How Weather Affects the Noise You Hear from Highways (2018)

Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

74 Factors for developing guidance on considering meteorological effects in highway traffic noise studies include the state of the practice in other countries, availability of models for evaluating meteorological effects, ease of obtaining the required model data, variations in meteorological conditions, effects on noise impact and noise abatement conclusions, and the extent to which considering meteorological effects could affect the decision-making process. State of the Practice in Other Countries The literature review report conducted as part of this research project (Appendix A) sum- marized existing requirements for including meteorological effects in highway traffic noise prediction either by statute, regulation, or guidance in the United States and abroad. Reg- ulatory bodies in the United States do not require consideration of meteorological effects. EU noise regulations, however, use annual average noise levels, i.e., noise levels representing annual average meteorological conditions. The EU Environmental Noise Directive (END) [76] [78] requires that member states develop noise exposure maps of cities for two noise indicators: Lden and Lnight. END also requires that each member state develop its own national noise regulation. The Netherlands uses the annual average Lden and the default noise limit is 48 dB Lden. For Lden levels exceeding 48 dBA, noise reduction measures should be applied, in principle. Many other countries use Lden and have regulations like the Netherlands. More information on these models is provided in Appendix A. Available Models As discussed previously, the FHWA TNM model does not account for meteorological effects. However, there are several other models that evaluate meteorological effects. The international standard ISO 9613-2 is a generalized method for calculating “attenuation of sound during propagation outdoors.” ISO 9613-2 adjusts sound levels for downwind conditions since down- wind sound levels are generally higher than upwind sound levels due to atmospheric refraction. Engineering models include the European programs Nord2000 and Harmonoise. Nord2000 includes effects for only eight atmospheric conditions and differs considerably from ISO 9613-2 because the model uses physical rather than empirical formulas for ground and barrier effects. The EU also developed the more robust Harmonoise to model road, rail, and industrial sound, and validated the model over a period of five years in three European countries. Australia has also adopted the model. Harmonoise predicts the effects of 30 different meteorological classes and calculates noise level deviations from the values for a neutral atmosphere. C H A P T E R 5 Guidance and Implementation

Guidance and Implementation 75 The EU decided in 2015 that future traffic noise mapping calculations will be performed with a harmonized noise model called CNOSSOS-EU. The EU considered three models for the propagation part of CNOSSOS-EU: • ISO 9613-2, • The French NMPB traffic noise model, and • Harmonoise. The EU selected NMPB as a compromise between accuracy and complexity. EU states must use CNOSSOS-EU beginning in 2022. Both the French NMPB traffic noise model and CNOSSOS-EU use an approach comparable to ISO-9613-2, so predicted sound levels represent a moderate downwind condition. The Dutch traffic noise model also is like ISO 9613-2. Advanced numerical models also exist for evaluating meteorological effects, including the PE model, the BEM model, and the Euler model. These are the most accurate predictors of meteo- rological effects, but these models require specific data on the range-dependent sound speed profiles. In addition, the models are computationally intensive, requiring hours of calculation for a single source-receiver pair and are not appropriate for engineering use. Obtaining Model Data Sound propagation models that consider meteorology require temperature, relative humid- ity, and barometric pressure data for atmospheric absorption calculations; these models also require wind gradients, temperature gradients, and turbulence for refraction calculations. How- ever, field-measured meteorological data are generally not available and meteorological data collection for most highway projects is not feasible given time and budgetary constraints. Atmospheric scaling techniques, like EPA’s AERMET, the preprocessor for its preferred air pol- lution model AERMOD, may provide a tool for obtaining the needed data at a relatively low cost. To our knowledge, all states use AERMOD for air quality modeling. As noted in Chapter 3, the research team created a tool to convert AERMET data into hourly vertical sound speed gradients. Variation in Meteorological Conditions The frequency and duration of meteorological conditions is also an important consideration. Neutral conditions may be unusual or upwind or downwind conditions may be prevailing. Thus, the frequency and variability of meteorological conditions at a site is important because that information is directly related to the effects on nearby land uses, and the percentage of time that a noise barrier may provide the desired noise reduction. Effects on Noise Impacts and Abatement Conclusions As discussed in Chapter 4, the research results indicate that noise impacts generally increase under downwind and inversion conditions but decrease under upwind conditions and condi- tions with negative temperature gradients. Noise abatement conclusions also could change. The results indicate that barriers are less effective under upwind and normal lapse conditions and much less likely to meet the reasonableness criteria in the SHA noise policy. Sound levels under these conditions also are lower than for neutral conditions and would not represent the “worst noise hour” as required by the FHWA noise regulation. The results also indicate that the barrier designed for the “worst noise hour” under neutral conditions is more effective under downwind and inversion conditions: ILs are higher, which

