Rockridge Soundwalls

State Route 24 Oakland, CA Pre-NBSSR Noise Study: Details

For the technically inclined, here are more details:

Noise threshold

According to Proposed Alameda County Congestion Management Agency (ACCMA) Freeway Soundwall Policy,

"The federal funding process applies to residences affected by an existing or predicted future noise level approaching an exterior sound level of 67 decibels… The term "approaching" is defined as 2 decibels below the federal criterion of 67 decibels. In the remainder of this document, a level of 65 decibels will be used to encompass the ACCMA's [Alameda County Congestion Management Agency] definition of 'approaching 67 decibels'."

The Pre-NBSSR Noise Study used the 65 dbA threshold. But Alameda County's noise threshold is not consistent with that of the State of California Department of Transportation. According to Caltrans Soundwall Synopsis,

"In order for the area to qualify [for soundwalls], it must meet all of the following criteria: … Has hourly noise levels that exceed the 67-decibel (Leg) threshold." The decibel is measured on a logarithmic scale; the dbA. is a decibel that is modified with a weighting filter. A difference of -2 dbA. means that a 65 dbA. threshold is only 63% of the intensity of a 67 dbA. level (calculated with the help of a decibel calculator). ACCMA considers this to be "approaching" 67 dbA.

Measured noise

The Pre-NBSSR Noise Study Long-Term Noise measurements (Table 1, Page 8) were used to correct the Short-Term Noise measurements (Table 2, Pages 10-11) for time of day. If the time of day that a short-term measurement was made was not the noisiest according to a nearby long-term measurement, the short-term measurement was scaled up accordingly.

Calculated noise

The Pre-NBSSR Noise Study also provides an estimate of Highway 24-generated noise by means of a computer calculation using the Federal Highway Administration (FHWA) Traffic Noise Model (TNM) software. Two sets of results are provided in Table 3, Pages 19-21: predictions for current noise and for noise in the year 2030. According to the Pre-NBSSR Noise Study, here are some of the assumptions that were made:

"TNM noise predictions are worst case scenario because the future projected traffic volumes used in the TNM model are independent of the time of day (i.e. eastbound PM and westbound AM peak hour volumes were used concurrently). Furthermore, peak traffic hour volumes were used, which are generally not necessarily the same as peak noise hour volumes. Free flow speed conditions (65 mph) are assumed; however, congestion due to peak traffic volumes, if that were to occur, would reduce vehicle speeds and consequently the projected noise levels…A speed limit of 65 mph was assumed along the entire study corridor, although the posted speed limit east of Golden Gate Avenue is 55 mph."

Note that the Noise Study states that "eastbound PM and westbound AM peak hour volumes were used concurrently". When this was pointed out to Steven L. Wolfe, one of the authors of the Study, he made the following comment: "The highest mainline traffic volume provided by Caltrans during the worst hour at each representative location was used in the model. It wasn't a combination of both." An official correction has not yet been published by Wilson, Ihrig, and Associates.

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