Representing Highway Noise Exposures—Confessions of a Contourtionist

This paper describes how a contourtionist is distinguished from someone who prepares airport noise contours. Contourtionist is defined in this paper as one who calculates contours; specifically: a highway noise analyst that is able to twist traffic noise contours into misleading positions. Noise contours along with a scaled geographic depiction of an airport and its surroundings are combined to create the noise exposure map required by the U.S. Federal Aviation Administration (FAA). Highway noise assessments performed to meet U.S. Federal Highway Administration (FHWA) or State departments of transportation requirements usually evaluate exposures at specific locations representative of noise-sensitive receptors. However, traffic noise contours are often desired by local land planning and zoning authorities to represent sound level exposures on land that is being considered for development, which is adjacent to highways. Airport noise contours are an indispensable tool for presenting concisely and comprehensibly noise exposure information for the large areas affected around an airport. Contours of aircraft sound levels can be calculated with reasonable accuracy without consideration of localized surface conditions because of the spatial variation of aircraft flight paths and the generally large aircraft elevation angles rendering ground surface structures and features insignificant. For highway traffic noise, simply estimated “contours” are a useful planning tool for preliminarily assessing noise impacts on undeveloped land and may be reasonably accurate if the topography adjacent to the roadway is fairly flat and unshielded. However, exactingly drawn contour lines can be misleading as a means of accurately determining noise exposures in developed areas—particularly if noise barriers exist. This paper will explore the influence of site model geometry and contour computation parameters on estimated contour shape using the FHWA Traffic Noise Model (TNM).