76 How Weather Affects the Noise You Hear from Highways increases the number of benefited residences. As a result, the barriers designed under neutral conditions to meet the requirements of the FHWA noise regulation and SHA noise policy would still be feasible and reasonable under downwind and inversion conditions. The noise impact implications would also be different for widening and new alignment proj- ects. For widening projects, the sound level increases would not change since the same adjust- ments would apply to both the existing and future sound levels. However, there may be no significant existing noise source for new alignment projects so sound level increase could be high and impact distances could extend much farther than under neutral conditions. The highest noise levels occur under downwind and inversion conditions. If these conditions existed during the time of day when the highest traffic volumes are free-flowing [i.e., level of service (LOS) C or D], then that could constitute the “worst noise hour.” Additional analysis of the com- binations of traffic, speed, and meteorological conditions may facilitate accurate identification of the worst noise hour for a project. The AERMET Sound Speed Profiler spreadsheet tool developed for this project provides a graph of the probability of upward and downward refraction conditions for one year of met data. This tool can be adapted for this purpose. These research results indicate that the barrier design for neutral conditions would still be effective, feasible, and reasonable for downwind conditions and perhaps also for inversion conditions. Differences exist between the measured adjustments and the NCHRP Report 791 adjustments developed using Nord2000, particularly for inversion conditions. Effects on Decision-Making Factors that could affect the noise abatement decision-making process on federal highway projects include: • Determination of the worst noise hour; • Changes in the number of impacts; and • Changes to the noise abatement conclusions (feasibility and reasonableness). Worst Noise Hour The FHWA noise regulation requires that noise levels represent the worst noise hour. Under neutral conditions, the worst noise hour occurs when traffic volumes and vehicle speeds are highest, typically when traffic is free-flowing and at or near LOS C or D. These conditions typically occur during the day and may be just before or just after the more congested peak travel periods. Upwind and lapse conditions reduce sound levels and would, therefore, not represent the worst noise hour. At the measurement locations used in the research, strong lapse conditions existed during almost all daytime periods due to the clear, sunny skies, which is not unusual for Phoenix. Actual sound levels would be lower than predicted by TNM during lapse conditions. This is also the case for upwind conditions that reduce sound levels. Downwind and inversion conditions increase sound levels. If these conditions occurred at the time when traffic volumes and vehicle speeds are highest, then the worst noise hour would occur then. The frequency and duration of downwind and inversion conditions can vary considerably even between locations in the same state. In general, inversions are more common at night. Although strong inversion conditions existed during all nighttime periods at the measurement sites, sound levels were generally lower during these periods, except during the morning peak traffic hours (Figure 24). However, during the winter in more northerly locations, when the sun sets early, temperature inversions also can occur during the afternoon peak hour.

Guidance and Implementation 77 As a result, the most typical actual worst noise hour condition could occur during either the morning or afternoon peak hour, depending on whether nighttime inversions have set in or at other times that are affected by downwind conditions. There may need to be additional analysis of the combinations of traffic, speed, and meteoro- logical conditions to accurately identify the worst noise hour for a project. Noise Impacts Fewer impacts occur under upwind and lapse conditions than under neutral conditions while more impacts occur under downwind and inversion conditions. If the objective is to identify the greatest possible number of impacts, then the analysis would evaluate downwind or inversion conditions during the hour when traffic volumes and speeds are highest even though this condi- tion may or not be typical at a given location. This method is consistent with European practices that predict sound levels for downwind conditions. Noise Abatement Conclusions If the desire is to mitigate the highest sound levels and greatest number of impacts, then the analysis would evaluate downwind or inversion conditions during the time when traffic volumes and vehicle speeds are highest. This condition would typically occur during the day and may be just before or just after the more congested peak travel periods. The research results indicate that a barrier designed under neutral conditions in accordance with FHWA requirements would increase benefits under downwind conditions. These results also indicate an increased likelihood that a barrier will meet the feasibility and reasonableness in a SHA noise policy. Therefore, if a barrier is deemed feasible and reasonable under neutral con- ditions, then it would still be feasible and reasonable under downwind and inversion conditions. Additionally, a barrier not feasible or reasonable under neutral conditions might be feasible and reasonable under downwind or inversion conditions because the same barrier increases the number of benefits. This could be a consideration for sites where downwind conditions prevail during the hour when traffic volumes and speeds are highest. Conversely, a barrier that is feasible and reasonable under neutral conditions may not be feasible under upwind and normal lapse rate conditions.