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

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P A R T I I Practitioner’s Handbook

139   National Cooperative Highway Research Program’s Project 25-57: “Breaking Barriers: Alterna- tive 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 abatement measure applied to address noise impacts. However, noise barriers cannot always be constructed due to site constraints, safety considerations, or federal and state policies on reasonable expenditure per benefited receptor. Past implementations of federal regulations (23 CFR Part 772) and some state policies allowed for a broader examination and application 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 NCHRP Project 25-57 was to develop resources detailing innovative approaches beyond use of noise barriers to minimize highway traffic noise, avoid traffic noise impacts, and address noise complaints. To meet the objectives, the research team conducted and summarized a literature review and a data review and then further investigated select combina- tions of strategies. The research and results are presented in the main NCHRP Research Report 984 and in Part II, which is the practitioner’s handbook. The handbook is accompanied by a set of flowchart sheets for use in identifying strategies appropriate for a given context. The flowchart sheets may be accessed for downloading by searching on the TRB website for “NCHRP Research Report 984”. C H A P T E R 1 1 Introduction

140 The practitioner’s handbook is intended to be a quick reference guide to innovative approaches that could minimize highway traffic noise, avoid traffic noise impacts, and address noise com- plaints. The handbook provides strategy descriptions, potential noise reduction benefits, cost considerations, and the context-appropriateness for multiple strategies that may be adopted specifically to address traffic concerns or that might be implemented for other reasons, such as safety or aesthetics, and provide noise reduction benefits. The practitioner’s handbook presents a summary of alternative noise reduction strategies. More detailed information is included in Part I of the report and in the report’s Appendix B: Summary of Noise-Reducing Strategies, including information about noise-reducing strategies that appear to be appropriate for a specific project based upon the information in the handbook. The handbook is intended for persons with some experience with traffic noise measurement, prediction, and abatement evaluation, such as state department of transportation (DOT) traffic noise analysts, highway design professionals, and others who are familiar with the terminology used in traffic noise studies. For those who do not have such experience, a review of appropriate federal, state, or local policies and procedures related to highway traffic noise is recommended. C H A P T E R 1 2 Purpose of the Practitioner’s Handbook

141   The practitioner’s handbook is intended to provide a procedural screening of alternative noise reduction strategies in Part I of the research report. This chapter provides a four-step process to determine which noise reduction strategies may be appropriate for a specific project (i.e., have merit) and which strategies are not likely to be appropriate for that specific project. To follow the four-step process, users will need the flowcharts, which are available for download by searching on the TRB website for “NCHRP Research Report 984”. Step 1 – Practitioner determines appropriate roadway type by reviewing the definitions in Chapter 14 and selecting the roadway type most similar to the specific project of concern (e.g., the project is a local road with two lanes in each direction and no center turn lane, so a 4-lane narrow street is most similar). Step 2 – Practitioner looks at the Roadway Types Versus Strategy Matrix (found in Chapter 14) and identifies strategies likely to be effective or very effective for the appropriate roadway type (e.g., for a 4-lane narrow street, quieter pavements, solid safety barriers, low-height berms, vegetated screens, and acoustically soft ground appear to have merit). Note that multiple strategies are likely to have merit in most cases. Step 3 – Practitioner reviews overview of strategies that have merit for the appropriate roadway type. Practitioner eliminates some strategies because they do not apply, cannot be implemented, or are not likely to provide enough noise reduction. For some strategies, the practitioner’s decision to do site-specific investigations or provide recommendations can be made at the conclusion of Step 3. For other strategies, such as solid safety barriers, low-height berms, and acoustically soft ground, which were investigated in detail by NCHRP Project 25-57, Step 4 will provide more information. Step 4 – Practitioner goes to a specific strategy flowchart and works through project specif- ics following the flow arrows. Note that flowcharts are available only for strategies with enough information to clearly define flowchart boxes/choices. For solid safety barriers, low-height berms, and acoustically soft ground, the practitioner can follow the flow for specific site/project param- eters (site with hard-soil or lawn ground type, street or freeway, number of lanes, percentage of heavy trucks, receiver heights) and specific strategy parameters (e.g., solid safety barrier or berm C H A P T E R 1 3 How to Use the Practitioner’s Handbook

142 Breaking Barriers: Alternative Approaches to Avoiding and Reducing Highway Traffic Noise Impacts height). At the conclusion of Step 4, the practitioner should be able to provide recommenda- tions or confirm it is appropriate to proceed with site-specific investigations. Prior to recommending any strategy for implementation, the practitioner must consider the policy implications (e.g., quieter pavements approved/available) and conduct site-specific investigations to more accurately predict the likely noise reduction for the specific project (e.g., modeling with project traffic and actual roadway/receiver geometry). The practitioner must also consider if the strategy has the potential to affect safety performance. Consultation with a roadway design professional is most likely required.

