Breaking Barriers: Alternative Approaches to Avoiding and Reducing Highway Traffic Noise Impacts (2022)

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1   Introduction National Cooperative Highway Research Program (NCHRP) Project 25-57: “Breaking Barriers: Alternative Approaches to Avoiding and Reducing Highway Traffic Noise Impacts” examined strategies other than traditional noise barriers to reduce highway traffic noise. Noise barriers are an effective way to reduce highway traffic noise and are the primary abate- ment measure applied to address noise impacts. Noise barriers, however, cannot always be constructed due to site constraints, safety considerations, or federal and state policies on reasonable expenditure per benefited receptor. Past implementations of U.S. federal regula- tions (23 CFR 772) and some state policies allowed for a broader examination and applica- tion of strategies to reduce noise, such as the construction of earthen mounds, lower speed limits, time-of-operation limits, horizontal or vertical alignment changes, or the creation of buffer zones to minimize noise impacts. A broader examination of current alternative noise reduction strategies could allow states to more effectively improve the noise environment in the vicinity of major highways and local roadways. The key objective of the project was to develop resources detailing innovative approaches beyond the use of noise barriers to minimize highway traffic noise, avoid traffic noise impacts, and address noise complaints. To meet the objective, the research team conducted and summa- rized a literature and data review, further investigated select combinations of strategies, and developed a flowchart-based handbook as a practical practitioner resource. The strategies discussed in this report were selected by the research team and panel; there may be other non-barrier traffic noise reducing strategies that are not covered in this report. The project was divided into two main phases. In the first phase, the researchers gathered information and data to summarize 14 primary strategies, examining them in terms of noise reduction, cost, and context appropriateness. For the second phase, the team further examined noise reduction, applying modeling techniques for three of the strategies: low- height berms, solid safety barriers, and acoustically soft ground. These three strategies were also examined in combination with secondary strategies. The research team investigated all three strategies using the FHWA Traffic Noise Model (TNM) v3.0, with supplemental investigations using other methods (strategy-specific and described in the applicable chapter) [Geo-decisions (Gannett Fleming) and the Volpe Center 2019]. S U M M A R Y Breaking Barriers: Alternative Approaches to Avoiding and Reducing Highway Traffic Noise Impacts

2 Breaking Barriers: Alternative Approaches to Avoiding and Reducing Highway Traffic Noise Impacts Strategy Noise Benefit Costs (scale $–$$$$$) Context Appropriateness On-Road Design Strategies Quieter bridge decks and joints: - Quieter bridge decks using diamond grinding or polyester overlays 5 to 10 dB (near source) ($$$$) Polyester overlay $10–$30 per ft2, geographically dependent; ($$) diamond grinding $1–$3 per ft2, geographically dependent Bridges or other structures Quieter bridge decks and joints: - Quieter bridge joints using patterned joint cover plates 6 to 9 dB (near source) ($$$–$$$$) 20% to 40% higher than conventional joints Bridges or other structures with expansion joints, particularly designed for seismic activity Quieter rumble strips with a sinusoidal pattern 3 to 7 dB (near source) ($–$$$) After minor equipment modifications, costs are similar to conventionally ground rumble strips; $0.15 to $0.60 per lineal ft Outside edges of travel lanes or centerline of undivided roadway Quieter pavement: - Diamond grinding Up to 7 dB ($$–$$$) $0.25 to $1.50 per ft2 All pavement surfaces (typically for concrete) Quieter pavement: - Open-graded or rubberized asphalt Up to 9 dB ($$–$$$$) $0.50 to $1.50 per ft2 All pavement surfaces Table S-1. Summary of alternate highway traffic noise-reducing strategies. Research Findings Table S-1 shows the summarized results from all strategies examined as part of this research project. There are 14 primary strategies (as seen in Table 2-1 in the main text), but some have been divided in Table S-1 to include sub-categories of the primary strategies. The information in the table includes potential noise reduction values and key considerations, relative cost esti - mates ($–$$$$$), and notes on context appropriateness. Highlights of the further investigations are included in the table for low-height berms, solid safety barriers, and acoustically soft ground, with additional information following the table. Appendix A provides highway traffic noise terminology and includes sound level metrics and abbreviations. A full literature review of the 14 primary strategies is available in Appendix B: Summary of Noise-Reducing Strategies. The information is summarized by strategy throughout this report. Further investigations for low-height berms, solid safety barriers, and acoustically soft ground are summarized in Part I of this report, and plots and tables of the predicted noise reductions are shown in Appendices C, D, and E. Part II comprises a practitioner’s handbook.

