State departments of transportation (DOTs) are required to consider highway traffic noise impacts from projects on existing and planned facilities. When these impacts exceed certain thresholds, 23 CFR 772 specifies a limited number of approaches to mitigate highway traffic noise. The most commonly used approach is noise barriers, which can be walls or, in some cases, earthen berms. To date, some 3,000 miles of noise walls have been constructed along U.S. highways at an average cost of $2 million per mile of wall.
However, noise walls are not always effective and appropriate. In some cases, a noise analysis indicates that a noise wall is not feasible and reasonable per 23 CFR 772. In other cases, a noise wall may provide only limited reductions of noise levels because of site conditions such as topography or traffic volumes. Even when a noise wall may provide an acoustical benefit, it may not meet cost-effectiveness criteria.
Notably, noise regulations and policies only address mitigation through noise abatement, although common practice to address environmental issues is to avoid or minimize impacts, before considering mitigation. As a result, transportation agencies have no means to address highway traffic noise outside the regulatory process and are thus constrained to respond to community needs.
Noise reduction targets established by policy or regulation can lead to situations where a transportation agency constructs a noise wall only for those portions of a community that qualify for abatement under regulatory requirements, while other portions of the same community receive no noise reduction benefit. Even if noise levels do not meet regulatory definitions of a noise impact, state DOTs and other transportation agencies may also seek to address complaints about/of excessive highway traffic noise. In addition, some communities may want reduced highway traffic noise, but oppose a noise wall. Conversely, communities may advocate for a noise barrier, although topography and other site conditions may mean that the barrier will function primarily as a visual screen and provide little reduction in noise.
To address these limitations, transportation agencies have sought alternative approaches to highway traffic noise reduction. For example, quieter pavements can be useful in certain applications or in conjunction with traditional noise walls (see NCHRP Report 738: Evaluating Pavement Strategies and Barriers for Noise Mitigation). Rumble strips can be quieter using newer designs. However, climate and road maintenance practices can limit the use of these and other on-road design choices. There is some existing research into the effectiveness of right-of-way design elements. For example, the Ohio DOT has evaluated the noise reduction capability of small height (3'- 6') berms (see Acoustical Performance of Small Height Earthen Berms, OTP 1.2:Ac185/2018. Available at: http://cdm16007.contentdm.oclc.org/cdm/ref/collection/p267401ccp2/id/16847),
which are not appropriate or feasible for all locations or contexts.
Research is needed to provide transportation agencies and their partners with information on how to avoid or reduce highway traffic noise using innovative approaches that are compatible with a wide range of contexts. This project will identify best practices and nontraditional and emerging approaches to increase design options for highway traffic noise solutions that are acceptable to adjacent communities and the transportation agency.
The objective of this research is to develop a resource of innovative approaches--beyond the use of noise barriers--to avoid and minimize highway traffic noise and address complaints. The resource should provide descriptions, ranges of noise reduction benefits, cost factors, and context-appropriateness for design choices and management strategies that may be adopted for other reasons, but that provide noise reduction co-benefits, as well as those adopted specifically to address noise.
The research team has provided a summary report documenting the results of a review of the literature and a survey of state DOTs. View or download the report here: http://onlinepubs.trb.org/onlinepubs/nchrp/docs/NCHRP25-57TechMemo.pdf