143   The Roadway Types Versus Strategy Matrix provides practitioners a quick way to determine if a given strategy has merit for a specific roadway type. For NCHRP Project 25-57, roadway types were generalized into eight roadway cases. These cases have the characteristics of common local roads, city streets, state routes, freeways, toll roads, or limited access highways typically encountered in traffic noise studies. For roadway types not listed below, and in the 14.1 matrix, the practitioner should find the roadway type that most closely matches the specific project based upon the number of travel lanes, shoulder widths, and median widths. 1. 2-Lane Narrow Street: city street, local road, or state route with one travel lane in each direc- tion. No shoulders and no median. 2. 2-Lane Wide Street: city street, local road, or state route with one travel lane in each direction. The outside shoulders were assumed to be 2 ft wide, and the center median/turn lane was assumed to be 12 ft wide. 3. 4-Lane Narrow Street: city street, local road, or state route with two travel lanes in each direc- tion. No shoulders and no median. 4. 4-Lane Wide Street: city street, local road, or state route with two travel lanes in each direction. The outside shoulders were assumed to be 2 ft wide, and the center median/turn lane was assumed to be 12 ft wide. 5. 4-Lane Narrow Freeway/Highway: controlled-access or limited-access major local roadway, state route, interstate, or toll road with two travel lanes in each direction. The outside shoulders were assumed to be 10 ft wide, and the center median was assumed to be 4 ft wide. 6. 4-Lane Wide Freeway/Highway: controlled-access or limited-access major local roadway, state route, interstate, or toll road with two travel lanes in each direction. The outside shoulders were assumed to be 10 ft wide, and the center median was assumed to be 60 ft wide. 7. 8-Lane Narrow Freeway/Highway: controlled-access or limited-access major local roadway, state route, interstate, or toll road with four travel lanes in each direction. The outside shoulders were assumed to be 10 ft wide, and the center median was assumed to be 4 ft wide. 8. 8-Lane Wide Freeway/Highway: controlled-access or limited-access major local roadway, state route, interstate, or toll road with four wide travel lanes in each direction. The out- side shoulders were assumed to be 10 ft wide, and the center median was assumed to be 60 ft wide. C H A P T E R 1 4 Roadway Types Versus Strategy Matrix

144 Breaking Barriers: Alternative Approaches to Avoiding and Reducing Highway Traffic Noise Impacts For NCHRP Project 25-57, project area surface conditions (default ground types) were gen- eralized into two cases. The default ground type substantially influences the noise reduction that may be obtained from a given strategy and must be considered along with the roadway type. • Hard Soil: an acoustically hard ground surface commonly found in dryer climates that has less noise reduction over a given distance than an acoustically soft ground surface [effective flow resistivity (EFR) 5,000 cgs rayls]. • Lawn: an acoustically soft ground surface commonly found in damper climates and which has more noise reduction over a given distance than an acoustically hard ground surface (EFR 300 cgs rayls).

14.1 Roadway Types Versus Strategy Matrix

146 This chapter provides an overview of each strategy so that practitioners can quickly under- stand what each strategy involves and the context where it may be appropriate. 15.1 On-Road Design Strategies 15.1.1 Quieter Bridge Decks and Joints The materials and textures used on bridge decks and the types of expansion joints used on a bridge can significantly affect noise that is generated by vehicles passing over the bridge. Technologies to reduce the noise of bridge decks are often the same or similar to those used for quieter pavements. Transverse tined and transverse broomed surfaces can result in some of the highest measured levels. Diamond ground decks or polyester overlays can result in the low- est measured sound levels. Impulse noise associated with bridge joints is commonly reported as a nuisance, even if total sound levels are not especially high. Modular joints without surface plates can result in rela- tively high noise levels. Cantilever finger joints, sliding finger joints, modular joints with noise- reducing plates, and single gap joints with noise-reducing plates can all reduce impulsive noise and overall noise levels. This strategy involves creating quieter bridge decks using diamond grinding or polyester overlays and quieter bridge joints using patterned joint cover plates. Context Appropriateness: Any roadway type where noise from vehicles passing over the bridge is a concern. Driver comfort, safety, and possible interference with bicycle traffic should also be considered. Noise Benefit: 5 to 10 dB near the noise source. Noise reduction at receptors is site dependent and the bridge height and distance to the receptor must be considered. Estimated Cost: Installing polyester overlay estimated at $10–$30 per square foot, diamond grinding $1–$3 per square foot. Patterned expansion joint cover plates estimated to be 20% to 40% higher cost than standard joints. All costs are geographically dependent. 15.1.2 Quieter Rumble Strip Design Quieter rumble strips can reduce noise in areas where conventionally ground rumble strips have been implemented for safety and particularly in areas where there are frequent vehicle strikes (curved roadways). NCHRP 16-68: Effective Low-Noise Rumble Strips (forthcoming 2022) C H A P T E R 1 5 Overview of Strategies That Reduce or Avoid Noise Impacts

