Evaluating Pavement Strategies and Barriers for Noise Mitigation (2013)

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57 Summary The methodology developed in this research provides a means of evaluating pavement strategies and barriers together for feasibility, reasonableness, effectiveness, acoustic longevity, and economic features. The primary elements of the approach include the following: • The use of OBSI data to account for the effect of pavement performance on tire noise source levels • The use of FHWA TNM with a modification to adjust for tire–pavement noise based on OBSI data • The use of LCCA to compare the costs of barriers, quieter pavement, and combinations of pavements and barriers. The methodology can be used immediately under the latest FHWA policy regarding feasibility and reasonableness. It can also be used directly under state policies that use a specified barrier cost per square foot in the cost-reasonableness analy- sis. However, if a barrier cost per square foot per benefited receptor is used, either a cost that could be used in the LCCA needs to be identified or a hybrid method for dealing with reasonableness needs to be developed. In the longer term, adjustments of the criteria for feasible and reasonable may be needed to reflect the use of quieter pavement and its noise reduction potential particularly when barriers cannot be justi- fied. Additionally, the concept of effectiveness would need to be explicitly defined. Life-Cycle Cost Analysis and Economic Features Accounting for the cost of noise abatement in the LCCA of pavement alternatives provides an ideal means for consider- ing the economic features of barriers and quieter pavements in the pavement–abatement selection process. This analy- sis allows consideration of both the initial and future costs of maintaining a specified level of noise reduction perfor- mance over time as opposed to only considering the initial cost of quieter pavement relative to barriers. Application of this approach may require collaboration between pavement engineers and environmental engineers. However, incorporating LCCA into this approach requires actual pavement structure costs because TNM Average Pave- ment is not an actual pavement structure and it does not have an associated cost. Implementing the approach relies on eval- uating actual pavement designs for both acoustic performance and cost. Therefore, the baseline for comparison of these two parameters would likely be that of the pavement with the low- est life-cycle cost. However, predictions based on TNM Aver- age Pavement may be useful for reference. The LCCA is greatly dependent on the rehabilitation cycle time and the project life, particularly for quieter pavements for which the noise performance can degrade more rapidly. In implementing the methodology developed in this research, the influence of these variables would need to be duly considered. Effectiveness With the integration of pavement into traffic noise abate- ment, effectiveness becomes a necessary concept. When only barriers are used for abatement, feasibility and reasonable- ness are based on barrier insertion loss. However, when pave- ment is also included, overall noise reduction becomes the primary metric for assessing the performance of abatement alternatives. In the example cases presented in this report, ranking the effectiveness of alternatives by amount of noise reduction provided was used as one measure of performance. However, this measure does not capture the complete con- cept of effectiveness as defined in this research. The intent of the effectiveness consideration is to avoid situations where post-project noise levels unknowingly become higher than the pre-project levels. This intent can be achieved by model- ing the existing levels based on the OBSI levels of the actual C H A P T E R 6 Summary and Suggested Research

