Implications of Rounding in Highway Noise Analyses and Policies

Abstract

In performing noise analyses, noise measurement equipment and noise predictive models usually report noise levels in tenths of a decibel. Customarily, these levels are reported as whole decibel numbers in state highway agencies’ noise reports. However, there is not a consistent rounding method used by state highway agencies (SHAs) that is applied to these levels in noise level calculations, in noise abatement feasibility and reasonableness evaluations, and in the reporting of results in SHA noise study reports. The objective is not to recommend one method of rounding, but to discuss the influence that several rounding methods may have on decisions regarding the feasibility and reasonableness of noise abatement devices and to offer recommendations and suggestions for SHAs to consider when determining the number of impacted receptors, calculating a noise barrier’s insertion loss, determining the number of benefited receptors, weighting benefited receptors, addressing noise reduction design goals, and calculating cost–benefit values. Nine recommendations are presented for consideration by SHAs, and rounding-related factors are suggested for consideration by SHAs in the modification of any noise policy or guidance document.