Overview of Strategies That Reduce or Avoid Noise Impacts 147   will provide recommendations on design and testing of low-noise rumble strips. Preliminary findings suggest that rumble strips with a specially designed sinusoidal pattern, sometimes termed “mumble strips,” appear to provide an optimum balance of reduced external sound levels and vehicle occupant alert and that existing installation equipment can be modified to create this sinusoidal pattern. Context Appropriateness: Any roadway type where noise from vehicles crossing rumble strips along the outside edges of travel lanes or the centerline of an undivided roadway is a concern. Motorcycle traffic and bicycle use should also be considered. Noise Benefit: 3 to 7 dB near the noise source. Noise reduction at receptors is site dependent. Estimated Cost: After minor equipment modifications, cost to install sinusoidal rumble strips is similar to conventionally ground rumble strips. 15.1.3 Quieter Pavements for Travel Lanes or Shoulders Over the last 40 years, strategies intended to reduce noise generated at the tire-pavement inter- face, and, to a lesser degree, increase noise reduction through absorption along the propagation path have been explored. For concrete pavements, diamond grinding of the pavement surface can reduce noise. Specific types of asphalt surfaces can also be used to reduce noise, including some open-graded and rubberized asphalt materials, along with thin bonded asphalt overlays. One common detail of quieter pavement alternatives includes the use of smaller aggregates (stones), which provides a more even surface with less texture. All pavement surfaces degrade with time and under traffic; therefore, both the initial noise reduction and acoustical performance through the life of the pavement should be considered when evaluating quieter pavements. Context Appropriateness: Any roadway type, all pavement surfaces. Noise Benefit: • Diamond grinding: up to 7 dB • Open-graded or rubberized asphalt: up to 9 dB • Thin bonded asphalt overlays: up to 6 dB. Note that this strategy can enhance noise reduction for many other strategies and was investigated as a secondary strategy to acoustically soft ground adjacent to the road. Estimated Cost: Applying diamond grinding estimated at $0.25 to $1.50 per square foot; open- graded or rubberized asphalt estimated at $0.50 to $1.50 per square foot; thin bonded asphalt overlays estimated at $0.50 to $1.50 per square foot. All costs are geographically dependent. 15.2 Highway Design Strategies 15.2.1 Horizontal and Vertical Alignment Horizontal alignment changes involve moving the roadway away from noise-sensitive sites. Vertical alignment changes involve raising or lowering the roadway elevation relative to the elevation of the noise-sensitive site. Horizontal and vertical alignment changes may involve the potential acquisition of additional right-of-way, the potential displacement of developed land uses such as homes, businesses, and institutions, and the potential negative impacts to regulated natural resources. Vertical alignment changes may also involve more complex drainage systems and extensive earth moving.

148 Breaking Barriers: Alternative Approaches to Avoiding and Reducing Highway Traffic Noise Impacts Investigation of horizontal and vertical alignment changes to reduce noise impacts will have to take into account the complex relationship of all other highway design and construction considerations (e.g., sight distance, curve radius, superelevation, grades, vertical curves, driveway adjustments, drainage, utility conflicts, available right-of-way, constructability, ease of mainte- nance, and environmental impacts). Given that horizontal alignment changes can nearly always be discounted because of potential right-of-way and construction costs, vertical alignment changes appear to be more realistic and can be modeled in the Federal Highway Administration Traffic Noise Model for specific project geometries. Context Appropriateness: Any roadway type with vacant land opposite the sensitive sites or where right-of-way is sufficient/available to accommodate shift. Potential impacts to existing and future land use, natural resources, highway drainage, nearby utilities, and constructability should also be considered. Noise Benefit: < 1 dB to 10+ dB at receptor depending on project specifics (extent of shift, site topography, and vehicle types). Note that this strategy can enhance noise reduction for shielding strategies by effectively increasing the shielding structure’s height; it was investigated as a secondary strategy to solid safety barriers. Estimated Cost: Horizontal alignment shift anticipated to be very expensive due to additional right-of-way and construction costs. Vertical alignment shift anticipated to be less expensive than horizontal shift if right-of-way is sufficient. Costs are geographically dependent. 15.2.2 Solid Safety Barriers In Lieu of Guardrails Constructing solid safety barriers instead of traditional steel guardrails can reduce traffic noise that is generated by vehicles traveling along any type of roadway. The amount of noise reduction achieved by replacing guardrail with a solid safety barrier is site geometry dependent. Taller safety barriers provide more shielding than shorter ones. This strategy involves installing concrete bar- riers that are already approved for use as safety shapes to reduce noise levels. Solid safety barrier heights, shapes, and dimensions can be found in various highway agency design guidance. Context Appropriateness: Any roadway type where the highway agency regulations permit the installation of approved concrete safety shapes. The need to maintain access to adjacent properties must be considered along with the potential for the solid safety barrier to be consid- ered a roadside crash hazard. Also, analysis of solid safety barriers needs to consider flanking noise (noise coming around the ends of the barrier) and loss of soft ground effects, if any. Noise benefit for 5-ft high receiver, 100 ft from near lane, 5% heavy trucks: • 2-lane narrow street with 4.8-ft high safety barrier: 4.0 dB (hard soil), 0.7 dB (lawn) • 4-lane wide street with 4.8-ft high safety barrier: 3.6 dB (hard soil), 1.2 dB (lawn) • 4-lane wide freeway with 6.8-ft high safety barrier: 6.4 dB (hard soil), 3.3 dB (lawn) • 8-lane narrow freeway with 6.8-ft high safety barrier: 6.3 dB (hard soil), 3.8 dB (lawn) Note that the benefits listed above and shown in flowchart format are from the NCHRP 25-57 project’s detailed investigations. Limited preliminary investigations conducted during the literature review portion of the project indicated somewhat more reduction for arterial roads and less for free- ways. The flowcharts in this handbook also provide values for additional noise reduction achieved when applying a roadway elevation increase as a secondary strategy. Estimated Cost: Anticipated to be a minor overall cost increase when replacing traditional steel guardrail. State DOTs or other highway agencies may provide a unit cost for standard height safety barriers.

