# Interregional Highways/Principles of landscape design

Washington, DC: United States Government Printing Office, pages 89–92
PRINCIPLES OF LANDSCAPE DESIGN
Highway design, in the broadest sense, rests upon landscape principles as well as upcn the more commonly recognized engineering principles of alinement, profile, grade cross-section, roadway and right-of-way width, drainage, and structural strength and durability. A balanced agreement with the two sets of principles characterizes the best design.
Flowing rather than abrupt change of gradient and alinement are necessary from the engineering standpoint for promotion of the safety and ease of vehicular movement and for increase in the highway’s traffic discharge capacity. They are equally necessary to fit the road gracefully into its natural environment, which is the essence of good landscaping.
Flattened slopes of excavation and embankment and a well-rounded cross-sectional contour are essential to prevent soil erosion and to minimize the risks of injury and damage when vehicles accidently or unavoidably leave the roadway. They are needful also to mold the highway into the terrain and to make it a harmonious feature of the natural landscape.
Marginal land strips, publicly owned or controlled, are required for the engineering reason of protection of vehicles moving on the highway against collision with entering vehicles, and of operators of moving vehicles against various roadside distractions. For landscaping reasons marginal land strips are needed to make possible a pleasing transition between the lines and plantings of the highway and the natural slopes and growth of the adjacent lands, to permit the screening of unsightliness, and to provide stopping space from which to view unfolded natural beauty.
If engineering principles require a certain monotony of smoothness and attention-lulling security in the roadway design, the appropriate application of landscaping principles can relieve the monotony and promote the safety of traffic by reawakening the interest and attention of drivers.
The interregional highways, in their rural sections especially, will serve a traffic composed in large degree of vehicles driven in the pursuit of pleasure or recreation. Sound landscape design will increase the pleasure and relieve the strain of all journeys.
In their urban and suburban sections, the interregional routes will carry a heavy, bustling traffic. Adequately landscaped borders will eliminate the traffic hazards of closely crowding buildings, and insulate adjacent residential and business properties, churches, and schools from the noise, dust, and fumes of traffic.
Landscaping for rural sections of the system.—Consideration of landscaping desiderata should pervade all stages of the location, design, and construction of rural sections of the interregional system, and a proper regard for landscape principles in the design will simplify and ​increase the effectiveness of maintenance processes and lower the cost of adequate upkeep.
Without sacrifice of distance or economy it will often be possible to bring the highway into view of a lake or river, an interesting rock formation or wooded hill. At no greater expense one location will provide frequent opportunities for distant vistas of natural charm that are unobtainable in an alternate location. For such enduring investments as the interregional routes there should be no sparing of whatever thought and care may be necessary to place these roads in locations of utmost fitness from every point of view, and this includes the fullest practicable development of scenic possibilities, consistent with the primary requirements of traffic service.
In the acquisition of right-of-way, thought should be given not only to the width required for the physical highway facility, but also to that required for protection against encroachment and protection and enhancement of the view from the highway.
The former will involve the creation of a space barrier Between the highway and roadside activity of any character whatever—space in which to screen from view disturbing or objectionable activity and space in which to control access to gasoline stations, restaurants, wayside stops, and other similar service facilities of an essential character.
The latter will involve the obtainment of space in which to screen the view of signboards and objectionable and unsightly objects, to blend the road verge into the natural landscape, to frame pleasing vistas, and to accommodate historical markers, overlooks and other halting and resting places in attractive surroundings.
The planning of clearing operations should provide for the conservation of desirable existing vegetation and trees and the saving of topsoil to the greatest practicable extent; and the grading should provide the flattened lateral slopes and rounded contours that are necessary to mold the highway into its natural surroundings.
On two-lane sections, the width of roadway and shoulders will be determined by traffic considerations, and the necessity of long sight distance to permit maximum facility of passing will limit the use of curvature for landscape effect. On such sections an ample right-of-way, variously and appropriately treated, can do much to relieve the monotony of driving over long, smooth stretches of straight highway, and will contribute largely to the safety as well as the pleasure of travel. Under these conditions the value of land is likely to be relatively low and the need of a reserve of space for future road widening will supplement the requirements of appropriate landscape treatment in support of the economy of a present liberal acquisition of right-of-way.