Summary 3   Solid safety barrier in lieu of guardrail: - For freeway/highway Preliminary investigations: 0.4 dB to 2.6 dB depending on project specifics: distance to receiver and site topography Further investigations: at 100 ft, 5 to 7 dB reduction, up to 8 dB with road slightly elevated (assumes tall freeway/highway safety barrier, 6.8 ft); best for hard ground sites and low % heavy trucks (% HTs) ($$) Minor overall project cost increase Limited-access highways if state standards allow Solid safety barrier in lieu of guardrail: - For street/arterial Preliminary investigations: 2.0 dB to 6.6 dB depending on project specifics: distance to receiver and site topography Further investigations: at 100 ft, 3 to 4 dB reduction, up to 6 dB with road slightly elevated (assumes tall street/arterial safety barrier, 4.8 ft); best for hard ground sites and low % heavy trucks ($$) Minor overall project cost increase State or local roadways if state or local standards allow Solid safety barrier in lieu of guardrail: - Low barriers and diffractors Up to 9 dB (similar to safety barrier with diffractor top added) Most effective close to sound source Further investigations: for near lane, diffractor can provide additional 3 dB reduction compared to short safety barrier alone (greatest in 500 to 1600 Hz -octave bands; could be tuned for further noise reduction). ($$$–$$$$) WHISwall (low barrier/diffractor) is ~$259/ft or ~$1.4M/mi; WHIStop (diffractor only) is ~$204/ft or ~$1.1M/mia May be limited to 2-lane road, although wider highway applications should be examined Including a separation zone between roadway and side path in TNM modeling ($) Minimal for modeling effort; construction and maintenance costs would vary by Roadways with sidewalks or shared-use paths When the surface in the separation zone was very different from the default ground type (pavement versus lawn), the separation zones can be modeled to accurately account for noise increases and decreases (although the decrease when switching from lawn to snow showed reductions only up to 0.3 dB). ground type and geographical area Strategy Noise Benefit Costs (scale $–$$$$$) Context Appropriateness Quieter pavement: - Thin bonded asphalt overlays Up to 6 dB ($$–$$$$) $0.50 to $1.50 per ft2 All pavement surfaces Highway Design Strategies Alignment shift: - Horizontal alignment shift < 1 dB to 10+ dB depending on project specifics: extent of shift, site topography, and vehicle types ($$$$–$$$$$) Very expensive due to additional Highways or local roadways with vacant land right-of-way and construction costs opposite the sensitive sites Alignment shift: - Vertical alignment shift < 1 dB to 10+ dB depending on project specifics: extent of shift, site topography, and vehicle types ($$$$–$$$$$) May be less expensive than horizontal shift if right-of-way is sufficient Highways or local roadways where right-of-way is sufficient Table S-1. (Continued). (continued on next page)

4 Breaking Barriers: Alternative Approaches to Avoiding and Reducing Highway Traffic Noise Impacts Right-of-Way Design Strategies Low-height berms: - Standard low-height berms (up to 6 ft high) The noise benefit of low-height berms ranges from 2 dB to more than 10 dB. The noise benefit strongly depends on the relative elevation of the source, berm, and receiver with the greatest noise reduction for receivers situated at a lower elevation than the roadway. A model of a 3.1-ft high berm showed a noise reduction of 2 to 5 dB with roadside and/or median berms. Further investigations: at 100 ft, 4 to 7 dB reduction for a 6-ft berm, up to 9 dB with road slightly depressed; best for fewer lanes and low % heavy trucks ($–$$$) In opening year, a concrete wall is about 2.6 to 3 times more expensive than an earthen berm. Assumes no right- of-way costs or fill costs for the berm. (Berms can be as or more expensive than a wall if material has to be brought in.) Maintenance costs for berms expected to be less than for noise walls. Highways or arterials with sufficient right-of- way (ROW) space for berm bases (up to 36 ft for a 6-ft high berm assuming a 3:1 slope on both sides) Low-height berms will be most effective when the berm can be placed close to the source or receiver, and when either the road or receiver decrease in elevation relative to the other (e.g., for a site that slopes down behind berm or for a depressed road). Low-height berms: These berms could reach taller heights closer to the noise source, ($$–$$$$) Engineered low- height berms - Engineered low- height berms with steeper slopes improving noise reduction compared to standard low-height berms. Further investigations: can influence reduction by up to 2 dB, but countering parameters (having a longer slope to help with soft ground effects versus moving the berm peak closer to traffic) need to be considered In 2011, costs were estimated to be $25 to $50 per sq. ft. If an irrigation system is needed to maintain vegetation, the costs would increase. could be implemented where there are ROW constraints for a standard berm. When the surface in the separation zone was similar to the default ground type, the addition of a ground zone for the separation zone made only a slight (0.1 dB or 0.2 dB) difference. Can increase accuracy up to 1.2 dB to 1.6 dB if separation zone is hard soil or pavement compared to lawn; for more absorptive surfaces than lawn, provides minimal noise reduction benefit (up to 0.3 dB). The noise reduction would be greater if separation zone is highly sound absorptive and the default ground type is not. Strategy Noise Benefit Costs (scale $–$$$$$) Context Appropriateness Table S-1. (Continued).