Overview of Strategies That Reduce or Avoid Noise Impacts 149   15.2.3 Separation Zones Between Vehicle Lanes and Side Paths for Non-motorized Users Some research has shown that accurately modeling roadway shoulders and other ground zones can improve model accuracy. Therefore, accurately modeling the width and surface type within a separation zone between travel lanes and side paths (sidewalks, bike trails, or shared- use paths) could change the project’s anticipated noise impacts. The separation zone (also called a buffer, tree lawn, or planting strip) is between the sidewalk or shared-use path and the adjacent roadway curb or shoulder. The effect of adding paths and separation zones to a noise model is geometry dependent. Reflections from the ground with these strategy elements might affect receptors farther from the roadway than at typical first row distances, and the noise reduction effect may be minor. Context Appropriateness: Any roadway type where the surface in the separation zone is very different from the default ground type (pavement versus lawn). Noise Benefit: Increased model accuracy up to 1.6 dB if separation zone is hard soil or pave- ment compared to default ground type lawn. The predicted noise level at a receiver should be lower if the separation zone is highly sound absorptive and the default ground type is not. Estimated Cost: Minimal for modeling effort. Construction and maintenance costs could vary considerably by ground type and geographical area. 15.3 Right-of-Way Design Strategies 15.3.1 Low-Height Berms Low-height berms are noise barriers up to 6 ft high and constructed from natural earthen materials in an unsupported condition. They can be desirable because they maintain a natural, aesthetically pleasing appearance, and they can be low-cost, especially if they can be constructed using surplus materials from a highway construction project. The disadvantages of low-height berms are the amount of right-of-way they require and the fact that a berm’s peak height cannot be located as close to the noise source compared to a similar-height concrete wall. Low-height berm location relative to the travel lanes, slope, and surface material can affect the potential amount of noise reduction. Context Appropriateness: Any roadway type with sufficient right-of-way space for berm bases (up to 36 ft for a 6-ft high berm assuming a 3:1 slope on each side). The need to maintain access to adjacent properties must be considered. Also, low-height berms will be most effective when the berm can be placed close to the source or receptor. If placed near the roadway, the practitioner must determine if the low-height berm has the potential to affect safety perfor- mance or create a possible roadside hazard. Noise benefit for 5-ft high receiver, 100 ft from near lane, 5% heavy trucks: • 2-lane narrow street with 6-ft high berm: 7.3 dB (hard soil), 3.2 dB (lawn) • 4-lane wide street with 6-ft high berm: 6.4 dB (hard soil), 3.0 dB (lawn) • 4-lane wide freeway with 6-ft high berm: 6.3 dB (hard soil), 3.3 dB (lawn) • 8-lane narrow freeway with 6-ft high berm: 6.2 dB (hard soil), 3.9 dB (lawn) Note that the benefits listed above and shown in flowchart format are from the NCHRP 25-57 project’s detailed investigations. The literature review portion of the project indicated a 2–10+ dB reduction may be possible with the noise benefit being strongly dependent on the relative elevation of the source, berm, and receiver; the higher reduction values were associated with cases where

150 Breaking Barriers: Alternative Approaches to Avoiding and Reducing Highway Traffic Noise Impacts receivers were situated at a lower elevation than the roadway. The flowcharts in this handbook also provide values for additional noise reduction achieved when applying a roadway elevation decrease as a secondary strategy. Estimated Cost: An earthen berm may be 2.6 to 3 times less expensive than a concrete wall of a similar height, assuming there are no right-of-way costs or additional costs for the earthen material used to construct the berm. The cost of an earthen berm can be equal to or more expensive than a similarly sized concrete wall if earthen material has to be purchased and transported to the project site. Costs are geographically dependent. 15.3.2 Vegetated Screens Vegetative screens (trees, shrubs, or bushes) can reduce highway traffic noise. Vegetative screens reduce low-frequency noise primarily because of ground absorption. Plants help keep the soil loose, leaf litter forms a soft forest floor layer, and shade prevents soil from becoming hard in hot, dry summers. High-frequency noise is reduced by vegetative screens due to reflec- tion and scattering from leaves, branches and trunks. Mid-frequency noise may be least affected by vegetated screens. The width of a vegetated screen and the vegetation density are important for reducing the noise. Also, tree heights, particularly as trees age, need to be considered because noise can travel under the tree canopy when it is high. Context Appropriateness: Any roadway type with enough right-of-way space to plant suf- ficiently dense vegetation. Noise Benefit: • 3 to 9 dB at receptor when vegetated screen is wide (> 65 ft) • 1 to 3 dB at receptor when vegetated screen is moderate width (< 65 ft) Even a single row of trees can have a positive effect because the visual screen can reduce per- ceived loudness of traffic noise. Note that coniferous trees provide noise reduction year-round, but deciduous trees can also be effective because leaves are generally present during backyard use and open-window season. Estimated Cost: Anticipated to range from $100 to $300 per installed tree. An example cost for two rows of trees planted 10 ft apart would be $0.5–1.5 million/mile plus the upkeep cost, if appli- cable. All costs are geographically dependent and likely to vary with the type of vegetation installed. 15.3.3. Vegetated Swales and Retention Basins Vegetated swales, ditches, or retention basins are constructed to reduce or store storm water run-off. These features may provide some noise reduction because they are a highly sound absorptive surface in a wide basin and in close proximity to a roadway. The feature’s geometry, placement, and material can all affect the noise propagation along a path from the roadway to adjacent receptor sites. Context Appropriateness: Any roadway type where a vegetated swale, ditch, or retention basin is needed to reduce or store stormwater run-off. Noise benefit for a 10-ft wide vegetated basin: • 2-lane street: less than 1 dB reduction at receptors within 300 ft of near lane. • 2-lane street: up to 2.4 dB reduction at receptors between 300 and 500 ft from near lane. • Any freeway type: less than 1 dB reduction at any receptor.

