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Chapter 4 Environmental Setting, Impacts, and Mitigation Measures As stated in the Thresholds of Significance, a 3.0 dBA CNEL increase is considered substantial if the noise increase would meet or exceed the City's 65 dBA CNEL noise level standard at high density residential uses, and a 5.0 dBA CNEL increase is considered substantial where existing noise levels are below the 65 dBA CNEL standard at high density residential uses. As shown in Table 4.8-11 (Traffic Noise Impacts for Year 2030) and Table 4.8-12 (Traffic Noise Impacts for Year 2035), none of the roadways within the Transit Zoning Code (SD 84A and SD 8413) area would result in a 3.0 dBA or 5.0 dBA CNEL increase over existing conditions with implementation of the proposed project under both scenarios. Likewise, the contribution of the proposed project to this increase under both scenarios would not exceed 3.0 dBA or 5.0 dBA CNEL. Therefore, this impact would be considered less than significant. Effects Found to Be Significant Threshold Would the project result in the exposure of persons to or generation of noise levels in excess of standards established in the local general plan or noise ordinance, or applicable standards of other agencies? Impact 4.8-8 Operation of the Southern California Regional Rail Authority's (SCRRA) rail line would potentially expose noise -sensitive land uses located within the Transit Zoning Code (SD 84A and SD 84B) area to noise levels that exceed the standards established by the City of Santa Ana General Plan. This is considered a significant and unavoidable impact. Under the Transit Zoning Code (SD 84A and SD 8413), additional sensitive uses (primarily residential structures) would be located in close proximity (approximately 100 feet) to the SCRRA rail line. As stated previously, typical commuter train noise produces a noise level of 80 dBA at 50 feet from the tracks, while a stopped commuter train would produce a noise level of 65 dBA. Per the Federal Railway Administration, noise levels associated with trains are anticipated to attenuate/reduce at a rate of 4.5 dBA for each doubling of distance. As such, potentially noise -sensitive uses, such as residential structures, would likely experience noise levels ranging from 60.5 to 75.5 dBA due to the physical movement and idling of commuter trains along the SCRRA rail fine. I Iowever, in addition to movement and idling noise levels, trains are required to use horns at any at -grade crossing for safety reasons. Depending on the type of horn used, noise levels could reach 110 dBA at a distance of 100 feet. Under the Transit Zoning Code (SD 84A and SD 8413), sensitive uses, both interior and exterior, could be located within areas that may experience excessive noise levels due to train horns. Interior uses would include predominantly residential structures, while exterior uses at new developments adjacent to the rail station and the SCRRA rail line may include communal open spaces, such as pocket parks or pedestrian walkways. It is expected that these uses could be located within the interior of new developments or on the opposite side of the development from the rail station and SCRRA tracks, thereby mitigating some of the noise generated by those transportation facilities. In terms of interior uses and as stated previously, under new construction practices, noise levels inside structures, such as residential buildings, can be expected to be 30 dBA less than exterior noise levels. As such, the instantaneous interior noise levels attributable to residential units located within 100 feet of the SCRRA 4.8-36 City of Santa Ana Transit Zoning Code (SD 84A and SD 8413) EIR