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1 Highway noise changes from day to day and hour to hourânot just because of varia- tions in traffic volumes, vehicle mix, and speed, but also because of the weather. In some cases, traffic noise can increase by more than 10 dB in as little as one hour due to changes in meteorology. This research project had the following objectives: 1. Measure and document the meteorological effects on roadway noise propagation under different atmospheric conditions. a. Develop best practices and provide guidance on how to quantify meteorological effects on roadway noise propagation and explain those effects to the public. The guidance developed under this research project: 1. Identifies the most critical atmospheric parameters that affect roadway noise propagation and the distances at which their respective influences may occur; and 2. Aids in determining when meteorological effects should and should not be considered in noise analyses. The research results help practitioners understand and explain roadway noise levels under different atmospheric conditions in a roadway noise analysis. Chapter 1 provides background on how meteorology affects highway noise. It includes a brief literature review and provides a primer on meteorological effects, showing how changes in temperature and wind speed with height affect the bending of sound toward or away from the ground. It discusses different highway noise propagation models used in the United States and Europe and their approaches to incorporating meteorological effects. Chapter 2 describes the data collection and modeling conducted for this research study. It discusses the program to collect sound data along Interstate-17 (I-17) north of Phoenix, Arizona. In all, over 34,000 valid 5-minute data points were collected quantifying 230 differ- ent variables, including traffic volume, speed, vehicle mix, sound levels by 1/3 octave band, temperature, humidity, barometric pressure, wind speed, wind direction, vertical profiles of temperature, wind characteristics, and sound speed. The collected data clearly show a sound level difference between periods with similar traffic volume and vehicle mix, a marked indica- tor of the meteorological effects at work. The chapter also shows the results of sound modeling using the European Harmonoise model and the FHWA Traffic Noise Model (TNM). Chapter 3 details various approaches to conceptual models that take meteorology into account. The research team evaluates what parameters should be included in the models and provides examples of models and tools that can be used to estimate the impacts of meteo- rology. These include a tool to use the U.S. Environmental Protection Agencyâs (EPAâs) S U M M A R Y How Weather Affects the Noise You Hear from Highways
2 How Weather Affects the Noise You Hear from Highways AERMET surface meteorology data files to estimate how many hours in the year will have adverse weather conditions with respect to highway noise. The research team also reviews statistical models, lookup tables, research models, approaches to creating an engineering model, and a framework for updating FHWAâs TNM, which currently assumes an acousti- cally neutral atmosphere. Chapter 4 discusses the implications for noise impact analyses in the United States. The research team investigates two âtypicalâ developments next to a highway to show what impacts meteorology can have on feasibility and reasonableness determinations in determining noise abatement for highway projects. Chapter 5 reviews the guidance around accounting for meteorological effects and how such guidance could be implemented in the United States. The research team reviews avail- able models, available data for those models, variation in meteorological effects, and effects on decision-making for noise abatement. Chapter 6 briefly introduces two public outreach tools developed through this research. The first is a four-page fold-out brochure that state highway agencies (SHAs) can cus- tomize for their own use. The brochure is titled âWhy is it so loud today? Understanding how weather affects traffic noise levels in your community.â An editable version of the brochure can be found on the TRB website [121]. The second public outreach tool is an interactive PowerPoint presentation to help practitioners explain weather effects to the public. It includes a primer on noise effects and interactive audio demonstrations under different weather conditions with and without a noise barrier. The interactive tool is avail- able for download on the TRB website [121]. Chapter 7 concludes the study and provides suggestions for further research. The technical appendices to the study provide additional detail on data collection, data analysis, modeling, and guidance.