Federal transportation authorities in May approved the use of a transparent sound barrier along part of the rebuilt Woodrow Wilson Bridge, a major section of the Interstate 95 system south of Washington, D.C.

The sound barrier will extend from the bridge's abutment to shield nearby residents in Alexandria, Virginia, from noise. Its transparency answered concerns over safety, maintenance and aesthetics that had made traditional sound walls unacceptable, according to Ronaldo "Nick" Nicholson, project manager on the bridge for the Virginia Department of Transportation. "The intention of Woodrow Wilson is to be a signature bridge, and the aesthetics of an opaque wall was in conflict with that," he says.

That makes the transparent barrier worth its cost--as much as three times more than a conventional wall. It is also worth it to near-by homes. A 1996 study by two researchers at the University of California, Davis, found that nationally 400,000 homes are close enough to highways to lose value due to noise.

This is the first bridge application in the United States, according to Cyro Industries, the company that makes the transparent acrylic panels. The lightweight product can be engineered so that if it gets hit and is dislodged from its holding, it doesn't fall, says Steve Barratt, Cyro's marketing manager.

Barratt adds that transparent sound walls are becoming common overseas--an engineer on the Wilson bridge spotted one in Europe, piquing interest in the technology--and similar sound walls are planned for upcoming projects in Alaska, Colorado and Ohio.