75C - PH - BRISTOL EIR FROM WARNER TO ST. ANDREW

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criteria in all cases except where front -yard impacts occur on corner lots. City standards require a 25 -foot setback of noise walls from a corner to allow for adequate sight distance. For this reason, the resulting location of barriers along Bristol Street may not fully attenuate noise levels to meet the federal criteria. Second Line Receptors Second line receptors are buildings that are currently acoustically shielded by a row of buildings located between them and the roadway. Since the proposed projects could ultimately result in the removal of 200 existing buildings adjacent to Bristol Street, there is the potential for adverse noise impacts at approximately 200 second line receptors. Barriers will be constructed, where feasible, to shield the second -line receptors that will exceed the 67 Leq standards (see Figure 19). In addition, barriers could be constructed per current Caltrans practices to shield those second -line receptors where exterior noise levels will approach the 67 Leq standard. After construction of the noise walls for the second line receptors, the attentuated noise levels will range from 59 to 63 Leq. It should be noted that final design may not include barriers at the locations identified as approaching the 67 Leq standard. Attenuation due to shielding is an important mechanism by which highway sound levels are reduced. Shielding occurs when the observer's view of a highway is obstructed or partially obstructed by an object or objects which significantly interfere with the propagation of the sound waves. Shielding can be provided by rows of buildings and /or existing barriers. The amount of attenuation provided by rows of buildings is determined by the portion of the row that is occupied by the buildings. For example, a 3.0 dBA additional attenuation is provided by the first row of buildings when the structures occupy 40 to 65 percent of the length of the row. A 5.0 dBA attenuation is afforded when the buildings occupy 65 to 90 percent of the length of the row. No attenuation is allowed for rows of houses which occupy less than 40 percent of the length of the row. Each successive row provides 1.5 dBA of additional attenuation until a total attenuation of 10.0 dBA for all rows is obtained. This is the maximum attenuation that this mechanism provides.Any excess attenuation by ground effects (i.e. 4.5 dBA /DD versus 3.0 dBA /DD) is assumed to end when the sound waves reach the first row of buildings. Consequently, the shielding provided by rows of buildings is only additive to the attenuation provided by geometric spreading (3.0 dBA /DD). IV -33 75C -168