Summary 5   Vegetated screens: - Thick vegetation belts (65 ft) Measured noise reduction of 3 to 9 dB; up to 10 dB noise reduction in computational model with optimized planting ($–$$$) Geographically dependent on type of vegetation used (example cost of $100 to $300 per tree, installed) and the need for upkeep; ROW cost could be significant. Highways with significant ROW available; areas that can support sufficiently dense vegetation Vegetated screens: - Moderate-to-low thickness vegetation screens (< 65 ft) Measured noise reduction of 1 to 3 dB; up to 6 dB noise reduction in computational model with optimized planting ($–$$) Geographically dependent on type of vegetation used (example cost for two rows planted 10 ft apart would be $0.5–$1.5 Areas that need relatively minor noise reduction; areas that can support sufficiently dense vegetation million/mile) and the need for upkeep; may be additional ROW cost for moderately thick vegetative belts of 10–20 m (33–66 ft) Vegetated screens: - Vegetation to improve adverse sound propagation effects A row of trees behind a noise barrier can reduce negative downwind noise effects. A vegetated belt can reduce the likelihood of a temperature inversion layer, which can bend sound downwards. ($) Geographically dependent on type of vegetation used and the need for upkeep; may be additional ROW cost Sites with a wall noise barrier, especially ones with prevalent downwind conditions Sites that can support vegetation, especially ones with frequent temperature inversion conditions Low-height berms: - Low-height berms with absorptive ground A more absorptive ground type could increase the noise reduction by up to 5 dB compared to packed dirt. Further investigations: could provide up to 2 dB additional reduction compared to hard surfaces, but only if the site ground type is hard (e.g., areas with hard-packed dirt or pavement) Costs would be geographically dependent based on the absorptive ground type selected. Highways with ROWs that can accommodate acoustically softer ground surfaces. Low-height berms: - Unusually shaped low-height berms For a 3.3-ft high berm next to an arterial road, unusually shaped berms did not provide a noise benefit compared to a traditional wall or berm. Unusually shaped berms have not been implemented; they were considered in a numerical model. Have yet to be implemented; considered in models only Strategy Noise Benefit Costs (scale $–$$$$$) Context Appropriateness Table S-1. (Continued). (continued on next page)

6 Breaking Barriers: Alternative Approaches to Avoiding and Reducing Highway Traffic Noise Impacts Sound-absorbing ground surface and ground treatment: - Acoustically soft ground TNM predictions show 1 to 2 dB for placement in ROW or median, however, more may be realized with gravel surfaces (multi-lane highway). Calculation of Road Traffic Noise (CRTN) predictions show 3 to 12 dB for placement 8.2 ft from the edge of the near travel lane, largest decrease for gravel and low flow resistivity grassland (2-lane road); soft surface extended 82 ft or more from road. ($–$$$) Geographically dependent – may need to maintain grass or gravel Highways with ROWs or medians that can accommodate acoustically softer ground surfaces Soft ground treatments are less effective as the receiver height increases. As source-receiver distance is increased, the insertion losses due to the near-source ground treatments do not decrease as they would with a traditional noise barrier. Further investigations: at 100 ft, 4 to 5 dB reduction, up to 6 dB combined with quieter pavement; best for fewer lanes, hard ground site, and low % heavy trucks. Sound-absorbing ground surface and ground treatment: - In-ground treatments Testing shows 2.4- and 4-dB reduction for two recessed lattice structures, one 0.95 m (3 ft) wide and 0.2 m (0.7 ft) deep, the other 1.9 m (6 ft) wide and 0.2 m (0.7 ft) deep, measured 39 ft from a single vehicle pass-by source. Calculations show 2 to 8 dB reduction for a recessed lattice structure 1-ft deep placed 8 ft from the nearest source; range of insertion loss depends on the width of the structure, ranging from 5–79 ft, widest being most effective. In-ground resonators may be tuned to reduce a specific frequency, although their effectiveness would likely be less than 2 to 3 dB (shoulder placement). ($$$) Varies with product used to construct As an example, WHISstone is ~$170 USD per 3.3 ft length along road (single row or 3-ft wide lattice). Highways with ROWs that can accommodate embedded structures close to the near travel lane (narrow shoulders, likely 2-lane road) Vegetated screens: - Vegetation to improve perception of noise with narrow vegetation screens No noise benefit; subjective reports of decrease in annoyance when more vegetation