transition curvature.—All horizontal circular curves on urban sections of the system sharper than 2° shall be approached by transition curves of a length consistent with the design speed and sufficient to permit the attainment of full superelevation within the length of the transition.

Superelevation of curves.—On urban sections of the system all curves sharper than 1° shall be superelevated, as hereinbefore recommended for rural sections, except that maximum superelevation shall be 0.10 foot per foot.

Sight distance.—Sight distance on urban sections of the system shall be at least as great as hereinbefore recommended for rural sections of the same design speed.

Vertical curvature.—Vertical curvature on urban sections of the system shall be as hereinbefore recommended for rural sections.

Number and width of traffic lanes and median strips.—All urban sections of the system expected to carry an average daily traffic of 20,000 or more vehicles shall be designed to provide, when it becomes necessary to do so, three lanes for traffic moving in each direction, each lane to have a width of 12 feet; and the lanes for traffic moving in opposite directions shall be separated by a raised median strip at least 4 feet wide.

Urban sections of the system expected to carry an average daily traffic of less than 20,000 vehicles shall be designed to provide at least 2 lanes for traffic moving in each direction, each lane to have a width of 12 feet; and the lanes for traffic moving in opposite directions shall be separated by a raised median strip at least 4 feet wide.

Transitions of median strip width.—Where narrowing or widening of the median strip is necessary, essential pavement alinement changes shall be accomplished over lengths sufficient to avoid hazard in vehicular operation at the design speed assumed, and to avoid the appearance of distorted or forced alinement.

Shoulders, curbs, and emergency standing areas.—Shoulders 10 feet wide and contrasting in texture and preferably in color with the adjoining pavement, shall be constructed on urban sections of the system, or in lieu thereof there shall be constructed, throughout the length of such sections and adjoining the outer lanes thereof, mountable curbs, outside and flush with the top of which, there shall be