Overview of Strategies That Reduce or Avoid Noise Impacts 151   Due to the relatively narrow width of swales and ditches, their anticipated noise reduction will likely be less than that of the 10-ft wide retention basin. Estimated Cost: Minimal or no additional cost if the vegetated swale, ditch, or retention basin is needed to reduce or store stormwater run-off. 15.3.4 Acoustically Soft Ground Adjacent to the Highway Acoustically soft ground is a soft, porous surface where noise transmission and/or reflection is affected by the ease with which air can move in and out of the ground surface. Acoustically hard ground (pavement, water) reflects traffic noise compared to acoustically soft ground (gravel, grass), which can reduce noise levels at a receptor adjacent to a highway. The acoustical “softness” of the ground is indicated by the airflow resistivity (high values make it difficult for the air to flow, low values allow easy airflow). Ground surfaces with the lowest EFR values provide the most noise absorption, and the depth of the layer of material can also affect the noise reduction. The ground types with the lowest EFR values are highly absorptive gravel (0.2 – 1 cgs rayls), standard gravel (1 – 10 cgs rayls), snow (10 – 50 cgs rayls), forest floor (20 – 80 cgs rayls), and grass (100 – 600 cgs rayls). Context Appropriateness: Any roadway type with sufficient right-of-way space or median widths to accommodate acoustically soft ground surfaces ranging from 5 ft to 30 ft wide. The need to maintain access to adjacent properties must be considered. Also, acoustically soft ground surfaces will be most effective when the soft surface strip is much more acoustically absorptive than the default ground type (lawn/gravel versus hard-packed dirt/pavement). Noise benefit for 5-ft high receiver, 100 ft from near lane, 5% heavy trucks: • 2-lane narrow street with 50-ft wide strip: 4.5 dB (hard soil), < 1 dB (lawn) • 4-lane wide street with 50-ft wide strip: 3.9 dB (hard soil), < 1 dB (lawn) • 4-lane wide freeway with 50-ft wide strip: 3.5 dB (hard soil), < 1 dB (lawn) • 8-lane narrow freeway with 50-ft wide strip: 3.1 dB (hard soil), < 1 dB (lawn) Note that the benefits listed above and shown in flowchart format are from the NCHRP 25-57 project’s detailed investigations. The literature review portion of the project indicated a 1–2 dB reduction may be possible when a somewhat absorptive surface is placed in the right-of-way, and extending soft ground out to 82 ft or more from a road may result in reductions of up to 12 dB for a highly absorptive gravel surface. The flowcharts in this handbook also provide values for additional noise reduction achieved when applying quieter pavement as a secondary strategy. Estimated Cost: Material and installation costs are geographically dependent based upon available material. Maintenance cost associated with gravel or grass should also be considered. 15.3.5 In-Ground Treatments In-ground treatments involve structures embedded in the ground, not constructed above the ground surface. Much like acoustically soft ground, in-ground treatments within a roadway right-of-way could reduce noise in nearby communities, but they avoid the safety concerns associated with above-ground structures like solid safety barriers or traditional noise walls. Examples are lattice structures or parallel short walls, approximately 1 ft deep and 3–6 ft wide, which might possibly be used as an extension of the shoulder, if drivable. At this time, it is not known if the embedded structures will affect safety performance. Context Appropriateness: Any roadway type that can accommodate embedded structures close to the near travel lane. Likely more appropriate for streets with narrow shoulders than freeways with wide shoulders.