On divided, four-lane sections in rural areas, variation of the width of the median strip, a permissible more liberal use of curvature, and separate adjustment of the grades of the divided roadways to the natural slopes of the terrain will add interest to the landscape treatment and often reduce the cost of construction. Where the location lies on the side of a hill or a gentle cross slope, for example, construction cost will usually be substantially lowered by building the separate roadways at different levels, and travelers on both roadways will have an unobstructed view of the countryside (see plate VII). A similar divergence in the alinement of the two roadways to take advantage of natural topographic conditions, such as location on the ​opposite banks of a stream or on the two sides of a local depression or rock outcropping, will likewise reduce costs and at the same time permit the conservation of interesting features of the natural landscape. And, even where there is no topographic reason for doing so, an opportune slight variation of the curvature of the two roadways will alter the width of the median strip and relieve the monotony of long parallel lines, without effect upon the total requirement of right-of-way width. An important result of all such variations in the lines and grades of the two roadways will be realized in reduction of the hazards of headlight glare in night driving.
As in the location and construction of the routes, design for utility and economy is found to go hand in hand with sound landscape design, so also a properly landscaped highway will be a highway easy to maintain. The flattened side slopes will favor the growth of vegetation, prevent erosion and thus remove the cause of much troublesome clogging of the drainage system. The easier slopes can be mowed by machine instead of by hand methods, and the streamlined contours of cut banks will reduce snow drifting and facilitate machine methods of snow removal.
It will be observed that there has been no mention in the foregoing of the tree planting that is so widely associated with the idea of roadside improvement. The omission has been intentional. There is no place in sound rural highway landscaping for the regular or row planting of trees. The objective should be the preservation or, where necessary, the re-creation of a natural foreground environment in harmony with the distant view. To that end, existing well-placed and beautiful trees should be preserved wherever possible; unpleasing and view-obstructing growth should be removed; and only where the irregular introduction of trees and other growth will serve to highlight the natural beauty of the roadside view or where it is especially desirable to screen unsightly or distracting objects or activity should the replanting of trees receive consideration. Trees replanted for such reasons should be invariably native to the environment.
The landscaping of urban sections.—In cities and their nearer suburban areas the opportunity for employment of the locational devices of landscape treatment will be more limited. But the general straightness of right-of-way alinement there necessary for avoidance of conflict with the existing street plan need not confine the roadways of the interregional routes to rigidly straight lines. Within a block-wide right-of-way the separate roadways may be constructed at different levels in adaptation to an existing transverse slope. The grades of both roadways may be gracefully rolled, dipping to pass under bridges at the crossing streets and rising between to approximate the level of the existing streets which form the local service ways. As they rise and fall the separate roadways may be caused to diverge and converge in alinement, thus varying the width of the median strip. Or the two roadways may be swung to one side of the right-of-way with only a narrow median strip intervening there, for example, to pass under a crossing bridge located off-center with respect to the right-of-way. To gain space for desirable widenings of the median strip or lateral park areas, retaining walls may be used to reduce the width required for slopes in depressed sections, but these should preferably be constructed at the edge of the service ways and never in cramping proximity to the roadways of the express route. The widened central or lateral areas may be used for appropriate plantings or for rest or playground area ​approached by pedestrian bridges or by steps from a crossing bridge or street. A treatment of this general character is suggested in plate X.
On urban sections of the routes the planting of trees in formal arrangement will be more appropriate than on rural sections. A tree screen may be used to separate the highway from an adjacent railroad, freight yard, or industrial siding, or to conceal other unsightly or objectionable roadside conditions. Trees in formal arrangement may be set against the straight lines of the local service ways to insulate bordering residential property from the restless movement of traffic on the expressway. But everywhere the effort should be made to avoid monotony and tiresome sameness in such plantings over long stretches of the routes.
Small flowering trees and vines may be appropriately set in the wider median or lateral areas and on the side slopes of depressed sections to vary the sameness of long stretches of uniform turfed banks. And every section of retaining wall at crossing bridges and against the local service ways will offer the opportunity for attractive groupings of small flowering trees, masses of colorful roses, and other low-growing plants in suitable relation.
All these things may be done in complete consistency with the utilitarian functions of the expressways. And, so treated, these new arterial ways may be made—not the unsightly and obstructive gashes feared by some—but rather elongated parks bringing to the inner city a welcome addition of beauty, grace, and green open space.