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2C H A P T E R 1 1.1 Introduction to NCHRP Project 25-34 Although the FHWA has provided substantial guidance for routine application of the Traffic Noise Model (TNM), sce- narios exist for which there remains limited or no technical guidance. The objectives of NCHRP Project 25-34 were the following: â¢ First, to supplement existing guidance on applying the TNM by identifying best practices to accurately, con sistently, and efficiently model (1) structure-reflected noise; (2) bridge expansion joints; (3) signalized interchanges; (4) inter- sections; (5) area sources, e.g., weigh stations, park-and-ride lots, toll facilities, and service plazas; (6) median barri- ers; (7) roundabouts; (8) parallel barriers; and (9) tunnel openings. â¢ Second, to determine the sensitivity and accuracy of meth- ods to model (1) multilane highways, (2) rows of build- ings, (3) topography, (4) ground zones, and (5) tree zones, and identify best practices for input parameters. These five research topics represent parameters that already exist within the TNM. â¢ Third, to synthesize the state of practice for analyzing the effects of (1) wind direction and (2) temperature on sound propagation. Although considerable data exist on the effects of meteorology on sound propagation, thus far these results have not been applied directly to use within the TNM. Together, these three objectives cover 16 research topics. Some of the research topics are related or have similar research approaches. Those have been grouped and addressed together. In conduct of the research, the topics were addressed in paral- lel through a seven-step process. Each step, or task, built upon the previous steps, ultimately forming the final report. The seven steps were as follows: 1. Determine the existence of any useful information, either modeling techniques or measurement data useful for valida- tion purposes. This was conducted by surveying AASHTO Standing Committee on Environmentâs Environmental Process Subcommittee, the NCHRP Project 25-34 research team, and the TRB ADC40 Committee members. Where appropriate and necessary, a literature search was conducted. 2. Compile modeling techniques and existing validation data for each research topic. 3. Identify candidate modeling techniques. 4. Prepare the interim technical report. 5. Process existing validation data and/or collect additional data. 6. Test and evaluate modeling techniques and identify best practices. 7. Prepare final technical report. 1.2 Purpose and Content of NCHRP Report 791 and Supporting Appendices The purpose of the research herein was to develop guid- ance for TNM users and highway noise analysts to address situations where guidance has not been provided in the past. Therefore, this report is intended and structured primarily as a guidance document. As a result, it does not attempt to fully justify or document how the guidance was developed; nor does this report list all of the information, studies, mod- eling practices, and results that were evaluated to develop the guidance. Instead, those details are included in Appendi- ces A through L of the contractorâs final report, which were prepared to accompany each of the research topic areas and which are available on the NCHRP Project 25-34 web page at http://apps.trb.org/cmsfeed/TRBNetProjectDisplay.asp? ProjectID=2986. Introduction