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eleven of which are encompassed by the 60 dBA Ldn contour, and five of <br />• which lie on the 65 dBA Ldn contour. Ldn sound levels are not predicted <br />to exceed 65 dBA at any residence. <br />It should be noted that the FHWA model predictions are rarely exact <br />predictions. Experience in correlating measured field data to model pre- <br />- dictions has indicated the model is generally accurate within two to three <br />-- d6. For this particular application, the validity of model predictions is <br />corroborated by the fact that the current production 55 dBA Ldn contour <br />extends approximately 200 feet to either side of SH 133. This result is <br />consistent with contours developed by Impact, Ltd. from field observations <br />of the operation at 650,000 TPY in 1977. Results of that study were pre- <br />rented in a report prepared for Thorne Ecological Institute in 1977 entitled <br />• "Colorado Westmoreland Noise Impact Study". <br />For this case, model predictions are expected to be over-estimates <br />,_ because several conservative factors are inherent in the analysis: <br />• Variations in topography were not considered excluding the <br />attenuation effect of increased distance and the barrier <br />.J <br />effect of existing structures and landforms. <br />r. <br />~ • A seven percent grade was assumed for all of Stevens Gulch <br />L. <br />Road and a corresponding adjustment made for increasing <br />ascending truck noise emissions. This assumption is appli- <br />cable only for portions of Stevens Gulch Road. <br />• The model does not account for the fact that the Savage Brothers <br />trucks are required by contract to comply with California noise <br />• <br />L <br />-24- <br />L <br />