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Noise Barrier Acceptance Criteria: Analysis

CHAPTER 4. CONCLUSIONS

This portion of the study, 23 CFR 772 Streamlining, Analysis, and Outreach, examines the feasibility and reasonableness factors in the FHWA noise regulation in Title 23 CFR Part 772. Those factors are:

As a starting point, the focus was on the range of factors, individually and in combination as contained in the SHA noise policies in use throughout the country.

Then, a sensitivity analysis was performed to test over 1,000 combinations of the factors' values for over 100 cases of one-row, two-row, and three-row communities with receptors spaced at 50, 100, and 200 feet from each other. Two series of reasonableness decision arrays are presented - one considering only the NRDG criterion and one that considers both NRDG and the CE criterion in terms of the APBR.

Finally, the findings were tested on four actual highway project study areas previously evaluated for noise abatement. Seven sets of feasibility factor values were tested, as were 84 combinations of reasonableness criteria values.

FEASIBILITY

All SHAs use 5 dB for the feasibility noise reduction. Feasibility is insensitive to the type of the quantity (number of impacted receptors, percentage of first-row impacted receptors and percentage of all impacted receptors) and its value. Positive decisions on feasibility were returned on all of the values considered, ranging from one impacted receptor to 75% of all impacted receptors.

While not directly studied in this task of this research, one concern of SHAs is the need to analyze abatement for cases of isolated single residences even though such cases will almost always result in a finding of "not reasonable" on the basis of CE. If one establishes in the policy that a noise barrier must achieve a minimum noise reduction of 5 dB at two or more impacted receptors, then any case of an isolated single residence would automatically return a decision of "not feasible" since there are not two or more receptors to be impacted.

The Task 4 report for this study addresses this approach as a possible "streamlining" technique. It should be pointed out that such an approach would not work for an SHA that uses the feasibility criterion as a design mechanism instead of a screening mechanism. However, if the NRDG was changed to be based on impacted receptors instead of benefited receptors, as discussed in the next section, then a feasibility criterion based on a number of receptors could be used for isolated receptor screening for those SHAs. The counter-argument for screening out an isolated receptor - and not doing a reasonableness analysis - is that there can be the relatively rare case where a barrier for a receptor very close to the road will be reasonable in terms of both the NRDG and CE criteria

REASONABLENESS

There are wide ranges of values used by the SHAs for each of the reasonableness factors studied. These ranges lead to very different decisions regarding abatement reasonableness for identical noise study area scenarios.

The decisions are not very sensitive to the NRDG and the NRDG quantity - up to a point. Meeting the NRDG for even a high percentage of first-row benefited receptors or a low-medium percentage of all benefited receptor was generally achievable. However, moving up to a requirement of over 50% of all benefited receptors results in "not reasonable" decisions for many cases that would otherwise be reasonable.

Having the NRDG quantity in terms of impacted receptors instead of benefited receptors would eliminate the problem of the number of benefited receptors changing in the design process as barrier height and/or length changes. Changing the number of benefited receptors changes the target number needed to meet a criterion based on percentage of all benefited receptors, complicating the design process. As an example of designing for impacted receptors, WSDOT uses its feasibility design goal, which needs to be achieved at greater than 50% of first row receptors, as a design criterion as well as a feasibility determinant.

Also, basing the NRDG decision on percentage of all benefited receptors can lead to counter-intuitive - and unfair - decision on reasonableness in cases where there are several rows of benefited receptors. A large number of benefited receptors coupled with a high percentage requirement for meeting the NRDG can lead to "not reasonable" decisions based on either or both of the NRDG and CE criteria. Yet, if those "extra" rows of houses were removed, then the total number of benefited receptors could not increase once all of the first-row receptors met the benefited noise reduction value, permitting the NRDG and CE criteria to be met. Either restricting the NRDG quantity to be a number or a percentage of first-row benefited receptors only or basing the NRDG on the number or percentage of impacted receptors will eliminate this problem.

Most SHAs use a CE criterion based on cost (in dollars) per benefited receptor (CPBR) and barrier surface area in square feet per benefited receptor (APBR). Several SHAs use sliding scales for determining the allowable values for either of these parameters. Some others use cost per benefited receptor per decibel of noise reduction.

The CE value in terms of CPBR for a project design is very dependent on the unit cost used by the SHA in developing that criterion and in computing the cost of a proposed abatement measure. As a result, the SHA must be very diligent in ensuring that any change in the unit cost is proportionately reflected in an adjustment to the allowable cost per benefited receptor (CPBR). The APBR approach factors cost out of the equation, as well as the ongoing need to keep unit costs and the CE criterion updated.

For the purposes of this study - in order to assess and characterize differences in the SHA policies - normalization for cost was performed (when abatement unit costs could be obtained from the SHAs) by dividing the CPBR by the unit cost to get an equivalent to the APBR.

The result is a very wide range in APBR criteria values, from the low end of the sliding scale at 250 SF/benefited receptor (for a case of in-fill development along a highway) to 2,750 SF/benefited receptor, a range of over 10:1.

The sensitivity analysis conducted in this research leads to the conclusion that the reasonableness decision is very dependent on the APBR. Values below 1,000 SF/benefited receptor for APBR resulted in "not reasonable" decisions even in many cases of dense receptor spacing. Conversely, high values of APBR resulted in very few "not reasonable" decisions, except when combined with a requirement for a high percentage of benefited receptors needing to meet the NRDG. APBR values no lower than 1,400-1,500 seem appropriate based on the results of this study.

This research has shown that there are a very large number of paths to the same decisions on reasonableness of abatement. There does not appear to a particular set of values for the various factors that is optimum. Instead, if extremes in the factors are avoided, then more uniform decisions from one SHA to the next and from one project to the next can be expected. Changing the feasibility quantity to two impacted receptors would eliminate the need to evaluate abatement unnecessarily in isolated receptor cases. Changing the NRDG quantity to be the percentage of first-row benefited receptors or impacted receptors would improve the design process and eliminate the problems with "not reasonable" decisions in study areas with multiple rows of houses. Not allowing low values for APBR or for normalized CPBR/unit cost will also lead to more consistent abatement decisions.

The charts in this report that display the results of the sensitivity analysis may help SHA analysts and policymakers understand the consequences of changes in the various factors covered by this study. The analysis tools resulting from this work, documented in a separate report, will allow the SHA analyst to examine the changes in decision-making for an individual project or a group of projects based on change in these factors.

Updated: 07/08/2014
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