Noise Abatement Analysis
Noise abatement measures (e.g., construction of a noise barrier) were considered for reducing noise impacts when the predicted peak-noise-hour levels were expected to approach or exceed FHWA Noise Abatement Criteria (NAC) for appropriate land use categories (see Table 3.5-1). For this project, noise abatement measures were considered and evaluated for effectiveness based upon the FHWA NAC.
The noise abatement analysis evaluated the acoustical effectiveness of noise barriers. Noise barriers must reduce future predicted noise levels at impacted receptors by at least five dBA to be considered feasible. The feasibility criterion is not necessarily a noise abatement design goal. Greater noise reductions are encouraged if they can be reasonably achieved. Feasibility may be restricted by topography, access requirements for ramps, other noise sources in the area and safety considerations. Reasonableness of noise barriers must be evaluated as well, including such additional factors as cost per benefited receptor, absolute noise levels, change in noise levels, viewshed preservation, appearance, views of impacted residents, input from other agencies, and other social, economic, environmental, legal and technological factors. Noise insulation of residential dwellings is typically only considered when severe traffic noise impacts are predicted and normal abatement measures are not physically feasible or economically reasonable. Because substantial noise impacts are not predicted for any of the project alternatives, noise insulation of structures was not considered. When affected receptors experience traffic noise levels or increases considerably greater than at other similar receptors, extraordinary abatement measures may be justified. Examples of such situations include private residential units with noise levels greater than 75 dBA Leq after project implementation or a predicted noise level increase of 30 dBA or more as a result of the project. The SFOBB East Span Project would not cause any such increases.
The effectiveness of a range of soundwall heights from 1.8 to 4.9 meters (6 to 16 feet) was tested using the criteria of reducing noise levels a minimum of five dBA at the receptor. In addition, for a soundwall to be considered effective, it must interrupt the line-of-sight between the receptor (assumed to be 1.5 meters [5 feet] tall) and an exhaust stack of a heavy-duty truck. The truck-stack height is considered to be 3.5 meters (11.5 feet) above the roadway surface. If soundwalls are installed, they should be constructed along the outer edges of the bridge decks for maximum noise abatement effectiveness.
Replacement Alternative N-2.
On the northern side of the bridge, two 4.27-meter (14-foot) high walls 245 meters (800 feet) long at each of the bridge decks exiting YBI would reduce noise levels 5 dBA or more and break truck-stack line-of-sight. Together, the walls would provide effective noise reduction for approximately 90 residential units at Location 2. Both the U.S. Navy and the City and County of San Francisco have expressed their desire to rent out these residential units on YBI. From Station 52+40 to Station 53+40 SW-2A and SW-2B would be 3.05 meters (10 feet) high, with a 3.66-meter (12-foot) transition area from Station 52+00 to Station 52+40. The soundwalls would need to be approximately 140 meters (459 feet) in length to provide effective noise reduction to the one residence at Location 3. On the south side of the bridge, walls 3.66 meters (12 feet) in height on both bridge decks exiting the tunnel would reduce noise levels 5 dB or more and break truck-stack line-of-sight. These walls would extend for 245 meters (800 feet) to the east of the tunnel portal. These walls would provide noise reduction for the one residence at Location 4.
Appendix P: Noise Abatement Analysis