is present. However, the relationship is difficult to show in subjective studies with test subjects at multiple vegetated or non-vegetated sites. ($) Geographically dependent on type of vegetation used and the need for upkeep Areas that do not qualify for noise abatement, but report high levels of traffic noise annoyance Vegetated biofilter basin Less than 1 dB effect on noise adjacent to a 6-lane highway; For a 2- lane highway, there is less than 1 dB effect out to 300 ft and up to 2.4 dB out to 500 ft. ($) No additional cost if the vegetated swale is already planned Highways or roads where a vegetated swale is needed Strategy Noise Benefit Costs (scale $–$$$$$) Context Appropriateness Table S-1. (Continued).

Summary 7   Solar panels If continuous panels assumed, then > 11 dB; however, gaps between arrays and panel angles need to be considered. ($$–$$$$) Cost for purchase, installation, and maintenance of panels Highways with ROW space Other Strategies Operations management strategies: - Speed restrictions For combined traffic flows of automobiles and heavy vehicles, an overall noise reduction of approximately 2 dB (LAeq) may be expected with a reduction in speed of 10 mph. ($–$$) Generally low; may impose costs of increased travel times. Other costs may include public education, enhanced enforcement, and related infrastructure improvements such as traffic calming measures. Applicable either to limited-access highways or local road networks Operations management strategies: - Truck restrictions Reductions in maximum pass-by levels (LAmax) of 10 dB or more. Overall LAeq noise reduction dependent on many factors, ranging from approximately 1 to 6 dB. ($–$$) Generally low; may impose indirect costs on truck operators and general public. Other costs may include public education, enhanced enforcement, and increased maintenance on designated alternative truck routes. Most commonly implemented on local road networks; also may be used on limited-access roads Sound-absorbing ground surface and ground treatment: - Above-ground treatments Boundary Element Method predictions show the following: - Low (< 1-ft) parallel walls (30-ft wide) can provide up to 9 dB reduction at 164 ft, - A low (< 1-ft) lattice wall structure (30-ft wide) can provide up to 11 dB reduction at 164 ft. Related notes: - Wider structures provide greater reduction. - Comparative soft ground for half the source-receiver ($$–$$$$) Varies with product used to construct – would need to construct and maintain low parallel or lattice walls Highways with ROWs that can accommodate above-ground low structures close to the near travel lane (narrow shoulders, likely 2-lane road) Note that above- ground structures may pose safety concerns when placed near travel lanes because they are not drivable. distance provides 9 dB reduction. Effectiveness is not affected by receiver height or distance. Strategy Noise Benefit Costs (scale $–$$$$$) Context Appropriateness Table S-1. (Continued). (continued on next page)

8 Breaking Barriers: Alternative Approaches to Avoiding and Reducing Highway Traffic Noise Impacts Sound-absorptive treatment: - Treatment on retaining walls Opposite side reflection: 1 to 2 dB with changes in spectral content potentially reducing adverse reflected noise effect Parallel barriers: predicted up to 2.5 dB Truck/barrier wall reflections: predicted up to 4 dB ($$–$$$) Varies by treatment/project (one example was $18–$22/ft2) Locations where retaining walls can reflect noise to sensitive receptors Sound-absorptive treatment: - Treatment on bridge substructures Highway measurements showed up to 6 dB, lab measurements up to 11 dB, and predictions up to 5 dB with sound-absorptive treatment. Low frequency vibration dampers can help to reduce noise from steel bridge structure. ($$–$$$$) Cost of material, installation, maintenance Elevated highway bridge structures or those over depressed highways where reflections can affect sensitive receptors Sound-absorptive treatment: - Treatment in tunnels Measurements showed 5 to 10 dB reduction for sound-absorptive treatment. Surface roughening predicted to reduce noise by 4 dB ($$–$$$$) Cost of material, installation, maintenance Highway tunnels where reflections can affect sensitive receptors Sound-absorptive treatment: - Other structure applications Sound absorption using Helmholtz resonators or metamaterials; engineered products that can be tuned to optimize traffic noise reduction Curvature of a wall should be considered to avoid focusing sound. Application of absorptive material to ramp and median safety barriers can reduce reflections. Green wall systems on walls and rooftops can reduce reflections. Example: for a green area of 3,972 ft2 - investment $23,236, maintenance $10,208 Locations where structure surfaces can reflect noise a January 13, 2021 conversion rate; based on the cost of 700/m for WHISwall and €550/m for WHIStop. Strategies implemented by receptors or local governments: - Site planning Up to 3 dB when distance to the roadway is doubled. 