Inexpensive Supplemental Short Noise Barriers
Community & Cultural Concerns, Environmental Process
Research Idea Scope
Recent advances in highway acoustic noise measurement technology have shown most vehicle noise generators are either at the tire/pavement interface or are positioned within 3.3 feet above the roadway surface. The National Academy of Sciences found all vehicle noise energy is lower in the vehicle profile than conventional wisdom has acknowledged. This means that less-expensive short noise barriers can potentially help lower traffic noise impacts. Short barriers have been completely overlooked and undervalued in the existing noise impact analysis process. Designed to address safety issues, solid-concrete-safety-shape barriers could, in some cases, also serve a secondary function to help attenuate traffic noise levels and provide a lower-cost option to taller expensive sound walls. Berms too have also been historically undervalued as a viable low-cost form of noise abatement. Two new, and more accurate, highway noise measurement technologies, OBSI and acoustic beam-forming, have now been developed and verified by the National Academy of Sciences, in NCHRP Research Reports 630, 635, and 842. These newer measurement technologies show that most all vehicle sound energy for light vehicles and heavy trucks is either generated by tire/pavement interaction or is mechanical/exhaust noise that is positioned just slightly above the pavement and very low in the overall vehicle profile. The current version of the FHWA Traffic Noise Model (TNM) distributes a large amount of noise energy tall in the vehicle profiles at 5 and 12 foot positions above the pavement. TNM significantly underestimates, and overlooks, the noise attenuation potential of shorter less-expensive noise barriers. This research would primarily examine how well short barriers could work at attenuating short-height vehicle noise. It would focus on capturing controlled pass-by data with/without short barriers in order to determine the insertion loss potential for short-height, 32 to 72-inch-tall, barriers and berms. This research would build on the prior NCHRP 630, 635, 842 studies and use acoustic beamforming measurements with supporting OBSI and SPL measurements to collect a comprehensive and representative matrix of short noise barrier field data. This project would help fill in knowledge gaps and determine if short barriers can provide a 1 to 5 dBA insertion loss and provide a supplemental low-cost noise attenuation design option. Tire/pavement noise is the dominate vehicle noise source, and short-concrete-safety-barriers, if positioned correctly, may furnish a quantifiable amount of noise reduction. Some SDOTs have also observed that low earthen berms provide much more attenuation than TNM calculations predict, and this strongly indicates that TNM energy positions are too tall and TNM assumptions are incorrect. Berms are another overlooked and inexpensive short barrier option that might provide significant traffic noise reduction-if the right-of-way can accommodate for berm footprints. Landscape architects could use this acoustic knowledge to lower traffic noise impacts in public green-spaces near roadways and intersections; highway engineers could use roadway cut material to lower traffic noise impacts on roadside communities. The findings of this research could have immediate application for SDOTs. The deliverables for this work would be a spreadsheet-based noise calculation model and accompanying documentation and guidance. EXAMPLE: An FHWA Type III bridge replacement project, doesn’t require a noise study, but it may create an unintended noise impact on receptors near and below the bridge. If the old outside concrete-safety-shape-barrier railing is replaced with a new, aesthetically pleasing, but acoustically transparent metal-bridge-railing, the noise levels near the bridge may be noticeably altered. When talking with the public on a bridge replacement project, knowing that a 42+-inch-tall solid-concrete-safety-shape barrier on an elevated roadway above sensitive noise receptors, will reduce the traffic noise by more than half (3 dBA) to all receptors that are less than 200 feet away, is critical project information. This research also has application to interchange connector ramps that are constructed three-or-four stories above surrounding residential communities and could benefit from a solid-concrete-safety-shape barrier that was designed for both safety and noise mitigation purposes. It is extremely useful for SDOTs noise specialists to know that noise impacts can be lowered (or not increased) by using short solid-concrete-safety-shape barriers instead of metal-beam-guard-railing. The short barriers can be used with confidence to lower tire/pavement noise exposure when elevated roadway cross sections are present. Metal-beam-guard-railing and metal-bridge-railing are acoustically transparent to vehicle noise, but solid-concrete-safety-shape barriers are acoustically beneficial if placed near elevated traffic. Under given roadway conditions, certain inexpensive design elements like solid-concrete-safety-shape barriers and earthen berms are very useful to alter the soundscape and lower noise impacts on sensitive receptors. Ultimately this research could be used to improve the computations in the FHWA TNM software; but this time is not budgeted for this proposed research.
Urgency and Payoff
Highway noise is always a top public concern on highway projects. Noise-impacted roadside communities increasingly have a high level of expectation for some form of noise abatement. Tall sound walls can cost as much as $2M/mile and are currently acoustically over designed. If walls do meet the acoustic performance requirements, they may be rejected for being too expensive. With the current version of FHWA TNM, shorter noise barriers are being rejected or overlooked because of internally-inaccurate-computational assumptions. SDOTs are paying too much for the few tall sound walls that do pass the noise analysis. Solid-concrete-safety-shape barriers maybe an alternative lower-cost and slightly less-acoustically effective practical solution. Potential noise impacts on bridge replacements are completely unaddressed. Solid-concrete-safety-shape barriers are already an established design component of highway infrastructure and could provide two functions: safety and noise attenuation. In California, concrete safety shape barriers typically cost 25% of what concrete masonry sound walls cost and if positioned above sensitive noise receptors could reduce traffic sound by half (3 dBA) or more, on roadside residences. This provides SDOTs with more less-expensive options to abate noise impacts.
Bruce Rymer AASHTO Noise Working Group and Caltrans 916-653-6073