Interregional Highways/Appendix 5

←

Appendix IV. New York Grade Crossing Elimination Act of 1928

Interregional Highways (1944)
National Interregional Highway Committee

Appendix V. Basic standards of road and structural design

Appendix VI. Standard design loadings for highway bridges

→

Washington, DC: United States Government Printing Office, pages 150–173

3985535Interregional Highways — Appendix V. Basic standards of road and structural design1944National Interregional Highway Committee

APPENDIX V

Basic Standards of Road and Structural Design

BASIC STANDARDS FOR RURAL SECTIONS

I. Roads.

Condition of access.—All rural 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. There shall be no crossings of railways at grade, and all railways that must cross the interregional highway shall be carried over it or under it by means of adequate structures.

On all rural sections of the system expected to carry an average daily traffic of 5,000 or more vehicles there shall be no crossings of other highways at grade, and all highways that must cross the interregional highway shall be carried over it or under it by means of adequate structures.

Wherever feasible on all rural sections of the system expected to carry an average daily traffic of less than 5,000 vehicles, grade intersection with other highways shall be avoided and all highways that must cross the interregional highway shall be carried over or under by means of adequate structures. If, in any case, the grade separation of a highway intersection is not immediately feasible, all necessary provision shall be made in the initial design for future conversion to the improved design when financially feasible. This initial provision shall include public acquisition of private property or acquisition of control of the use and improvement of private property essential to conversion to the improved design. Where separation of grades at highway intersections is not feasible, and at all points where vehicles may be expected to cross, enter, or leave the interregional highway, the design shall be such as to insure a high degree of safety in crossing, entering or leaving it, without installation of traffic control signals, which shall in no case be employed.

Location.—The location between control points shall be as direct as feasible and shall conform to the topography in such manner as to avoid the appearance of forced alinement. Where four or more traffic lanes are to be constructed, two distinct one-way roads should be provided rather than a divided highway of fixed cross section, wherever advantages of alinement, construction cost, or traffic facility may be expected to result from such provision.

Alinement.—Alinement of rural 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 and the expected traffic volume. The design speeds assumed for adjacent sections of the highway shall not differ widely.

Horizontal curvature.—Horizontal curvature shall be under all conditions of the lowest practicable degree, and at no point sharper than the degree shown in the column headed “Absolute maximum”? in the following table, corresponding to the design speed assumed for the section. Preferred limits are shown in the column headed “Desirable maximum.”

Table 1.—Maximum horizontal curvatures at various design speeds (rural)

Design speed of section	Curvature limits
Design speed of section	Absolute maximum	Desirable maximum
	Degrees	Degrees
75 miles per hour . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	3	2.5
70 miles per hour . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	4	3
65 miles per hour . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	5	4
60 miles per hour . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	6	5
55 miles per hour . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	7	6
50 miles per hour . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	9	7

Curvature of lower degree than that shown in the above table may be necessary to meet sight-distance requirements where a cut slope or other obstacle obstructs the view across the inside of a curve.

Transition curvature.—All horizontal circular curves on rural sections of the system sharper than 2 degrees 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 a curves.—All curves sharper than 1 degree shall be superelevated. The maximum superelevation shall be 0.12 foot per foot. Where snow and ice may be expected to cause a frequent slippery condition of the road surface, the maximum superelevation shall be 0.08 foot per foot. On all curves the superelevation shall be such as to counterbalance completely the centrifugal force of a vehicle traveling at three-fourths of the design speed of the road, except that it shall not exceed the above stated appropriate maximum limit.

Superelevation shall be attained gradually, and in such manner that the difference in slope between longitudinal profiles separated by the width of one lane shall be not greater than 1 in 200.

Sight distance.—On all rural sections of the system the design shall be such as to afford from a height of 4.5 feet above any point on the road surface, a continuously unobstructed view for the following minimum nonpassing sight distances to the top of an object 4 inches high placed on the road surface.

Table 2.—Minimum nonpassing sight distances that shall be provided at various design speeds (rural)

Design speed of section:	Minimum nonpassing sight distance^[1]
75 miles per hour . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	800 feet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
70 miles per hour . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	700 feet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
65 miles per hour . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	600 feet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
60 miles per hour . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	525 feet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
55 miles per hour . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	450 feet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
50 miles per hour . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	400 feet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

↑ Measured between one point 4.5 feet and another 4 inches above the road surface.

Within these sight distances passenger vehicles as presently constructed can be stopped from the speeds shown, and trucks and combinations can be stopped from speeds ranging from 35 miles per hour where the sight distance is 400 feet to about 50 miles per hour where the sight distance is 800 feet.

On rural two-lane sections of the system, whenever it is financially feasible to do so, the road shall be so designed in its horizontal and vertical curvature and other features as to afford a continuously unobstructed view for at least the following distance between points 4.5 feet above the road surface.

Table 3.—Minimum sight distances for passing at various design speeds (rural)

Design speed of section:	Minimum sight distance for passing^[1]
65 to 75 miles per hour . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	2,600 feet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
60 miles per hour . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	2,200 feet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
55 miles per hour . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	1,800 feet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
50 miles per hour . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	1,500 feet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

↑ Between points 4.5 feet above the road surface.

On sections so designed, drivers of vehicles moving at the design speed will have the assurance of a sufficient length of road, if no oncoming vehicle is in view, in which to accomplish the passing of another vehicle moving in the same direction at a speed 10 to 15 miles per hour slower. Passing at speeds above 65 miles per hour on two-lane roads generally cannot be considered safe, unless the vehicle passed is traveling at a speed considerably slower than that of the passing vehicle.

On rural two-lane sections of the system expected to carry an average daily traffic volume of 2,000 vehicles or more, where it is not feasible to provide the minimum passing sight distance recommended in table 3, two lanes shall be provided for traffic in each direction.