152 Breaking Barriers: Alternative Approaches to Avoiding and Reducing Highway Traffic Noise Impacts Noise benefit of recessed lattice structure, receiver 39 ft from a single vehicle pass-by: • Measured 2.4 dB reduction with 3-ft wide and 0.7-ft deep lattice. • Measured 4.0 dB reduction with 6-ft wide and 0.7-ft deep lattice. • Calculations showed noise reduction increases as lattice width increases. Effectiveness will be dependent on placement, ground type, material, or configuration, number/ placement of traffic lanes, and vehicle mix. Estimated Cost: Varies with product used to construct lattice or parallel short walls. As an example, the cost of WHISstone is estimated to be about $170 per 3.3 ft length (3-ft wide lattice). Maintenance cost should also be considered. 15.3.6 Above-Ground Treatments Above-ground treatments involve structures constructed above the ground surface. Much like acoustically soft ground or in-ground treatments, above-ground treatments could reduce noise in nearby communities. Because they are shorter than solid safety barriers or traditional noise walls, there would be less visual intrusion. Examples are lattice structures or parallel short walls, approxi- mately 1 ft high with varying widths. However, above-ground structures may pose safety concerns when placed near travel lanes because they are not drivable, it is not known if above-ground struc- tures are crashworthy, or it is not known if they would be considered a roadside hazard. Context Appropriateness: Any roadway type that can accommodate above-ground struc- tures close to the near travel lane. They are likely to be more appropriate for streets with narrow shoulders than for freeways with wide shoulders. Noise benefit for receiver 164 ft from roadway: • Parallel walls 1 ft high and 30 ft wide predicted to provide up to 9 dB reduction. • Lattice wall structure 1 ft high and 30 ft wide predicted to provide up to 11 dB reduction. Estimated Cost: Unknown; construction and maintenance costs would need to be considered. 15.3.7 Solar Panels Solar panels placed within highway rights-of-way have the potential to reduce traffic noise in communities adjacent to the highway. Some states have installed, or are considering install- ing, arrays of panels or a single row of panels intended as a renewable energy source and as photovoltaic noise barriers. FHWA provides guidance on, and must approve use of, renewable energy within a federally funded highway right-of-way. In addition, solar panel installation may be dependent on various state or local regulations and policy. It should be noted that Arizona DOT has conducted research on photovoltaic systems and their applicability to noise reduction, as described in Part I. The Arizona DOT research predicted that solar panel arrays can provide more noise reduction than standard noise barriers. Context Appropriateness: Any roadway type with sufficient right-of-way space for installation of solar panel arrays or a single row of panels. However, the practitioner must determine if the solar panels have the potential to affect safety performance or create a possible roadside hazard. Noise Benefit: Continuous panels may provide noise reduction of 11 dB or more at a receptor. Solar panel/array height, gaps between arrays, horizontal and/or vertical panel gaps, and panel orientation, which can vary throughout the day, will likely change the anticipated noise reduction.

Overview of Strategies That Reduce or Avoid Noise Impacts 153   Estimated Cost: Construction cost may be low if solar panels are being proposed in order to create a power supply. Construction cost would be high if solar panels are specifically for noise mitigation, but installation cost might be offset by return from power sales. Maintenance cost would need to be considered. 15.4 Operations Management Strategies 15.4.1 Speed Restrictions Reducing vehicle speed reduces tire-pavement noise and thereby should reduce traffic noise levels in communities adjacent to a highway. Speed restrictions intended to reduce traffic noise would likely be implemented by a state or local government entity with the authority to implement lower speed limits for other purposes, such as pedestrian safety. Measures to implement speed reductions include traditional speed limit signage, as well as public educational/ informational materials, enforcement by either law enforcement officers or automated sys- tems, roadway design features such as speed humps/tables or pavement narrowing, and vehi- cle technology that alerts the driver or automatically slows the vehicle to a safe speed or legal speed limit. Context Appropriateness: Any roadway type. Noise Benefit: For traffic at speeds ranging from 45 mph to 65 mph, a 2 dB reduction in traffic noise at a nearby receptor can be expected with a reduction in speed of 10 mph. Estimated Cost: Low for changes to speed limit signage. Costs for public educational/ informational materials, enforcement, installing roadway design features, and implementing vehicle technology would need to be determined on a case-by-case basis. The cost to vehicle operators associated with increased travel times would need to be considered. 15.4.2 Truck Restrictions Because one heavy truck has generally been considered to be as loud as about 10 automobiles at highway speeds, truck restrictions (removing heavy truck traffic) have the potential to provide substantial reductions in noise levels for communities adjacent to the highway. Truck restric- tions would reduce average sound levels and should also have the additional benefit of reducing maximum noise levels from individual truck pass-by events, thereby reducing the potential for annoyance, speech interference, and sleep disturbance. Practical obstacles related to busi- ness operations and required delivery schedules may limit a state or local government entity’s options for implementing truck restrictions. Context Appropriateness: Any roadway type. However, truck restrictions (removing heavy truck traffic) may be most practical on city streets or local roadways because of the need to trans- port goods at any time on freeways or the interstate system. Noise Benefit: Overall noise reduction at a nearby receptor expected to range from 1 dB to 6 dB, dependent on the percentage of heavy trucks on the roadway prior to removal. Maximum individual truck pass-by noise levels at a nearby receptor could be reduced by 10 dB or more. Estimated Cost: Low for installing signage that restricts truck usage of a specific roadway. The potential increased costs for truck operators associated with increased travel times, enhanced law enforcement, and increased maintenance on designated alternative truck routes would need to be considered.

