Parallel barrier effectiveness under free-flowing traffic conditions

The U.S. Department of Transportation, Research and Special Programs Administration, Volpe National Transportation Systems Center, in support of the Federal Highway Administration and seventeen sponsoring state transportation agencies, is conducting a National Pooled-Fund Study (NPFS), HPandR 0002-136, "Evaluation of Performance of Experimental Highway Noise Barriers." The first publication supporting the NPFS, FHWA-RD-90-105, "Parallel Barrier Effectiveness, Dulles Noise Barrier Project", presented measured results and evaluations of parallel barriers subject to controlled traffic conditions. This document is the second publication supporting the NPFS. It presents the results of a measurement study performed at a highway noise barrier site located along Interstate 495 in Montgomery County, Maryland. The objective of the study was to measure the degradation in acoustic performance of a highway noise barrier due to the close proximity of a parallel barrier on the opposite side of the roadway. The test site selected for measurements consisted of a contiguous arrangement of two parallel reflective noise barriers followed by a single noise barrier. Five-minute, energy-averaged, A-weighted noise levels were computed from data measured simultaneously at identical heights and offset positions behind the single and parallel barrier arrangements. Measured results show barrier insertion loss degradations of .6 to 2.8 dBA, dependent on microphone height and offset distance behind the barrier. In addition, three parallel-barrier prediction programs were used to model the study site (BARRIER, BARRIER-X, and IMAGE-3). A comparison of the measured and predicted results is presented.\n