58 pavement in the project area and comparing them to the lev- els predicted after the project is complete. However, if both the existing pavement and the candidate pavement alterna- tives produce higher levels, an effective solution will not be obtained. This situation was demonstrated in the case study in which the random transversely tined PCC pavement was found to meet the feasible and reasonable criteria only when compared to a random transversely tined PCC baseline. To set a more calibrated definition of effectiveness, it may also be appropriate to relate effectiveness criteria to TNM Aver- age Pavement predictions or another absolute level such as the NAC. Acoustic Longevity Using OBSI measurements in TNM to predict traffic noise levels provides a straightforward method of consider- ing acoustic longevity. As illustrated in some of the example cases, the increase in traffic noise due to aging of the pave- ment can be easily predicted by inputting the appropriate OBSI levels for a given age of pavement. Ideally, these data would be generated through research projects conducted by the agency. However, they could initially be based on data by others and then followed up by periodic measurements of the pavement in the actual projects. The rehabilitation cycle could be initially set based on these data and confirmed over time, possibly with a provision for a shorter cycle if the pavement acoustic performance deteriorates faster than anticipated. Other Issues The case studies considered in this research have shown that minor differences in agency requirements/criteria play a large part in demonstrating the need for noise abatement and identifying the preferred alternative. While this research was not intended to develop or propose agency policies, it identi- fied some actions that could be taken when establishing cri- teria for abatement measures that would include pavements. These actions are as follows: • Define the existing traffic noise levels based on GLSS- modified TNM levels with actual current pavement OBSI data • Predict future noise levels based on the OBSI levels of the pavement types proposed for the project • Encourage the consideration of quieter pavement when impacted receptors are identified and barriers are not fea- sible or reasonable • Develop effectiveness criteria for selecting abatement options that consider the overall noise reduction of the alternatives not just insertion loss • Reference effectiveness to some absolute level of perfor- mance defined by TNM Average Pavement or some other criteria such as the NAC • Account for the benefit to receptors on both sides of the highway when pavement is considered as part of the abate- ment alternative Findings from Example Cases Although the case studies presented in this project were not intended to provide conclusions regarding the pavement type, quiet pavement versus barriers, or the specifics of the case, they revealed the following findings relevant to application of the methodology: • A design goal criterion of 7 dB is generally not achievable with current quieter pavements alone except in extreme cases where a much quieter pavement is compared to a noisier pavement [e.g., an RAC(O) or ground PCC surface is compared to a transversely tined PCC surface]. • Quieter pavement alternatives can be both feasible and cost reasonable in some situations (even under current state agency criteria). • Maintaining the acoustic performance of quieter pave- ments can result in a significant increase in life-cycle cost. • In some situations where a barrier alone would not meet criteria, feasible and reasonable noise abatement alterna- tives can be obtained by combining quieter pavement with a barrier. • Feasible and reasonable alternatives can be developed using pavements and barriers in combination, in some cases achieving comparable effectiveness. • In some situations, a more effective and lower cost alterna- tive can be obtained by combining quieter pavement with shorter barrier heights. • In some instances, barriers do not meet criteria because of low receptor density or geometric factors, but the quieter pavement would provide noise-level reduction of 3 to 4 dB to impacted receptors at low enough cost. • In cases where there are impacted receptors on both sides of a highway, it may be appropriate to evaluate them together when quieter pavement or combinations of barriers and pavement are considered. OBSI and TNM Based on the analysis performed in this research, it appears that using SRTT OBSI levels in the modified version of TNM offers a viable approach for accounting for differences in tire– pavement noise in traffic noise predictions. Studies have shown that the range of CTIM levels from the transversely tined PCC to ARFC is comparable to the range in OBSI levels (17). With

59 the example cases, the predicted levels for the noisier and quieter pavements were about as expected relative to TNM Average Pavement. The use the GLSS-modified TNM with SRTT OBSI data could be used in future traffic noise studies as developed in this research. Accounting for some of the effect of pavement is more desirable than not, particularly in cases where the pave- ment under consideration will produce levels higher than TNM Average Pavement. Suggested Research To advance the methodology developed in this research, interested state agencies may consider using it on a trial basis, in parallel to traditional noise studies. This trial would help the agencies familiarize themselves with the methodology and provide feedback on the approach and how it relates to the current practice. It would also provide feedback on how application of this methodology might affect the outcome of highway studies. Parallel studies would provide insight on the use of fewer and/or shorter barriers, use of quieter pavement when barriers alone are not feasible or reasonable, and use of barrier–pavement combinations. Documenting the existing acoustic conditions for Type 1 projects that are not all new construction will help provide information for “calibrating” TNM model results to field measurements. There is an immediate need to collect comparisons of OBSI and CTIM measurements and TNM results with the GLSS adjustment and further examine these relationships. Studies performed to date have been limited to projects that were not designed explicitly to make these comparisons and limited OBSI data using the standard SRTT test tire were available. To better evaluate the potential of quieter pavements for noise mitigation, there is a need for a research effort to better understand the acoustic characteristics and sound-absorptive effects of the different pavement surfaces. This effort would require CTIM, OBSI, and EFR measurements and TNM cal- culations to quantify the effect of sound-absorbing pavement on traffic noise. As noted in some of the example cases, current quieter pavements will not generally achieve a 7 dB design goal. Thus, there is a need for research into quieter pavement technolo- gies to explore innovation that would produce more quieter pavement surfaces with high resistance to degrading acoustic performance with time.