Case Study in Public Perception of Noise Barrier Effectiveness

I-97 and US-50 are the two major limited-access highways that connect Washington, D.C., and Baltimore to Annapolis (Maryland’s state capital). Three communities situated along US-50 and at the interchange with I-97 were considered for Type I noise barriers as the result of proposed widening and ramp additions to the highway network. Ultimately, precast concrete noise barriers were constructed in five sections on both sides of the highway in the area. Located just west of the US-50/I-97 interchange is the retirement community of Heritage Harbour. During development and construction of the community in the early and mid-1980s, the developer constructed two noncontiguous earth berms adjacent to but outside the highway right-of-way line. The noise barrier design concept developed for this project incorporated these existing earth berms into the overall US-50/I-97 barrier system. Barrier wall sections were designed to tie into both ends of the berms to mitigate flanking noise and ultimately to provide a “closed system” for the adjacent communities. STAMINA 2.0/OPTIMA modeling identified that the noise barrier construction would provide an additional 3- to 7-dBA insertion loss for homes situated near the berms. After construction of the US-50/I-97 noise barrier system, community members living directly behind the berms claimed that the new barrier walls were “funneling” noise over the top of the berms and causing the noise to be louder than it was before the barrier was constructed. The Maryland State Highway Administration monitored postbarrier noise levels in response to the community’s request. The monitored levels confirmed OPTIMA predicted noise levels and did not support claims that noise levels had become worse after barrier construction. Under prebarrier conditions, it was hypothesized that noise particularly from heavy vehicles unshielded by the existing berms was substantial enough that vehicle noise passing over the berms was being masked, so that the source direction was not always discernible. Once the “flanking noise” was abated by the barrier walls, vehicle noise passing over the berms dominated. The results of the noise monitoring and an explanation of identified phenomenon were successfully presented to the community. The case study presents the US-50/I-97 berm/barrier public perception issues, discusses the project’s applicability to previous “public noise perception” research, and suggests public involvement techniques that could prepare communities for unique acoustic situations.