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Qz types of Carriers are suited for all cli- mates, and local conditions may cause significant differences in the mainte- nance cost of the various barrier types. The cost questions must be evaluated on a local basis. Some of the frequently used materials for fence and wall construction are masonry, precast concrete, and wood. Masonry noise barriers can be made of concrete blocks, brick or stone. A con- crete block barrier might range in cost from $10 a linear foot for a 6 -ft. high wall, to $75 a linear foot for a 12 -ft. high wall. This latter figure includes a safety rail- ing. In general, a concrete block wall would cost $50 to $60 a linear foot.l To alleviate the monotony of a long run of wall, pilasters can be used: a 20 ft. high concrete wall with pilasters might cost $300 per linear foot.2 Brick and stone are extremely expensive and should only be used for special aesthetic considera- tions.3 Precast concrete panels offer opportuni- ties for cost reduction. A 13'4" high wall in Fairfield, California constructed of pre -cast concrete panels cost only $29.50 per linear foot. Wood noise barriers are another possibil- ity. They tend to be less expensive than other methods but are not as durable. An estimated cost for a 6' high 518" plywood fence is $5.00 per linear foot.4 C) Plantings Plants absorb and scatter sound waves. However, the effectiveness of trees, shrubs, and other plantings as noise reducers is the subject of some 1 Figure provided by an official of the California Highway De"n- ment. =Representative cost estimates of materials and labor of construc- tion but excluding real estate acqutaklon; private debate.: conclusions can, however, be drawn. • Plantings in a buffer strip, high, dense, and thick enough to be visually opaque. will provide more attenuation than that provided by the mere distance which the buffer strip represents. A reduction of 3-5 dBA per 100 feet can be expected. Shubs or other ground cover are neces- sary in this respect to provide the re- quired density near the ground. • The principal effect of plantings is psy- chological. By removing the noise source from view, plantings can reduce human annoyance to noise. The fact that people cannot see the highway can reduce their awareness of it, even though the noise remains. • Time must be allowed for trees and shrubs to attain their desired height. • Because they lose their leaves, decidu- ous t rees do not provide year-round noise protection. In general, plantings by themselves do not provide much sound attenuation. It is more effective, therefore, to use plant- ings in conjunction with other noise re- duction techniques and for aesthetic enhancement. The cost of plantings varies with the species selected, the section of the country, the climate, and the width of the buffer strip. For deciduous trees and evergreens, costs range from $10 to $50 a linear foot. The width of such a strip would be approximately 40 feet for deci- duous trees and 20 feet for evergreens. Planting shrubs between the trees so as to form a dense ground cover would double the price. 3Caiifornia Division of Highways, Highway Noise Control, Value En- gineedng Sfudy, (October 1972), p. 33. aCanfornia Division of Highways, .40wary Noise Control. Valor En- 54 0)Combinations of Various B& 7esigns Of ten, the most economical, stically t acceptable, and aesthetically pleasing barrier ,s some combination of the barrier types previously discussed. F;;t example, the Milwaukee: County Eyoressway and Transportation Commis- sion feels that barriers constructed of ;,re -cast concrete on top of an earth berm provide maximum benefit for the cost.5 They estimate that such a co Tibination costs $.51 per linear loot. In addition to cost advantages an earth berm with a barrier wall on top of it pos- sesses several other advantages over both a wall or a berm alone: 1) it is more visually pleasing than a wall of equivalent height; 2) the berm portion of this com- bination is less dangerous for a motorist leaving the roadway; 3) the non -vertical construction of the berm does not reflect noise back to the opposite side of the highway the way a wall does; 4) the combination requires less land than would be required for a berm of equiva- lent height and slope; and 5) the wall provides a fencing function not provided by a berm. Another combination to be considered is that of plantings in combination with a barrier. Not only do plantings and ground cover provide some additional noise at- tenuation, but they also increase visual appeal. 4.5 Conclusion Figure 4.19 provides a summary of the physical techniques which can be used by designers, builders, and developers to reduce highway noise impacts. Some gireenng Study. iOctober 1972) p• 46• S Milwaukee County Expressway and Transportation Commission, Norse Impact Study of the Airport Spur. V. 0: Technical Report, (March 1973). pp. 7-21.