10 dB or more when non-sensitive buildings or ($) Minimal when considered early New development privacy walls are used to shield sensitive sites or areas. Strategies implemented by receptors or local governments: - Building design Up to 13 dB when noise sensitive rooms are placed farthest away from the highway ($) Minimal when considered early New development or redevelopment Strategies implemented by receptors or local governments: - Construction methods Up to 35 dB interior ($$–$$$$) Expensive due to materials required New development or redevelopment Strategy Noise Benefit Costs (scale $–$$$$$) Context Appropriateness Table S-1. (Continued).

Summary 9   More details of the three further investigations are summarized as follows. Low berms were examined in terms of berm height, width, placement, shape, top width, and berm surface ground type. They were also combined with roadway depression to help reduce highway traffic noise. Low berms are considered to have a maximum height of 1.8 m (6 ft). The following general trends were found: • For roadways with moderate heavy truck volumes (approximately 5% of total traffic volume), low berms can provide up to an approximately 4–7 dB reduction. In general, reductions are slightly greater (by approximately 1 dB) for 2–4-lane streets than for 4–8-lane freeways. • When combined with roadway depths (depression) of up to 0.9 m (3 ft), low berms can provide up to an approximately 9 dB reduction for 2–4-lane streets and up to an approxi- mately 7 dB reduction for 4–8 lane freeways. • Berm height and receiver height are the two most important parameters. 1.8-m (6-ft) high berms provide up to approximately 4 dB more reduction than 0.9-m (3-ft) high berms. Similarly, reductions at 1.5-m (5-ft) high receivers are up to approximately 4 dB greater than at 4.6-m (15-ft) high receivers. • At sites with default hard soil ground type, reductions typically are about 2–4 dB greater than at sites with default lawn. • Soft berms are more effective than hard soil berms, providing up to approximately 2 dB higher reductions. • Although in some cases berm shape, including slope and top width, may affect reductions by up to 2 dB, these parameters typically are less critical than those related to height and ground type. Solid safety barriers (SSB) were examined in terms of barrier height and also combined with roadway elevation to help reduce highway traffic noise. In addition, a separate analysis was conducted for a barrier diffractor top. The following general trends were found: • At 30 m (100 ft): For streets, the solid safety barrier can provide up to 3–4 dB reduction. The combined barrier and small road elevation strategies can provide up to 4–6 dB reduction. For freeways/highways, the barrier can provide up to 5–7 dB reduction (assuming taller safety barrier than for streets). The combined strategies can provide up to 5–8 dB reduction. • Solid safety barriers are more effective for reducing traffic noise when roadways are narrow, ground types are hard soil, and heavy truck percentages are low. When roadways are wide, ground types are lawn, and heavy truck percentages are high, an SSB may be effective only for receptors located relatively close to the highway. • Solid safety barriers merit consideration for any street case with hard-soil ground type and when receptors of concern are less than 30 m (100 ft) from the roadway and ground type is lawn. Solid safety barriers also merit consideration for any freeway case with hard soil ground type and when receptors of concern are less than 90 m (300 ft) from the free- way and ground type is lawn. • Solid safety barrier effect, regardless of barrier height or roadway elevation, is 2 dB or greater at all distances with hard soil than with lawn ground type at 1.5-m (5-ft) high receivers. • Solid safety barrier effectiveness increases as the barrier height increases. Increasing road- way elevation increases the effectiveness of a solid safety barrier in all cases. • Solid safety barriers provide a discernable noise reduction at all distances regardless of the percent heavy trucks. However, solid safety barrier effectiveness is greatly reduced for receivers beyond 30 m (100 ft) when heavy trucks are 15%. • Solid safety barriers provide more noise reduction at 1.5-m (5-ft) high receivers than 4.5-m (15-ft) high receivers closest to the roadway, but the SSB attributable reduction is nearly the same for receivers more than 45 m (150 ft) from the roadway, regardless of the receiver height.