On all other two-lane sections of the system where the provision of the recommended minimum passing sight distance is not financially feasible, the longest practicable sight distance shall be provided.

Vertical curvature.—Vertica] curvature on all rural sections shall be of sufficient length over crests and in sags at underpasses to provide at least the minimum sight distance previously recommended for the assumed design speed, and, in other sags, to provide for safe and comfortable travel at the assumed design speed.

Number and width of traffic lanes and median strips.—All rural sections of the system expected to carry an average daily traffic of 15,000 or more vehicles shall provide three and not more than 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 median strip at least 15 feet wide.

All rural sections of the system expected to carry an average daily traffic of 3,000 but less than 15,000 vehicles shall provide at least two 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 median strip at least 15 feet wide. On sections which permit crossing or entering at grade at intersecting highways or private entrances, the median strip at public road crossings shall be at least 40 feet wide, and opposite private-property entrances, at least 25 feet wide.

All rural sections of the system expected to carry an average daily traffic of 2,000 but less then 3,000 vehicles shall provide at least a two-lane pavement 24 feet wide. On all parts of such highways where it is not feasible to provide a sight distance at least equal to the minimum passing sight distance specified in table 3, the design of the highway shall provide two Janes for traffic moving in each direction. 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 for passing trucks and tractor combinations, with maximum limits under various conditions as given in table 4.

Table 4.—Maximum gradient limits under various traffic volumes and topographical conditions (rural)

Average daily traffic (all vehicles)	Surrounding topography	Maximum desirable grade
		Percent
Less than 1,000 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	Relatively level . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	3
	Rolling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	^[1]4
	Mountainous . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	6
1,000 to 2,000 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	Relatively level . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	3
	Rolling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	^[1]4
	Mountainous . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	6
2,000 to 3,000 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	Relatively level . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	3
	Rolling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	^[2]6
	Mountainous . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	6
3,000 to 5,000 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	Relatively level . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	3
	Rolling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	^[3]6
	Mountainous . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	6
5,000 and more . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	Relatively level . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	3
	Rolling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	^[4]4
	Mountainous . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	^[4]^[5]5

↑ ^1.0 ^1.1 This limit desirable on the 2-lane highways to be provided for this volume of traffic, in order to permit the maximum feasible speed of trucks and tractor combinations and correspondingly reduce the frequency of passing.
↑ This limit permissible because a divided 4-lane highway is to be provided for this volume of traffic where minimum passing sight distance Previously recommended is not feasibly obtainable, thus permitting passing of slow-moving trucks and tractor combinations at all points.
↑ This limit permissible because a continuously divided 4-lane highway is to be provided for this volume of traffic, thus permitting ready passing of slow-moving trucks and tractor combinations in the number to be expected.
↑ ^4.0 ^4.1 These limits desirable because of the greater number of slow-moving trucks and tractor combinations to be expected and the consequent necessity to permit the maximum feasible speed of such vehicles to reduce the frequency of passing.
↑ No existing roads conforming to the recommended interregional system in mountainous topography carried this volume of traffic before the war.

In general, extremely long grades should be less steep, and very short grades and grades to be traveled only in the downward direction on one-way roads may be steeper, than the limits given in table 4, but none shall exceed 7 percent.

Width of right-of~vay.—On rural sections of the system, the width of right-of-way to be acquired by purchase or outright condemnation shall be at all points at least sufficient to include the road surfaces or pavements and median strip, the shoulders, gutters, or ditches, and the side slopes of the road, constructed in accordance with the foregoing recommendations with full allowance for the widening and conversion of the traveled way and other cross-section features estimated to become necessary within a period of 20 years.

In addition, public control shall be obtained, either by purchase or outright condemnation or by the acquirement of highway development rights, over a strip of land of sufficient width to prevent the erection of any private structure or sign within a distance of not less than 100 feet from the outer edge of the road surface or pavement as it is likely to be constructed or converted within-a period of 20 years.

Substantial conformity with these right-of-way standards will require the obtainment of public control, in the manner and degree determined to be necessary, over a strip of land not less than 224 feet wide in the case of the most lightly traveled sections of the system to be improved initially with two-lane surfaces or pavements, and not less than 288 feet in all other cases.

Wherever feasible, it is desirable on rural sections of the system that public control be obtained at the outset over a strip of land 300 feet wide without regard to the expected traffic volume on the highway.

Where it is necessary at the time of construction or where it will probably be necessary at a later date to provide service roads to permit use of the interregional highway as a limited-access highway, sufficient width for the construction of such service roads shall also be included in the width of right-of-way to be initially acquired.

Foundations and bases.—All road foundations and bases on rural sections of the system shall be capable of supporting the recommended maximum loads of vehicles as such loads are transmitted by surfaces or pavements of adequate design, without reduction of load or speed at any season of the year.

Surfaces and pavements.—All road surfaces and pavements on rural sections of the system shall consist of such material and shall be of such thickness as will enable them, when placed on bases and foundations of adequate design, to support the recommended maximum loads of vehicles, without reduction of either load or speed at any season of the year; and shall be capable of retaining under traffic of the expected weight, speed, and volume, with a reasonable expenditure of maintenance effort, a uniformly dustless, mudless, and smooth but skid-resistant surface.

Shoulders.—All road shoulders on rural sections of the system shall contrast in texture and preferably in color with the adjoining surface or pavement. They shall be capable of supporting the recommended maximum loads of vehicles standing on them or passing onto them infrequently and in emergency at high speed, and shall be capable of retaining under such usage, with a reasonable expenditure of maintenance effort, a reasonably mudless and even surface, without dangerous difference of level at the line of junction with the road surface or pavement.

II. Bridges and culverts.

Definitions.—All structures of a length between abutments greater than 20 feet, measured along the center line of the road, shall be defined as bridges.

All bridges of a length between abutments greater than 100 feet shall be classed as long bridges.