154 Breaking Barriers: Alternative Approaches to Avoiding and Reducing Highway Traffic Noise Impacts 15.5 Sound Absorptive Treatments 15.5.1 Retaining Walls Adding sound-absorptive treatment (acoustically absorptive material) to existing retaining walls can reduce reflected traffic noise from a specific retaining wall or multiple reflections created by several retaining walls along a depressed highway. An absorptive surface is defined as having a noise reduction coefficient of at least 0.8 on the roadside. Context Appropriateness: Any roadway type where existing retaining walls reflect noise to sensitive sites opposite a retaining wall. The practitioner should determine if the acoustically absorptive material has the potential to affect safety performance or create a possible roadside hazard. Noise Benefit: 1 dB to 2 dB reduction at a receptor. Although broadband sound levels are minimally reduced by installing acoustically absorptive material, the change in sound quality and background sound level could make highway traffic noise more acceptable. Note that greater reduction may be possible for cases with parallel retaining walls (both with absorptive treatment) or where there are reflections between trucks and walls, affecting receptors on the same side as the retaining wall. Estimated Cost: An example project identified a cost of $18–$22/square foot. However, the cost will vary by the type of absorptive material, installation details, and other project specific consid- erations. Maintenance cost of the acoustically absorptive material should also be considered. 15.5.2 Understructure of Bridges The understructure of bridges can reflect traffic noise. This can occur when the bridge is elevated and noise reflects directly down to noise-sensitive receptors or when the bridge is over a depressed roadway creating multiple reflections from other surfaces such as bridge abutments or retaining walls. Additionally, at locations where a noise barrier has been constructed between the recep- tor and the roadway, reflected noise from the understructure of elevated bridges can become the primary noise source. Absorptive treatments for bridge understructures include panels mounted flush with the understructure on elevated bridges or bridges over depressed roadways and hanging panels on the understructure of elevated bridges. Context Appropriateness: Any roadway type with elevated bridge structures or with bridges over depressed roadways where reflected noise is a concern at sensitive receptors. Noise Benefit: On-site measured reductions and predicted reductions were as much as 5 dB to 6 dB at the receptor, and laboratory measured reductions were up to 11 dB. Hanging panels provided substantial reduction, particularly when there is direct line-of-sight from the bridge to a sensitive receptor that is not from the highway. Estimated Cost: Varies by the type of the acoustically absorptive material, installation details, and other project specific considerations. Maintenance cost should also be considered. 15.5.3 Tunnels Sound-absorptive treatment can also be applied to tunnel surfaces to reduce the traffic noise radiating from the tunnel opening. In general, installing sound-absorptive material near the tunnel opening is recommended. In addition to installing acoustically absorptive material, wall roughening, skewing the tunnel opening, and the shape of the tunnel opening can influence the traffic noise.

Overview of Strategies That Reduce or Avoid Noise Impacts 155   Context Appropriateness: Any roadway type where traffic noise radiating from the tunnel opening is a concern at sensitive receptors. Noise Benefit: On-site measured reductions were 5 dB to 10 dB at the receptor. The reduction from sound absorptive material depends on the angle of view to the tunnel opening. For small angles to the tunnel axis, the effect is small, but for greater angles, the effect can be significant. Generally speaking, the greater the angle and the greater the length of absorptive material, the greater the noise reduction that results. Estimated Cost: Varies by the type of the acoustically absorptive material, installation details, and other project-specific considerations. Maintenance cost should also be considered. 15.6 Strategies Implemented by Receptors or Local Governments The U.S. DOT FHWA’s Title 23 Code of Federal Regulations, Part 772 Section 17 (23 CFR 772.17) requires highway agencies to inform local officials of noise-compatible planning concepts so that communities can protect future land development from becoming incompatible with anticipated highway noise. 15.6.1 Site Planning Site planning is a noise-compatible planning concept involving potential action outside the highway right-of-way. Therefore, it is likely a function of local government entities or private developers. Site planning involves reducing noise impacts by utilizing natural terrain, open space, and building placement on a parcel to shield residential or noise-sensitive areas from highway noise. For example, buildings that are not sensitive, such as garages, can act like noise barriers and privacy walls constructed around a development or home can also act like noise barriers. Context Appropriateness: New development near any roadway type. Noise Benefit: Up to 3 dB when distance to the roadway is doubled. 10 dB or more when non-sensitive buildings or privacy walls are used to shield sensitive sites or areas. Estimated Cost: Minimal when considered early in site development process. 15.6.2 Building Design Building design is a noise-compatible planning concept involving potential action outside the highway right-of-way. Therefore, it is likely a function of local government entities or private developers. Building design involves considering existing and future highway noise when devel- oping room layout, window placement and balcony or courtyard/open space locations. An example is orienting the building such that the most noise-sensitive rooms or windows face away from roadways. Context Appropriateness: New development or redevelopment near any roadway type. Noise Benefit: Up to 13 dB (interior noise level) when noise-sensitive rooms are placed farthest away from the highway. Estimated Cost: Minimal when considered early in site development or redevelopment process.

