Choosing Plants
Plants that are to be used along the roadside must be chosen with care, particularly with respect to the type of environment into which they will be placed. The highway environment presents a number of adverse situations that must be tolerated by the plants in order to survive. One condition that has been identified in recent years is often hazardous to both plants and man. That condition is air pollution.A major cause of air pollution today is the internal combustion engine used by millions of people in vehicles along our highways. Hazardous conditions are most likely to occur in dense, urban situations, and under atmospheric conditions which prevent or hinder air movement. Some plants are sensitive to this condition; others tolerate it. In urban situations where air pollution is likely to be a problem, plants which are resistant should be chosen.
Droughtis another problem which is likely to occur along highways. In areas where annual rainfall is low, it is near impossible to maintain plantings without artificial irrigation. The costs for this are high; however, the benefit gained is often worth the price.
When properly used in a noise abatement system, plantings serve a distinct and necessary function. Since plantings contribute to the visual quality of noise barriers, proper steps to ensure their survival should be taken as part of the planning process. Drought resistant plantings should be used in arid regions; however, often even these will have difficulty in surviving along the highway, particularly for the first few years after planting. Some type of irrigation to augment natural rainfall should be included as part of the noise abatement project.
The use of a thick layer of mulch, such as wood chips, has proven to be a valuable aid to survival of plantings. A mulch will hold moisture, but also acts as an insulator, which helps to reduce evaporation losses resulting from the soil temperature on hot days. Mulches have the added advantage of inhibiting the growth of weeds, which compete with plantings for available moisture.
Plants that are relatively resistant to disease and pestsare more likely to survive along the highway without requiring additional maintenance. Most highway plantings are likely to receive a minimum of maintenance, in the form of pruning, weed control, or preventative pest control. For this reason, native plants which are disease and pest resistant should be chosen for highway use, since they are more likely to thrive under low maintenance.
In northern areas of the country which have severe winters, plants along the highway are subject to damage by saltused on the roads. Some plants, particularly evergreens, are very susceptible to excessive salts in the soil. Other plants tolerate this condition fairly well. The designer should consider the possibility of salt damage to plants where this is utilized on the roads and confine the use of evergreens and other susceptible plants to areas that are not likely to receive water runoff from the highway. Salt spray from vehicles is also a problem which requires that susceptible plants be located at a sufficient distance from the pavement. Salt tolerant plants should be used wherever possible to minimize damage or plant loss.
There are a number of sources which are of use to the designer in choosing plant materials suitable for the highway environment. The following list is intended as a basic guide which should be augmented with locally available information. This may be available through state agricultural extension offices or state university horticultural departments, which should be able to make recommendations as to plant materials native to the area. In addition, most highway departments employ horticulturists who are familiar with plant species for the local hardiness zone and with highway planting requirements. For further information:
Cathey, H., Growing Ornamentals in Urban Gardens,USDA, Washington, D.C. 1971.
Lumis, G. P., Hofstra, G., and Hall, R., "Salt Damage to Roadside Plants", Journal of Arbori-culture,Volume 1, Number 1, 1975, pp. 14-16.
Murphy, L. M., Foote, L. E., and Doerr, Landscape Planting Species Survival,Minnesota Department of Highways, Investigation Number 628, 1971.
Sucoff, E., Effects of De-icing Salts on Woody Vegetation Along Minnesota Roads,Minnesota Agricultural Experiment Station, Technical Bulletin 303, 1975.
United States Department of Agriculture, Landscape for Living,U.S. Government Printing Office, Washington, D.C., 1972.
Wyman, Donald, Shrubs and Vines for American Gardens,Macmillan, New York, 1969.
Wyman, Donald, Trees for American Gardens,Macmillan, New York, 1969.
Zak, John M. et al., A Handbook for the Selection of Some Adaptable Plant Species for Massachusetts Roadsides,University of Massachusetts, Massachusetts Department of Public Works, Report 24-R5-2656, August, 1972.
