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72 C H A P T E R 1 0 This chapter focuses on guidance on the use of tree zones within FHWA TNM. Appendix I (available on the NCHRP Project 25-34 web page at http://apps.trb.org/cmsfeed/ TRBNetProjectDisplay.asp?ProjectID=2986) provides sub- stantial detail on the methods and test cases that were used to develop the guidance. 10.1 Overlaid Loose-Soil Zone Not Needed with Tree Zones Prior to this current research, it was believed that TNM would incorrectly compute attenuation for tree zones placed on ground types other than loose soil. Were this true, then a loose-soil ground zone would need to be input, overlaid on each tree zone, for TNM to compute correct tree attenuation. No overlaid ground zone of any type is needed for TNM input to properly compute tree attenuation. However, it is important to use default ground type or a ground zone type for the tree zone that is consistent with the actual ground present under the vegetation. 10.2 Guidance for Narrow Tree Zones Figure 48 shows a narrow tree belt that intervenes between roadways and a receiver. As the text in Figure 47 indicates, an observer at the receiver position can see some traffic when looking perpendicular to the roadway, toward its closest por- tions. However, when looking at a significant skew angle, no traffic is generally visible due to the much longer path through the trees. Computations conducted for such narrow tree belts show that belts of trees up to 25 ft thick (and marginally up to 50 ft thick) provide no attenuation to the nearest portions of a roadway. Therefore, it may seem unnecessary to include narrow tree zones as TNM input. Further, TNM guidance is that vegetation must be âsufficiently dense to completely block the view along the propagation path.â Nevertheless, these computations also show that attenu- ation down the roadway from the receiver is significant, automatically per TNM, because of the extra depth of trees in those skew directions. Therefore, with narrow vegeta- tion zones, less sound energy will reach receivers from more distant sections of a highway than will if such zones are not included. As a result, omitting narrow vegetation belts (up to 50 ft in width) as TNM inputs could result in a noise barrier that was longer than necessary, thereby increasing noise barrier costs at each end of the barrier. Appendix I provides some examples from test cases run with TNM. 10.3 Attenuation Dependence on Visibility through Tree Zones An especially useful article by Fang and Ling examines the dependence of tree/shrub attenuation on degree of visibility through a wide variety of different kinds of vegetation.37 The measurements conducted and described in the article were compared with TNMâs built-in tree attenuation to develop guidance for TNM users who are (1) comparing TNM com- putations with field measurements at tree/shrub locations along a project roadway or (2) deciding when to include TNM tree zones in TNM computations for a project roadway. Fang and Ling developed the equation below to summa- rize their extensive measurements of attenuation through vegetation: Tree Zones 37 Fang, C.-F., and D.-L. Ling, âInvestigation of the Noise Reduction Provided by Tree Belts,â Landscape and Urban Planning, Vol. 63, pp. 187â195 (2003). Avail- able at http://ir.lib.ncut.edu.tw/bitstream/987654321/2472/1/2003-Investigatio n+of+the+noise+reduction+provided+by+tree+belts.pdf.
73 that site perpendicular to the roadway. This is measured by walking into the vegetation until no longer visible from the outside and then averaging this visibility distance over three tries at each of two locations. To interpret the measurements, analysts should do the following: 1. Determine TNMâs vegetation attenuation by computing TNM with and without an intervening tree zone. 2. Compute a visibility-based attenuation using Fang and Lingâs equation (above). 3. Compare the two attenuations: â If they are both nearly zero, then a tree zone is not needed. â If they are not zero but are nearly the same, then the TNM tree zone is computing well enough. â If they are not zero and are not the same, analysts should compare the TNM-computed attenuation and the visibility-based attenuation, and, based on this comparison, an appropriate tree-attenuation value should be chosen. Accurate computation at specific sites may require combined use of TNM tree zones and an additional negative adjustment factor to compensate for TNMâs under-prediction of tree attenuation. A DVeg Vis dBA ft ft50 4 08 2 87 1 2 32= â ï£«ï£ï£¬ ï£¶ ï£¸ï£· +. . log . log LVegProp ft1 ï£« ï£ï£¬ ï£¶ ï£¸ï£· ï£® ï£°ï£¯ ï£¹ ï£»ï£º where AVeg is the vegetation attenuation (in dBA per 50 ft). DVis is the visibility distance into the vegetation (in ft), which was measured by walking into the vegetation until no lon- ger visible from the outside and then averaging this vis- ibility distance over three tries at each of two locations. LVegProp is the vegetation path length (in ft). Whenever vegetation intervenes at a TNM measurement site, analysts should measure visibility into the vegetation at Some traffic visible in this direction No traffic visible in this direction Trees Roadways Figure 48. Roadway visibility through a narrow tree belt.