All bridges of a length of 100 feet or less shall be classed as short bridges.

All structures of a length between abutments of 20 feet or less, measured along the center line of the road, shall be defined as culverts.

Alinement of bridges.—All bridges, wherever feasible, shall be so located as to fit the over-all alinement and gradient of the highway and shall be subordinated thereto. Where structural or architectural requirements make it desirable to adjust the alinement and gradient of the highway, the changes shall be such that the highway will meet all the basic standards for rural sections of the system, recommended herein under “I. Roads.”

Width of bridges.—The width between vehicular curbs on all bridges built on tangents of rural sections of the system shall be at least 6 feet greater than the width of the surface or pavement of the approach highway, and the lateral distances between the edges of the pavement of the approach: highway and the faces of the vehicular curbs shall be at least 3 feet.

On all bridges the lateral distance. from the face of the curb to the face of the bridge rail or any structural member shall be at least 18 inches and as much more as is necessary for walk space, which shall be provided on all long bridges.

On short bridges the lateral distance from the edge of the pavement of the approach highway to the face of the bridge rail, any structural member, or the curb shall be not less than the width of shoulder on the approach highway.

On sections of the system improved with divided highways, one bridge to accommodate both roadways and the median strip or two separate bridges, each to accommodate one roadway, may be used. On short bridges the two roadways shall be accommodated without deviation from their normal alinement, and on long bridges the two roadways shall be so accommodated if feasible.

Where, on long bridges, the median strip is reduced in width, the change in alinement shall be accomplished in such manner and over such lengths as will avoid hazard in vehicular operation at the design speed and the appearance of distorted or forced alinement. Where the median strip is reduced in width over structures the two roadways shall be separated by a raised, but mountable, dividing strip not less than 4 feet wide.

Auxiliary lanes on bridges.—Where auxiliary lanes or designs for other purposes provide a pavement over bridges that extends more than 3 feet outside the normal edge of through-traffic lanes, the vehicular curb shall be continuous with the curb on the approach highway; or in the absence of an approach curb, the vehicular curb shall be located at least 2 feet outside a line extending from the edge of the widened pavement on the approach.

Bridges on curves.—On all bridges on curves the clearances shall be at least as great as those required for all other bridges, and the lateral clearance to the face of the bridge rail or any structural member shall be as much more as shall be necessary to maintain the minimum sight distance used in the design of the section of highway on which the bridge is located.

Clear height of bridges.—The clear height of all bridges over the entire width between outer curbs shall be not less than 14 feet.

Openings under bridges.—All bridges over railways shall provide a clear height and length between abutments to be determined by agreement with the railway companies concerned.

All bridges over public roads or highways shall provide a clear normal distance between abutments not less than the sum of the widths of the surfaces or pavements, median strip and shoulders or sidewalks of the underpassing road or highway, such distance to be clear of all obstruction except that a central pier may be founded within the median strip of a divided underpassing highway if the sides of such pier, parallel to the direction of the underpassing highway are not less than 4 feet from the inner edges of the adjacent pavements. All bridges over roads or highways shall provide a clear height above the surface or surfaces of the underpassing road or highway of not less than 14 feet, and of not less than 12 feet above the outer edges of the shoulders of such highway.

Pavements on bridges.—All portions of bridge pavements, between lines joining the edges of the pavements of the approach highways at the bridge, shall be of a color identical with that of the pavement on the approach highway. The color of portions of bridge pavements outside of such Fines shall approximate the color of the shoulders of the approach highways.

Structural design of bridges.—All bridges constructed on rural sections of the interregional highway system shall be of steel or reinforced concrete, and shall be designed for the standard H20-S16 design loadings specified by the American Association of State Highway Officials, as defined in appendix VI. All short bridges shall be of deck construction, and long bridges shall preferably be of that type of construction.

Width of culverts.—The over-all clear width of all culverts shall be equal to the sum of the widths of the surfaces or pavements, median strip, and shoulders of the section of the interregional highway in which they are installed.

On divided highways the two roadways shall be separated, over culverts, by a median strip of the width provided on the highway approaching the culvert. In the case of culverts supercharged with an earth embankment, shoulders and slopes shall be carried over the culvert, identical in design with the shoulders and slopes of the approach highway.

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. ll portions of the pavement over culverts outside of such lines shall approximate the color of the shoulders.

Structural design of culverts.—All culverts on rural sections of the interregional system shall be constructed of steel, reinforced concrete, or stone masonry, or pipes composed of material of a probable durability of not less than 50 years, and all culverts, regardless of the material employed, shall be designed to carry with complete safety the recommended H20-S16 design loadings and the weight of any supercharged embankment, as such loads and weight are transmitted to the supporting structure.

III. Underpasses.

Clear width of underpasses.—All underpasses on rural sections of the system shall provide a width for passage of the interregional highway equal to the sum of the widths of the surfaces or pavements, median strip, and shoulders of the interregional highway approaching the underpass, such width to be clear of all osbtruction except that a central pier of the underpass structure may be founded within the median strip of a divided highway, but the sides{of such pier, parallel to the direction of the interregional highway, shall be not less than 18 inches from the faces of nonmountable curbs which shall be not less than 3 feet from the edges of the pavements of the interregional highway.

Where provision is to he made for walks, the space may be provided in the clear width provided for the highway or may be provided beyond the supports adjacent thereto.

Auxiliary lanes at underpasses.—Where auxillary lanes or designs for other purposes provide a pavement at undercrossings that extends beyond the normal edge of through traffic lanes, the face of abutment walls or other support shall be at least 4 feet outside the edge of the widened pavement.

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.