Noise Absorption by Highway Noise Barriers
The current theories of highway noise abatement advocate the use of solid barriers which are reflective, in that they redirect a sound wave which strikes the surface of the barrier. In many situations, this type of noise abatement is acceptable. However, in certain situations, this may result in an unwanted reflection of noise, with a corresponding increase in noise on the opposite side of the roadway. Considerable research has been undertaken in recent years in order to develop noise absorbing barriers. Absorptive barriers have been tested and are in use along highways in Europe and appear to be beneficial in reducing noise levels, particularly in dense urban situations. Noise reflection by tall buildings adjacent to the roadway often results in higher noise levels within the right of way and between the roadway and adjacent buildings. In similar urban situations, consideration should be given to the use of absorptive material for barriers.
The highway environment prohibits the use of a wide variety of noise absorptive materials which are used indoors, in protected situations. Rain, ice, snow, dust, dirt, and temperature extremes combine to reduce the effectiveness of the absorptive material, or hasten deterioration, which makes these materials economically unfeasible for highway use. Research undertaken as part of this study has identified several materials and potential designs which may have an application for highway noise abatement and are presented herein for information purposes. The proposed designs require additional study, including the construction and testing of prototypes, in order to determine the performance and feasibility of these designs.
Noise Absorptive Materials Which are Currently Available
"STARKUSTIC", product of Stark Ceramics, Inc., Canton, Ohio 44711
"STARKUSTIC" ceramic is a structural tile that employs a glazed, perforated face on one side. The tile is hollow and four inches in thickness. The horizontal core is filled with a fiberglass pad which is vermin resistant, chemically inert, and steam clean-able. Moisture drains through the perforations.
The glazed ceramic surface of STARKUSTIC tile resists oils, grease, and other stains. Colors are fired in and will not fade-minimum maintenance is required. The tile and absorbent lining will not support combustion. Both a smooth face and a textured face are available. The tile is available in 5" x 12" and 8" x 16" face sizes.
"SOUNDBLOX", product of the Proudfoot Company, Inc., Post Office Box 9, Greenwich, Connecticut 06830
"SOUNDBLOX" are structural masonry blocks which employ vertical slots in one face of the block; the slots open to a cavity similar to other concrete blocks. The hollow chamber acts as a resonator to absorb noise. The cavities contain a fibrous filler or metal septa; the latter acts as an additional resonator. The blocks can be manufactured locally by approved concrete block manufacturers and are installed in a similar manner to other concrete block masonry walls.
"SOUNDBLOX" have been installed in outdoor situations, primarily to reduce noise from electrical transformers, and have proven to be efficient in reducing noise levels below that which could be expected with conventional masonry blocks.
"BROAD BAND SOUND ABSORBER" (NFA-1) developed by Lord Corporation/Allforce Acoustics, 3016 West Lake Road, Erie, Pennsylvania 16512
NFA-1 is a non-fibrous absorber designed to absorb sound by virtue of its geometry rather than the nature of the material from which it is constructed. It is similar to a Helmholtz resonator, in that it employs a triangular, hollow chamber, which traps the noise. However, this chamber contains an array of cells which vary in depth in a honeycomb arrangement. The variety in cell depth accounts for the broad range of frequencies that can be absorbed.
The chamber can be manufactured from a variety of materials, including aluminum, resin impregnated fiberglass, and impregnated paper. At this writing, Lord Corporation is actively engaged in the development and testing of additional materials from which to manufacture this product. Advantages of this type of sound absorber are that it potentially can be applied in outdoor environments, there is no fibrous filling to clog or deteriorate, and the cells can be readily cleaned by steam or high pressure water.
"NOISHIELD" product of Industrial Acoustics Company, Bronx, New York 10462
"NOISHIELD" is a modular absorptive barrier constructed of galvanized sheet metal. It employs a fiberglass filling that is protected by a perforated or porous material on one side and a solid, impervious surface on the other. "NOISHIELD" modules are also available in Cor-Ten Steel. The absorptive materials are claimed to be mildew and vermin proof and resist oil, dust, and water. The modules are four feet by twelve feet and are four inches thick. "NOISHIELD" barriers of this type are currently in use in railroad switchyards, and their performance in the outdoor environment can be readily assessed.
