Federal Highway Administration (FHWA) regulations for mitigation of highway traffic noise in the planning and design of federally aided highways are contained in Title 23 of the United States Code of Federal Regulations, Part 772. Requirements during the planning and design of a highway project include identification of traffic noise impacts, examination of potential mitigation measures, inclusion of reasonable and feasible noise mitigation measures, and coordination with local officials. The regulations contain noise abatement criteria for different types of land uses and human activities. Reasonable and feasible efforts must be made to provide noise mitigation when the criteria are exceeded. Compliance with the regulations is a prerequisite for securing federal-aid highway funds for construction or reconstruction of highways. Further details may be found in the FHWA Noise Standards.
Computer programs based on mathematical models have proven very useful for predicting noise levels and designing noise barriers. The FHWA has released an entirely new, state-of-the-art computer program known as TNM® that provides a traffic noise model for predicting noise impacts in the vicinity of highways. Replacing older models (Stamina and Optima), the new program uses advances in personal computers and software to improve the accuracy and ease of modeling highway noise, and the design of effective, cost-efficient highway noise barriers. Included are the following components:
• Modeling of five standard vehicle types, including automobiles, medium trucks, heavy trucks, buses, and motorcycles, as well as user-defined vehicles
• Modeling of both constant-flow and interrupted-flow traffic using a 1994/1995 field-measured database
• Modeling of the effects of different pavement types, as well as the effects of graded roadways
• Sound level computations based on one-third octave-band database and algorithms
• Graphically interactive noise barrier design and optimization
• Attenuation over/through rows of buildings and dense vegetation
• Multiple diffraction analysis
• Parallel barrier analysis
• Contour analysis, including sound level contours, barrier insertion loss contours, and sound-level difference contours
Local criteria may be more restrictive than federal criteria. In Minnesota, for example, daytime criteria in residential areas are an hourly L10 of 65 dBA and an hourly L50 of 60 dBA. L10 refers to the sound level that is exceeded 10 percent of the time over the period under consideration (1 h, in this case); L50 refers to the level exceeded 50 percent of the time. Noise abatement projects strive for a minimum reduction of 10 dBA in L10 and 6 dBA in L50 from existing traffic noise levels.