156 Breaking Barriers: Alternative Approaches to Avoiding and Reducing Highway Traffic Noise Impacts 15.6.3 Construction Methods and Materials Considering noise reduction when selecting construction methods and materials is a noise- compatible planning concept involving potential action outside the highway right-of-way. Therefore, it is likely a function of local government entities or private developers. The planning concept involves considering potential noise transmission through building materials such as walls, windows, doors, ceilings, and floors in material selection. Context Appropriateness: New development or redevelopment near any roadway type. Noise Benefit: Up to 35 dB (interior noise level) when noise-sensitive rooms are placed farthest away from the highway. Estimated Cost: May be expensive due to cost of materials with high sound transmission class ratings.

157   Strategy Effectiveness Flowcharts Flowcharts (described in the Strategy Effectiveness Flowcharts Overview in Chapter 16) were developed for several strategies identified in Chapter 15 with the intent of providing the prac- titioner a quick reference to the noise reduction that can be anticipated given specific project parameters. For strategies such as solid safety barriers, low-height berms, and acoustically soft ground, which were investigated in detail by the NCHRP team for Project 25-57, the flow- charts provide more detailed information. For strategies such as vegetated screens, in-ground/ above-ground treatments, and sound absorptive treatments, the flowcharts provide more general information based upon data collected during the literature review conducted for NCHRP Project 25-57. Note that flowcharts were developed only for strategies with enough applicable information in the NCHRP Project 25-57 report to clearly define possible outcomes of specific decisions. For strategies without enough information to create a flowchart, additional detailed investigation would be necessary. The flowcharts in Table 16.1 summarize and simplify information for a variety of project parameters. More information, however, for many other project parameters is included through- out the complete NCHRP Project 57-25 research report. As mentioned earlier, users will need to access the flowchart sheets, which are available for download by searching on the TRB website for “NCHRP Research Report 984”. C H A P T E R 1 6

158 Breaking Barriers: Alternative Approaches to Avoiding and Reducing Highway Traffic Noise Impacts 16.1 Strategy Effectiveness Flowcharts Overview Flowchart Information Flowchart Sheet # Strategy Detailed General Not Included On-Road Design Strategies: Quieter Bridge Decks and Joints X Quieter Rumble Strip Design X Quieter Pavements X Highway Design Strategies: Horizontal / Vertical Alignment X Solid Safety Barriers X 1A, 1B, 1C, 1D Separation Zones X Right-of-Way Design Strategies: Low-Height Berms X 2A, 2B, 2C, 2D Vegetated Screens X 4 Vegetated Swales and Basins X Acoustically Soft Ground X 3A, 3B In-Ground Treatments X 5 Above-Ground Treatments X 5 Solar Panels X Operations Management Strategies: Speed Restrictions X Truck Restrictions X Sound Absorptive Treatments Retaining Walls X 6 Understructure of Bridges X 6 Tunnels X 6 Strategies Implemented by Receptors or Local Governments: Site Planning X Building Design X Construction Methods and Materials X

Abbreviations and acronyms used without definitions in TRB publications: A4A Airlines for America AAAE American Association of Airport Executives AASHO American Association of State Highway Officials AASHTO American Association of State Highway and Transportation Officials ACI–NA Airports Council International–North America ACRP Airport Cooperative Research Program ADA Americans with Disabilities Act APTA American Public Transportation Association ASCE American Society of Civil Engineers ASME American Society of Mechanical Engineers ASTM American Society for Testing and Materials ATA American Trucking Associations CTAA Community Transportation Association of America CTBSSP Commercial Truck and Bus Safety Synthesis Program DHS Department of Homeland Security DOE Department of Energy EPA Environmental Protection Agency FAA Federal Aviation Administration FAST Fixing America’s Surface Transportation Act (2015) FHWA Federal Highway Administration FMCSA Federal Motor Carrier Safety Administration FRA Federal Railroad Administration FTA Federal Transit Administration GHSA Governors Highway Safety Association HMCRP Hazardous Materials Cooperative Research Program IEEE Institute of Electrical and Electronics Engineers ISTEA Intermodal Surface Transportation Efficiency Act of 1991 ITE Institute of Transportation Engineers MAP-21 Moving Ahead for Progress in the 21st Century Act (2012) NASA National Aeronautics and Space Administration NASAO National Association of State Aviation Officials NCFRP National Cooperative Freight Research Program NCHRP National Cooperative Highway Research Program NHTSA National Highway Traffic Safety Administration NTSB National Transportation Safety Board PHMSA Pipeline and Hazardous Materials Safety Administration RITA Research and Innovative Technology Administration SAE Society of Automotive Engineers SAFETEA-LU Safe, Accountable, Flexible, Efficient Transportation Equity Act: A Legacy for Users (2005) TCRP Transit Cooperative Research Program TDC Transit Development Corporation TEA-21 Transportation Equity Act for the 21st Century (1998) TRB Transportation Research Board TSA Transportation Security Administration U.S. DOT United States Department of Transportation

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