Abstract
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.
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