Presenting highway noise exposures : The downfall of Contourtionists

This article is intended as a cautionary guide to estimating noise contours for highway noise exposures and to the limitations of contours for presenting noise exposure information. It is a consolidation of two recent papers – one explored the influence of site model geometry and contour computation parameters on estimated contour shape using the U.S. Federal Highway Administration (FHWA) Traffic Noise Model (TNM).1 The second examined the significance of a substantial upgrade to TNM released after the preparation of the initial paper and extended the previously reported contour modeling analyses.2 A contourtionist should not be confused with a contortionist: con·tor·tion·ist: one who contorts; specifically : an acrobat able to twist the body into unusual postures (Merriam-Webster’s 11th Collegiate Dictionary). con·tour·tion·ist: one who calculates contours; specifically: a highway noise analyst able to twist traffic noise exposures into misleading positions.1 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) in its Part 150 Airport Noise Compatibility Planning process. Highway noise assessments performed to meet U.S. Federal Highway Administration (FHWA) or state transportation departments 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 and 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 considering 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 flat and unshielded. However, exactingly drawn contour lines are misleading as a means of determining noise exposures in developed areas – particularly if noise barriers exist.