Using Onboard Sound Intensity Measurements to Interpret Results of Traffic Noise Modeling

With the evaluation of noise impacts and noise abatement under the requirements of 23 CFR 772, Procedures for Abatement of Highway Traffic Noise, traffic noise levels are calculated with FHWA's Traffic Noise Model (TNM). The TNM calculates traffic noise levels on the basis of the geometry of the site and traffic volumes, mix, and speeds. The TNM does not currently account for pavement type or condition, atypical vehicle noise populations, transparent shielding, reflections, or meteorological conditions. For these reasons, noise measurements are conducted, and traffic noise model adjustments and calibration factors are developed. For several highway studies recently conducted in California, onboard sound intensity (OBSI) measurements were made concurrent to the traditional wayside noise measurements along the length of the freeway to be studied, to account for the existing pavement type and condition of the freeway. Use of OBSI data, when compared with the calculated TNM average pavement OBSI level, has helped to interpret calibration factors that had been developed in the TNM on the basis of standard modeling practices. A description is given of how the OBSI can be used as a supplementary tool to help interpret calibration factors developed for a TNM model.