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• For empty (eastbound) trucks, the lowest passby noise levels were measured at speeds of 38-43 mph, with engines in 8th gear at 700-800 rpm. The mean SEL under such conditions was computed to be 81 dB SEL at 40 feet. • For loaded (westbound) trucks, the lowest passby noise levels were measured at speeds of 33-35 mph, with engines in 61h or 7`h gear at 1400-1600 rpm. The mean SEL under such conditions was computed to be 83 dB SEL at 40 feet. The noise level results cited above for the optimized heavy truck operating parameters are 5 dB and 3 dB lower than the previous mean heavy truck passby test results for empty and loaded trucks, respectively. Therefore, by instructing the truck operators to utilize the combinations of speed, rpm, and gear cited above, an average reduction of 4 dB can be realized over non- optimized operating procedures. This 4 dB reduction results in a reference heavy truck passby SEL of 82 dB at a distance of 40 feet from the centerline of the roadway. The truck passby noise optimization tests also revealed that noise due to the truck passbys decreased at a rate of approximately 4.5 dB per doubling of distance from the roadway, which is the expected theoretical value based on analysis of moving point sources. The remainder of this analysis assumes project truck drivers will be instructed to operate their vehicles in the quietest manner possible, thereby generating SEL values of approximately 82 dB at a distance of 40 feet from the truck passby. It was further assumed that sound radiating away from the truck passby would decrease at a rate of 4.5 dB per doubling of distance from the source. Environmental Noise Analysis Munn& Perkins Expanded Hours of Operations—San Joaquin County Page 28