Where, on rural sections of the system expected to carry an average daily traffic of 2,000 but less than 3,000 vehicles, grades are separated and access facilities are to be provided, the interregional highway shall be widened to provide two lanes for traffic moving in each direction, the lanes for traffic moving in opposite directions shall be separated by a median strip at least 4 and preferably 15 feet wide, and ramps or connections between the intersecting roads shall be provided in the same manner as at grade-separated intersections with access facilities, on sections of the system designed with 4 or more traffic lanes.

Where, on rural sections of the system expected to carry an average daily traffic of less than 2,000 vehicles, grades are separated and access facilities are to be provided, the ramps or connections between the intersecting roads shall be constructed in such manner as to permit exit from or entrance to the interregional highway by right-turning movements only.

Access at crossings at grade.—Where, on rural sections of the system designed with four or more through-traffic lanes, access is to be provided from an intersecting highway and it is not deemed financially feasible immediately to separate the grades of the intersection, separate channels shall be provided for all right-turning movements, the median strip of the interregional highway shall be not less than 40 feet wide at the intersection, and adequate space shall be provided for all crossing and left-turning vehicles to stop clear of the through-traffic lanes and proceed across the interregional highway or merge with and emerge from the through traffic on the interregional highway in safety. (See plate VII.)

Where, on rural sections of the system expected to carry an average daily traffic of 2,000 but less than 3,000 vehicles, access is to be provided from an intersecting highway and it is not deemed financially feasible immediately to separate the grades of the intersection, the interregional highway shall be widened to provide two lanes for traffic moving in each direction, the lanes for traffic moving in opposite directions shall be separated by a median strip at least 15 feet wide, and the intersection shall be designed in the same manner as at grade crossings on sections of the system designed with four or more traffic lanes.

Where, on rural sections of the system expected to carry an average daily traffic of less than 2,000 vehicles, access is to be provided from an intersecting highway, and it is not deemed financially feasible to separate the grades of the intersection, the intersection shall be designed to provide adequate space for right-turning vehicles safely to merge with and emerge from the through traffic on the interregional highway. Such space shall also be sufficient to permit vehicles turning left from the interregional highway to halt if necessary at the center of the intersection before completing the turning maneuver and to permit through traffic to pass such halted vehicles in safety.

Alinement of ramps or connections.—Ramps at grade-separated intersections on rural 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 highway and, if deemed feasible, at the intersecting highway also shall be designed as one-way roads separated by suitable channelizing islands. Entrances and exits shall be located at sufficient distances from the grade-separating structure to provide sight distances adequate for safety under the conditions of vehicular speed to be anticipated.

All ramps and connections shall be designed to enable vehicles to leave and enter the through-traffic lanes of each highway at 0.7 of its design speed, except where “stop” control is necessary. On all four-lane sections of the system, on all two-lane sections expected to carry an average daily traffic of more than 2,000 vehicles, and wherever feasible on all other two-lane sections, the curvature of ramps and connections shall preferably not exceed 45 degrees (radius approximately 125 feet), and under no conditions shall exceed 70 degrees (radius approximately 80 feet). All curves shall be eased by transition or compounding.

Widths of pavements and shoulders and side slopes.—All ramps and connections shall have a width of at least 16 feet if designed for one-lane operation, and at least 26 feet if designed for two-lane operation. Widths greater than these minima shall be provided on sections of ramps and connections of sharp curvature.

A shoulder at least 6 feet and preferably 8 feet wide shall be provided along the right side of all ramp pavements (right in the direction of traffic movement).

Side slopes on ramps shall be not steeper than 1 foot measured vertically to 2 feet measured horizontally, and shall be rounded at the top and bottom to merge by curves of natural appearance with the adjacent land slopes or shoulders.

Added space for turning maneuvers.—All rural sections of the system shall be so designed, at the approach to entrances and exits on the interregional highway, as to provide space outside the through-traffic lanes of the interregional highway for emerging vehicles to decelerate and entering vehicles to accelerate, and, in general, to maneuver as required so that they may safely emerge from or merge with the through traffic stream. The added space may take the form of a taper, a pavement gradually increasing in width, or a taper combined with a lane of full added width. Tapers shall be smoothly alined and of a length consistent with probable speed of travel. Where a full width of added lane is required, it shall be at least 10 feet wide. If deemed necessary and feasible, similar provision shall be made on the intersecting highway.

Where an exit to an inner loop is provided on one side of a grade-separating structure and an entrance from an inner loop is provided on the opposite side of the structure, an added lane shall be carried over the bridge or through the underpass to connect both inner loops and serve as added space for the maneuvers of entering and leaving the interregional highway.

At exits from a through-traffic lane, added width of pavement and a taper shall be provided beyond the nose at the fork to enable vehicles which start to turn off to return safely and conveniently to the through-traffic lane if desired. A curb of high visibility should. be used around the nose at the fork and along these widened pavements.

All pavement surfaces of ramps and connections and all added pavement width provided for maneuvering shall contrast in color and preferably in texture with the pavement of the through-traffic lanes.

Gradients on ramps.—The gradient on ramps shall not exceed 6 percent on upgrades and 8 percent on downgrades.

Sight distance at ramps and connections.—On all ramps and connections the combination of grade, vertical curves, alinement, and clearances of lateral and corner obstructions to vision shall be such as to provide sight distance along such ramps and connections and from their terminal junctions along the interregional highway and intersecting road consistent with the probable speeds of vehicle operation.

At all grade intersections on rural sections of the interregional system, vehicles approaching on the intersecting highways shall be required, before crossing the interregional highway, to come to a complete stop at a point off the through-traffic lanes of the interregional highway. From such point of stopping, the sight distance in each direction along the interregional highway and from the rear along the intersecting highway, shall be not less than the safe stopping distance corresponding to the probable speed of traffic on each high- way, as hereinbefore recommended.

Access for busses.—Bus stops shall be prohibited on all rural sections of the interregional system. Access connections for bus stops off the interregional highway shall be designed to the same standards as other access connections.

