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Limitation of Reflective Noise Analysis <br />• Under CEQA, a 3 dB increase is considered significant where noise levels are <br />above 60 dBA (day-night). Where noise levels are below 60 dB, a 5 dB <br />increase or greater is considered to be significant. FHWA and Caltrans use <br />much higher thresholds ~12 dB <br />• 3 d6 is considered barely perceptible in the environment and has been <br />established as a measure of significance for most local governments <br />• The modeling is conservative and assumes that the new sound walls would act <br />as acoustic "mirrors." Multiple shielding features (e.g., housing rows) that would <br />reduce noise further were not taken into account in the modeling <br />• The greatest increases measured in the field are 2.5 dB in research studies and <br />some walls that were studied had immeasurable effects (i.e., less than 1 dB) <br />• The use of absorptive materials will add 25-30% to the cost <br />