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Rh Each lane shall have a width of 12 feet, and the lanes for traffic moving in opposite directions shall be separated by a median strip at least 4 and preferably 15 feet wide. The conversion from two to four lanes shall be safely graduated and appropriately and conspicuously marked.

All rural sections of the system expected to carry an average daily traffic of less than 2,000 vehicles shall provide a' two-lane pavement 24 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.

Width of shoulders and gutters or ditches.—The shoulder width shall be considered as the transverse distance from the edge of the road surface or pavement to the inside of the guard rail or, in the absence of a guard rail, to the beginning of rounding into the slope of the embankment or the inside slope of the gutter or ditch.

On rural sections of the system the shoulder width shall be 10 feet, and this width shall be provided at all points, except as follows:

(a) In mountainous topography, where for reasons of expense a 10-foot width is not feasible.

(b) Where the two roadways of a divided highway are widely separated or constructed at different elevations and left shoulders are required, the width of such left shoulders may be less than 10 feet.

In no case shall the shoulder width be reduced to less than 4 feet.

In excavation, gutters or ditches of adequate capacity shall be constructed outside of the shoulder width provided, and the slope from the edge of the shoulder shall be not steeper than 1 foot measured vertically to 4 feet measured horizontally.

Side slopes in excavation and embankment.—In general, the sides of all excavations, except in solid rock, shall have a slope not steeper than 1 foot measured vertically to 2 feet measured horizontally, modified as deemed desirable to meet landscape requirements. The sides of all excavations shall be rounded at the top and bottom to merge by curves of natural appearance into the slopes of the adjoining land and those of the gutter or ditch. At the ends of sections in excavation the side slopes shall be flattened as the depth of excavation decreases.

The sides of all embankments 10 feet or less in height shall have a slope not steeper than 1 foot measured vertically to 4 feet measured horizontally, except where the adjoining land lies on a steeper downward slope or where landscape considerations may justify modification of this requirement.

All embankments more than 10 feet in height and all embankments built on ground having a natural downward slope steeper than 1 foot measured vertically to 4 feet measured horizontally, shall have a slope not steeper than 1 foot measured vertically to 2 feet measured horizontally, except where the adjoining land lies on a steeper downward slope, in which case slope protection or a retaining wall shall be constructed.

Gradient.—The gradient of rural sections of the system shall be adapted to the surrounding topography, the volume of traffic (especially of trucks and tractor combinations), and the relative necessity