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As a portion of the overall research work for NCHRP Project 1-44, field tests were performed on in-service pavements to demonstrate the applicability of the onboard sound intensity (OBSI) method for quantifying the effect of different pavement types in relation to total vehicle noise emissions measured with the controlled pass-by (CPB) and statistical pass-by (SPB) methods. Testing included the simultaneous measurement of OBSI on two specific candidate test tires, CPB on test vehicles equipped with those tires, and SPB of both light and heavy-duty vehicles. With the SPB data, the relationship between OBSI tire–pavement noise and average vehicle noise emissions were examined for both light vehicles and heavy trucks. To account for site-to-site variation, pass-by results were normalized by using the measured relationships between OBSI and CPB levels for each site. On the basis of these results, it was found that SPB levels for both light vehicles and heavy trucks could be predicted accurately with a standard deviation of 0.8 dB for both vehicle categories for an average of sites that used OBSI data obtained from either of two candidate light-vehicle test tires.

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A highway-widening project was completed on US-101 near San Rafael in Northern California with the addition of a high-occupancy vehicle lane in each direction, relocation of the southbound existing sound wall, and the addition of a multipurpose path. Because there was an existing sound wall in the northbound direction, additional conventional noise abatement was not considered feasible under California Department of Transportation and FHWA policies for this Type 1 project. However, two elements were included to improve aspects of existing noise environment. The elements included overlaying all lanes of the completed project area with quieter, open-graded asphalt and applying absorptive treatments to the traffic side of both barriers. To document the potential benefit of these features, the Transit Authority of Marin commissioned a study of preproject and postproject noise levels. Included were measurements of wayside noise levels above and behind the northbound sound wall and of tire–pavement noise using the onboard sound intensity method. Measurements indicated that 6- to 7-dB reductions in wayside noise levels both above and behind the barrier could be accounted for by reduction in tire–pavement noise. Effectiveness of added absorption could not be directly isolated in the measurements, so the benefit of this feature was assessed through acoustic modeling. Results indicated that additional reductions of 3 to 5 dB could occur for receptors elevated above the roadway, shielded from ongoing traffic but with a view of the opposing sound wall, and at distances of 500 ft or more. Near the highway, reduction provided by absorptive treatment was 1 to 1½ dB or less.