"PERMA-DELTA SOUND BARRIER" product of Permapost Products Company, P.O. Box 121, 25600 S.W. Tualatin Valley Highway, Hillsboro, Oregon 97123
The Perma-Delta Sound Barrier consists of a series of vertical, triangular shaped chambers which are similar to Helmholtz resonators in design. The chambers are constructed of treated plywood and are mounted upon a backing panel of plywood which serves as support and additionally as a noise attenuation surface. Sound waves are admitted to the interior of the chambers through narrow slots between chambers. The barrier is constructed in modular form. Each modular panel can be supplied in 2' x 4' or 4' x 8' sections. The manufacturer claims up to 13 decibels in noise reduction can be achieved with this product. Initial testing was completed by Oregon State Highway Acoustics Division; further testing of this product in a highway situation is recommended.
"COUSTIVIEW" product of Ferro Corporation, Composites Division, 34 Smith Street, Norwalk, Connecticut 06852
"COUSTIVIEW" is a transparent noise barrier material constructed from PVC, which is claimed to be resistant to yellowing, fading, and clouding. The material is a limp sheet of transparent film which must be supported in a framework of some type. PVC is normally used in manufacturing industries for visual monitoring of machinery, while providing some degree of noise protection. There are many situations where a transparent highway noise barrier would be advantageous, particularly where required sight distances would be adversely affected by a solid barrier. This material should be tested under outdoor environmental conditions in order to determine its ability to withstand temperature extremes, etc. "COUSTIVIEW" is available in 48 inch by 20 yard rolls in 0.04" or 0.08" thicknesses.
Manufacturers of Highway Noise Barriers
Armco Steel Corporation, Metal Products Division, Middletown, Ohio
Armco offers several types of steel noise barriers constructed of "steelox" structural panels. Panels to be used in horizontal applications are either flat or sculptured rib design and are mounted to exposed vertical steel posts with self-tapping screws. Vertical panels are available in a trapezoidal corrugated rib pattern, and are mounted to vertical steel posed vertical steel posts with self-tapping screws. Vertical panels may be applied as a double skin, or in an alternating pattern, to cover the exposed vertical supports. Steel corner flashings, top caps and end caps are available and recommended for use with a vertical panel wall.
Panels are available in nine colors of fluoropolymer enamel, which is resistant to weathering, salts, and stains.
For severe environments, an acrylic film coating is available which is further resistant to corrosion by salts used along the highway. Base metal is galvanized or may be specified with an aluminum coating for further resistance to deterioration.
Fanwall Sales Corporation, Box 868, Framingham, Massachusetts
Fanwall is a pre-cast concrete modular panel system which can be constructed in a variety of configurations. By assembling the panels at an angle to one another, an extremely stable wall which requires no footings can be constructed. Fanwall claims that noise barriers of this type can be constructed quickly and at a cost competitive with other barrier types. A variety of surface textures are available including exposed aggregate, striated, simulated wood, stone and brick. Fanwall has developed a transparent barrier of similar design using polycarbonate sheets in either a continuous or staggered arrangement. Standard colors available with cast concrete may be applied to any Fanwall panel during the casting process.
Maccaferri Gabions, Inc., Box 43A, Governor Lane Boulevard, Williamsport, Maryland 21795
Gabions supplied by Maccaferri are constructed of woven wire mesh which is available in a galvanized finish or galvanized with a black PVC coating. PVC coating is recommended in locations where extreme corrosion due to salts is likely. A variety of gabion sizes are available. The manufacturer recommends that free standing walls be constructed in a pyramid arrangement if wall height is to exceed 9 feet; otherwise gabions may be stacked directly upon one another to form a vertical wall. Rock fill to be used with gabions should be approximately 4" x 8" in diameter and preferably should be washed.