10 Breaking Barriers: Alternative Approaches to Avoiding and Reducing Highway Traffic Noise Impacts Acoustically soft ground strips were examined in terms of strip width and sound absorp- tion value. The strips were also combined with quieter pavement to help reduce highway traffic noise. The following general trends were found: • An acoustically soft ground strip of gravel alone provides several dB reduction for sites with acoustically hard ground. It is not recommended as a strategy for sites with acousti- cally soft ground, where minimal reduction is predicted and only very close to the road. • An acoustically soft ground strip of gravel combined with quieter pavement increases the noise reduction for hard ground sites. For soft ground sites, the combined noise reduc- tion is dominated by quieter pavement. • The noise reduction decreases with distance at a hard ground site (region of maximum influence within 61 m or 200 ft from center of near travel lane). • For streets, the strip can provide up to 4–5 dB reduction. The combined strip and quieter pavement strategies can provide up to 5–6 dB reduction. [More reduction is expected for pavements quieter than open-graded asphalt concrete (OGAC).] • For freeways/highways, the strip can provide up to 3–4 dB reduction. The combined strip and quieter pavement strategies can provide up to 4–5 dB reduction. (More reduction is expected for pavements quieter than OGAC.) Practitioner’s Handbook In addition to the literature/data review and further investigations, this project also pro- duced a practitioner’s resource, which is Part II of the research report. The practitioner’s hand- book provides a procedural screening of alternative noise reduction strategies included in this report. A four-step procedure is applied, starting with roadway context and working through other elements of context appropriateness, related costs, and then estimated noise reduction. The estimated noise reduction is not intended to replace project-specific predictions, which must be made prior to implementation of a chosen noise-reduction strategy. The handbook is shown in Part II. Recommendations Results of this NCHRP project research are intended to provide stakeholders with infor- mation about alternative strategies to reduce highway traffic noise. Reasons to apply alter- native strategies include: (1) when a barrier cannot be constructed due to site constraints, safety considerations, or federal and state policies on reasonable expenditure per benefited receptor; or (2) when applying the strategies may prevent noise impacts. The report and practitioner’s handbook allow identification of viable noise-reduction strategies for an array of project parameters. Some strategies are currently implementable and some require further investigations. Each would need to be evaluated with project specifics to know the effective- ness at reducing noise within the project’s study areas. In addition to the investigations conducted as part of this NCHRP project, further investi- gations would help to better understand some of the noise reduction strategies, either for the purpose of better understanding applications or limitations, or for testing the viability of a strategy. The following six promising additional research topics are described in this report: 1. Effectiveness of low barriers to reduce noise generated by different types of highway vehicles; 2. Effectiveness of solar panels to reduce highway traffic noise; 3. Effectiveness of gravel in right-of-way to reduce highway traffic noise; 4. Effectiveness of vegetated screens to reduce highway traffic noise, and when to include effects in modeling; 5. Effectiveness of in-ground treatments to reduce highway traffic noise; and 6. Effectiveness of absorptive treatment on a bridge understructure to reduce highway traffic noise.