Access for roadside businesses.—Roadside businesses, such as parking areas, gasoline filling stations, restaurants, etc., shall be prohibited from fronting directly on rural sections of the interregional system. Access connections for such businesses off the interregional highway shall be designed to the same standards as other access connections.

V. Tunnels.

Tunnels on rural sections of the interregional highway system shall accommodate, if financially feasible, the same number and width of traffic lanes that are provided on the highway approaching the tunnel. Tunnels on two-lane highways shall provide space for a 2-foot flush median strip between the two lanes, and shall have an over-all width not less than 10 feet greater than the pavement or surface width of the highway approaching the tunnel, to provide for the 2-foot center separation and not less than 4-foot side clearances beyond the edges of the pavement.

Tunnels on divided highways shall be constructed to accommodate the same number of lanes as the approaches, in either a single or twin bore. In the former case the lanes for traffic in opposite directions shall be separated by a raised but mountable median strip not less than 4 feet wide; and the over-all width of the tunnel shall provide space for this strip and two lateral clearances of not less than 4 feet, in addition to the aggregate width of traffic lanes of the same number and width as on the highway approaching the tunnel. In the case of twin bores, the over-all width of each bore shall be not less than 8 feet greater than the total width of the lanes to be accommodated, as provided on the highway approaching the tunnel.

The clear height provided in tunnels shall be not less than 14 feet across the entire paved width.

Standards of gradient and curvature for tunnels shall be identical with those previously recommended for divided highways.

Wherever their length requires, all tunnels shall be-artificially ventilated and lighted in such manner as to provide amply safe conditions of air and light. For an appropriate distance inward from the portal the intensity of artificial light provided during the day shall be sufficient to afford such a transition between outer daylight and the normal tunnel lighting as will permit safe entrance into the tunnel without reduction of the speed of vehicles.

VI. Pedestrian facilities.

On rural sections of the system pedestrian use of road surfaces and pavements and shoulders shall be prohibited. Adequate pedestrian paths shall be provided whenever the need justifies.

Wherever other public highways are carried over or under a rural section of the interregional system, provision shall be made for safe pedestrian crossing of the interregional highway, if necessary, by means of adequate walks outside the vehicular curbs of underpassing highways or overpassing bridges. At other points where need is found to exist, special pedestrian underpasses or overpasses with connecting walkways shall be provided.

VII. Landscaping.

On all rural sections of the system the design, wherever feasible, shall conserve desirable and irreplaceable landscape features, avoid needless damage to desirable trees and other growth and to lake and stream shores, and preserve natural sites for the development of overlooks, picnic areas, and other desirable wayside attractions. Unnecessary construction scars shall be avoided. Borrow pits shall not be permitted within sight of the road unless they are adjusted and recovered to avoid unsightliness. All ground surfaces disturbed by construction shall be appropriately recovered with suitable vegetative growth, and additional landscaping shall be done where deemed necessary.

The design, combined with re-covering of disturbed surfaces and other landscaping, shall be planned to protect the highway against erosion by wind and water, to reduce maintenance to a minimum, and to enhance the natural appearance of the road and the wayside.

VIII. Signs and pavement markings.

The design of rural sections of the interregional system shall be such as to reduce to a practicable minimum the necessity of cautionary signs and pavement markings. The installation of traffic control signals shall be prohibited.

The form, dimensions, color, and style and size of all lettering of all signs, the legend of all cautionary signs, and the form, dimensions, character, and significance of all pavement markings shall be uniform throughout all rural sections of the interregional system in all States.^[1]

Route markers.—It is recommended that all interregional highways be incorporated in the United States system of numbered highways, and that all rural sections of the system be marked with standard U S route markers appropriately illuminated or reflectorized for night visibility. If two or more U S numbered routes incorporate the same section of any interregional highway, standard U S route

↑ It is recommended that the details of design of all signs and pavement markings, as herein generally described, be defined by the joint committee on uniform traffic control devices, appointed by the American Association of State Highway Officials, the Institute of Traffic Engineers, and the National Conference on Street and Highway Safety; and that signs and markings of the character so defined be adopted and used on all parts of the interregional system in all States.

markers designating each route shall be erected on the section so incorporated, but no route markers other than standard U S route markers shall appear on any interregional highway, except at points of exit.

Destination signs.—On rural sections of the system, all points of entrance or exit that are located at grade-separated intersections with other highways shall be designated as “interchanges,” and each shall be identified by the name of a single nearby important city or area within a city or by the number of the intersecting highway. Warning of approach to an interchange shall be provided by a sign located at a suitable distance in advance of the interchange, bearing the legend “(Name) interchange ahead.”

At each point of turning at each interchange, a sign shall be provided bearing the name of a single nearby important city or locality served by the intersecting highway with which the particular ramp connects, or the number of the intersecting highway and direction if no geographical name is appropriate, or both if necessary. Immediately beyond the interchange on the interregional highway, a confirmatory U S route marker shall be placed, and also a sign showing the distance to the next important city or interchange. The size, legibility, and location of all such signs shall be appropriate for the conditions of placement and probable speed of traffic on the interregional highway.

Except as above recommended, no direction or distance indications shall be given at interchanges by signs on the interregional highway. Such additional directions and distances as may be deemed desirable shall be indicated by signs located at the junction of the ramp with the intersecting highway.

At interchanges that are lighted at night, all such signs located on the interregional highway shall be illuminated. At other interchanges all signs shall be illuminated where feasible, or if not illuminated, shall be reflectorized.

Signs at grade crossings.—At important grade intersections with other highways, a similar system of signing shall be used on the interregional highway, modified as necessary to indicate also important nearby cities or localities, or highway number and direction, or both, in the direction of permitted left turns as well as right turns. Warning signs preparatory to such turns shall bear the legend ‘“U S (or State) (route number) ahead.” On all highways intersecting interregional highways, appropriate signs and marking shall be installed to provide necessary information for traffic approaching the interregional highway, and to insure the maximum degree of safety to approaching traffic and to traffic on the system.

Where traffic is required to come to a full stop before entering or crossing an interregional highway, a stop line shall be marked on the pavement of the intersecting highway and a standard stop sign bearing the legend “Express highway” shall be erected.

Whether or not a stop sign is required there shall be erected at an appropriate advance location on the intersecting highway a warning sign bearing the legend “Express highway ahead.”

All stop lines shall be reflectorized, and all stop and warning signs shall be illuminated where feasible, and, if not illuminated, shall be reflectorized.

At intersections of insufficient importance to require destination signs, a distinctive sign shall be erected at a suitable distance in advance to indicate merely the approach to a minor intersection.

Speed control signs.—Wherever the design of the interregional system provides for maximum safe speed less than 70 miles per hour, the following signs shall be provided:

At the beginning of a section on which the maximum safe speed under normal conditions is between 60 and 70 miles per hour, there shall be erected a sign bearing the legend, “Maximum speed 60,” and signs of this character shall be repeated at approximately 1-mile intervals throughout the section. At such point as a 70-mile speed again becomes safe, a sign shall be erected bearing the legend, “End 60-mile speed.”

At the beginning of a section on which the maximum safe speed under normal conditions is between 50 and 60 miles per hour there shall be erected a sign bearing the legend, “Maximum speed 50,” followed by similar signs at approximately 1-mile intervals throughout the section. At the end of such section one of two signs shall be used, (1) if 70-mile speed is again safe, a sign bearing the legend “End 50-mile speed,” or (2) if the section terminates in a section on which the maximum safe speed is between 60 and 70 miles per hour, a sign bearing the legend, “Maximum speed 60,” followed by similar signs throughout that section.

Where the traffic conditions anticipated at interchanges on rural sections of the system require reduction in speed below 50 miles per hour, the safe speed shall be indicated by a sign erected at the beginning of each such reduced-speed section, bearing the legend “Slow to ——,” with following signs located at appropriate intervals throughout the section, bearing the legend “Speed ——.” The end of such a reduced-speed section shall be marked as above, according to the safe speed permitted by the following section of the highway.

Where State or local traffic regulations, rather than alinement of the highway or traffic conditions, govern the maximum speed, such maximum speed limits shall be indicated at appropriate intervals by signs bearing the legend “Speed limit ——.”

All speed control signs shall be reflectorized for night visibility, except at points where artificial illumination is preferable and feasible.

Signs to control or prohibit passing.—At the beginning of any rural two-lane section of the system on which the sight distance is insufficient for safe passing but more than 1,000 feet, there shall be erected a sign bearing the legend, “Passing unsafe,” and similar signs shall be erected at approximately one-half mile intervals throughout the length of the section.

At the beginning of any rural two-lane section of the system on which the sight distance is less than 1,000 feet, there shall be erected a sign bearing the legend, “No passing.” At the end of such section there shall be erected a sign bearing the legend, “End no passing zone.”

Sections on which passing is unsafe or prohibited shall be indicated independently for each direction.

Pavement markings.—All pavement markings shall be reflectorized.

A. Lane lines: On rural two-lane sections of the system there shall be a continuous 4-inch, white center line. On all four- and six-lane divided sections the lanes shall be defined by 4-inch, white dashed lines.

Arrows, route numbers, or other pavement markings may be used when required, particularly on four- and six-lane sections, to supplement directional or other signs, but no warning or direction shall be conveyed by pavement marking alone.

B. No-passing zones: On rural two-lane sections of the system a 4-inch barrier line, preferably yellow, shall be marked on the pavement parallel and adjacent to the center line wherever the sight distance is less than 1,000 feet. Such barrier lines shall be marked independently for each direction of traffic and shall be placed on the right of the center line in the direction of traffic affected. Barrier lines shall be used in conjunction with “No passing” signs above recommended.

C. Special treatment at interchanges: Where, on multilane sections, it is desirable to confine traffic to particular lanes, as at interchanges, continuous white lines shall be used in lieu of dashed lane lines.

Location and information signs.—The use of location and information signs shall be confined to points of general importance or significance. Such signs shall be of such size and shall be so located as not to detract from, or confuse the significance of other signs as herein recommended.

Discouragement of other signs and markings.—On rural sections of the interregional system that are designed in accordance with the standards herein proposed, it is recommended that the erection of signs and the marking of pavements, except as above proposed, be strongly discouraged.

IX. Lighting.

At all points on rural sections of the system where traffic speeds are required to be reduced because of merging traffic or where an unusual degree of caution is required because of traffic or other conditions, the interregional highway and, as necessary, its connections, shall be lighted by fixed-source illumination to provide a maximum degree of safety and convenience of movement at night. In all such cases appropriate transitional illumination between the lighted and unlighted sections shall be provided.

It may also be desirable to illuminate throughout their length rural sections of the system expected to carry large volumes of traffic, particularly if the traffic includes a large percentage of commercial vehicles.

X. Provision for public utilities.

The erection of electric light, power, and telephone poles within the right-of-way of rural sections of the system, except those necessary for service of the highway or its appurtenant facilities, shall be discouraged.

The construction of underground electric conduits and the laying of water-supply and sewerage pipes and pipes for other public-utility purposes, within the right-of-way of rural sections of the system, except those necessary for the service of the highway or its appurtenant facilities, shall likewise be discouraged. ere it is necessary to use the right-of-way of the system for electric facilities, underground construction shall be preferred to the erection of pole lines. Wherever underground electric, water, sewerage, or other facilities are constructed within the right-of-way, they shall in no case be constructed, except for crossing the highway, beneath any portion of the right-of-way to be used immediately or eventually for the construction of a pavement.

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.

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 provided, if financially feasible, an area not less than 10 feet wide, which shall be reserved for the temporary accommodation of disabled or other stationary vehicles.

Drainage.—For the removal of drainage from the pavements, median strips, shoulders, or standing areas, and adjacent slopes of urban sections of the system, an underground drainage system shall be constructed, entrance to which shall be provided at suitable intervals and in appropriate places by means of drop inlets of adequate design and capacity, in such manner as to avoid all possible hazard to traffic and reduction of the traffic capacity of the pavements.

Gradient.—The gradient of urban sections of the system shall preferably be not steeper than 3 percent and shall in no case exceed 5 percent. In general, extremely long grades should be less steep and very short ones may be steeper than grades of moderate length. Grades to be traveled only in the downward direction on one-way roads may be steeper than the limits recommended above.

Width of right-of-way.—The right-of-way to be acquired for urban sections of the interregional system shall be at least sufficient to permit the construction of pavements, median strips, areas for deceleration, acceleration, and maneuvering, standing areas, side slopes, ramps, retaining walls, barrier strips, and service streets, or such of these facilities as may be required at any point, all constructed in accordance with the foregoing recommendations. The required right-of-way shall be acquired in its entirety by outright purchase or condemnation in accordance with the need for the planned ultimate development of the highway.

Foundations and bases, surfaces, and pavements.—All road foundations and bases, and all road surfaces and pavements on urban sections of the system shall conform to the basic standards hereinbefore recommended for foundations, bases, surfaces, and pavements on rural sections of the system.

II. Bridges and culverts.

Definitions.—All structures shall be classed as long bridges, short bridges, or culverts as hereinbefore recommended for rural sections of the system.

Alinement of bridges.—All bridges, wherever feasible, shall be so located as to fit the over-all alinement and gradient of the highway and shall be subordinated thereto. Where structural or architectural requirements make it desirable to adjust the alinement and gradient of the highway, the changes shall be such that the highway will meet all the basic standards for urban sections of the system recommended herein under “I. Roads.”

Width of bridges.—The width between vehicular curbs on all bridges built on tangents of urban sections of the system shall be at least 4 feet greater than the width of the pavement of the approach highway, and the lateral distances between the edges of the pavement of the approach highway and the faces of the vehicular curbs shall be at least 2 feet. Where the approach pavements have curbs adjoining the outer lanes thereof, curbs on the approaches and on the bridge shall be continuous.

On all bridges the lateral distance from the face of the curb to the face of the bridge rail or any structural member shall be at least 18 inches and as much more as is necessary for walk space. Where curbs on a bridge are continuous with curbs adjoining the outer lanes. of the approach pavement, the lateral distance from the outer edge of the approach pavement to the face of the bridge rail or any structural member of the bridge shall be 3 feet 6 inches.

On short bridges over streams, railways, and minor intersecting roads and streets, the lateral distance from the edge of the pavement of the approach highway to the face of the bridge rail, any structural member, or the curb shall be not less than the width of shoulder or the emergency standing area on the approach highway.

On sections of the system improved with divided. highways, one bridge to accommodate both roadways and the median strip or two separate bridges, each to accommodate one roadway, may be used. On short bridges the two roadways shall be accommodated without deviation from their normal alinement, and on long bridges the two roadways shall be so accommodated if feasible.

Where the median strip is reduced in width, the change in alinement shall be accomplished in such manner and over such lengths as will avoid hazard in vehicular operation at the design speed and the appearance of distorted or forced alinement. Where the median strip is reduced in width over structures the two roadways shall be separated by a raised, but mountable, dividing strip not less than 4 feet wide.

Auxiliary lanes on bridges.—Where auxiliary lanes or designs for other purposes provide a pavement over bridges that extends more than 2 feet outside the normal edge of through-traffic lanes, the vehicular curb shall be continuous with the curb on the approach highway or, in the absence of an approach curb, the vehicular curb shall be located at least 2 feet outside a line extending from the edge of the widened pavement on the approach.

Bridges on curves.—On all bridges on curves the clearances shall be at least as great as those required for all other bridges, and the lateral clearance to the face of the bridge rail or any structural member shall be as much more as shall be necessary to maintain the minimum sight distance used in the design of the section of highway on which the bridge is located.

Clear height of bridges.—The clear height of all bridges over the entire width between outer curbs shall be not less than 14 feet.

Openings under bridges.—All bridges over railways and_ public streets, roads, or highways shall provide a clear height and length between abutments or piers as hereinbefore recommended for bridges on rural sections of the system.

Pavements on bridges.—All bridge pavements shall be constructed as hereinbefore recommended for bridges on rural sections of the system.

Structural design of bridges.—All bridges shall be structurally designed as hereinbefore recommended for bridges on rural sections of the system.

Width of culverts —The over-all clear width of all culverts shall be equal to the sum of the widths of the surfaces or pavements, median strip, and shoulders or emergency standing areas of the section of the interregional highway in which they are installed.

On divided highways the two roadways shall be separated, over culverts, by a median strip of the width provided om the highwe approaching the culvert. In the case of culverts supercharged with an earth embankment, shoulders or emergency standing areas, and slopes shall be carried over the culvert, identical in design with the cross section of the approach highway.

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 highway and, if deemed feasible, at the intersecting highway also shall be designed as one-way roads separated by suitable channelizing islands. Entrances and exits shall be located at sufficient distances from any grade-separating structure to provide sight distances adequate for safety under the conditions of vehicular speed to be anticipated.

All ramps and connections shall be designed to enable vehicles to leave and enter the through-traffic lanes of each highway at 0.7 of its design speed, except where “stop” control is necessary. The maximum curvature of ramps and connections shall preferably not exceed 45°, and under no conditions shall exceed 70°. All curves shall be eased by transition or compounding.

Width of pavements and shoulders and side slopes.—All ramps and connections shall be made as wide and side slopes shall be made as flat as hereinbefore recommended for rural sections of the system. Shoulders may be omitted under appropriate conditions; but, if provided, shall conform to the standards recommended for rural sections of the system.

Added space for turning maneuvers.—Added space for turning maneuvers shall be provided, and shall be designed as hereinbefore recommended for rural sections of the system.

Gradients on ramps.—The gradient on ramps shall not exceed 6 percent on upgrades and 8 percent on downgrades.

Sight distance at ramps and connections.—On all ramps and connections the combination of grade, vertical curves, alinement, and clearances of lateral and corner obstructions to vision shall be such as to provide a sight distance along such ramps and connections and from their terminal junctions along the interregional highway and intersecting road or street consistent with the probable speeds of vehicle operation.

Access for busses.—Bus stops shall be prohibited on all urban sections of the interregional system. Access connections for bus stops off the interregional highway shall be designed to the same standards as other access connections.

V. Tunnels.

Tunnels on urban sections of the interregional system shall accommodate the same number and width of traffic lanes as are provided on the street or highway approaching the tunnel and shall conform to all recommendations hereinbefore made for tunnels on rural sections of the system.

VI. Pedestrian and recreational facilities.

On urban sections of the system, pedestrian usa of road surfaces and pavements shall be prohibited.

Pedestrian use of shoulders or standing areas shall be prohibited wherever feasible. Where walks are provided in these areas, they shall be separated from the edge of the pavement for vehicles by a curb and a strip at least 6 feet wide.

Where median strip or border areas of the interregional highway are of dimensions sufficient to permit their safe use for pedestrian purposes or for recreational facilities, such as rest and comfort facilities and playgrounds for both road users and nearby residents, their use for such purposes shall be encouraged either by the provision of these facilities or the assignment of space for their future development in accordance with an approved development plan. If so used, adequate steps, ramps, or walks shall be provided to give access to such areas from the adjacent service streets or ways, and, if necessary, barriers such as fences shall be provided to prevent pedestrian encroachment on the pavements for vehicular travel.

Adequate crosswalks for pedestrians shall be provided on all bridges and within all underpasses carrying intersecting streets or highways over or under the interregional highway, and steps or ramps may be provided from such bridges or underpasses to give pedestrian access to median strip or border areas capable of safe recreational or other pedestrian use.

In addition to the pedestrian-crossing facilities provided at intersecting streets or highways, special bridges or underpasses for pedestrians shall be provided at such intervals as may be necessary for the convenience of pedestrian crossing of the pavements of the highway.

Paths for pedestrians shall be equal in surface smoothness and accessibility to the surfaces provided for vehicular travel.

VII. Landscaping.

On all urban sections of the system the landscaping design shall conform, wherever feasible, to the recommendations hereinbefore made for rural sections of the system.

VIII. Signs and pavement markings.

The installation of traffic control signals shall be prohibited on urban sections of the system.

On all urban sections of the system, signs and pavement markings shall be provided as hereinbefore recommended for rural sections of the system, except that speed-control signs shall be used only where the maximum safe speed under normal conditions is less than 50 miles per hour, in which case signs warning of the approach, the presence, and the termination of such sections shall be provided in a manner similar to the recommendations made for rural sections of the system. Where State or local traffic regulations govern the maximum speed, such maximum speed limits shall be indicated at appropriate intervals by signs bearing the legend “Speed limit ——.”

IX. Lighting.

All urban sections of the interregional system shall be lighted by artificial fixed-source illumination to provide the maximum degree of safety and convenience of movement at night. At all connections illumination shall be provided for such distance and in such degree as may be necessary to provide a safe transition between the normal system lighting on the interregional highway and the normal degree of illumination on the connecting street. Illumination of the interregional highway shall not terminate abruptly at the limits of urban sections but shall be extended in diminishing degree for such distance as may be necessary to insure safe transition from lighted to unlighted sections or to sections on which the illumination is of lower degree.

X. Provision for public utilities.

The erection of electric light, power, and telephone poles and the construction of underground. utilities shall be restricted on urban sections of the system as hereinbefore recommended for rural sections of the systems.

Where underground utility lines, which require regular and not infrequent maintenance, repair, and replacement, cross urban sections of the system they shall be placed in service tunnels under the pavements to insure continuous and undisturbed operation of traffic on the interregional highway.

[1] Measured between one point 4.5 feet and another 4 inches above the road surface.

[2] Between points 4.5 feet above the road surface.

[n1-3] 1.0 ^1.1 This limit desirable on the 2-lane highways to be provided for this volume of traffic, in order to permit the maximum feasible speed of trucks and tractor combinations and correspondingly reduce the frequency of passing.

[4] This limit permissible because a divided 4-lane highway is to be provided for this volume of traffic where minimum passing sight distance Previously recommended is not feasibly obtainable, thus permitting passing of slow-moving trucks and tractor combinations at all points.

[5] This limit permissible because a continuously divided 4-lane highway is to be provided for this volume of traffic, thus permitting ready passing of slow-moving trucks and tractor combinations in the number to be expected.

[n4-6] 4.0 ^4.1 These limits desirable because of the greater number of slow-moving trucks and tractor combinations to be expected and the consequent necessity to permit the maximum feasible speed of such vehicles to reduce the frequency of passing.

[7] No existing roads conforming to the recommended interregional system in mountainous topography carried this volume of traffic before the war.

[8] It is recommended that the details of design of all signs and pavement markings, as herein generally described, be defined by the joint committee on uniform traffic control devices, appointed by the American Association of State Highway Officials, the Institute of Traffic Engineers, and the National Conference on Street and Highway Safety; and that signs and markings of the character so defined be adopted and used on all parts of the interregional system in all States.

[1]

[1]

[1]

[2]

[3]

[4]

[5]

[1]

Interregional Highways/Appendix 5

Navigation menu

Search