Breaking Barriers: Alternative Approaches to Avoiding and Reducing Highway Traffic Noise Impacts (2022) / Chapter Skim
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Conduct of Research P A R T I
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13 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.
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14 Breaking Barriers: Alternative Approaches to Avoiding and Reducing Highway Traffic Noise Impacts • Chapter 8: Findings: Sound-Absorptive Treatment Strategies; • Chapter 9: Application of Findings; • Chapter 10: Conclusions and Suggested Research; • Bibliography; and • Part II: Practitioner's Handbook.
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15 For this research project, there were two main phases: (1) literature and data review for multiple strategies; and (2)
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16 Breaking Barriers: Alternative Approaches to Avoiding and Reducing Highway Traffic Noise Impacts Noise-Reducing Strategy Category Strategy On-road design choices Quieter bridge decks and joints Quieter rumble strip design Quieter pavements for travel lanes and/or shoulders Highway design choices Horizontal and vertical alignment Solid safety barriers in lieu of guardrail (alone and combined with roadway elevation and diffractor top) Separation zones between vehicle travel lanes and side paths for non-motorized users Right-of-way design choices Low-height berms (alone and combined with roadway depression and acoustically soft ground on berm)
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Research Approach 17 applicable chapter) [Geo-decisions (Gannett Fleming)
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18 Breaking Barriers: Alternative Approaches to Avoiding and Reducing Highway Traffic Noise Impacts 2.2.2 Base Cases Each strategy investigation starts with eight general roadway cases, listed in Table 2-3. These roadway cases are further described in Table 2-4, with roadway types represented by each case.
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Research Approach 19 divided into four groups, shown as the following ranges (with labeling in quotes as it appears in the results) : • 25 ≤ distance ≤ 100 ft, "25–100" ft • 100 < distance ≤ 250 ft, "100–250" ft • 250 < distance ≤ 500 ft, "250–500" ft • 500 < distance ≤ 1000 ft, "500–1000" ft Essentially, for DOEs, a researcher holds one or more elements constant and varies the other parameters with all relevant permutations, then averages the results for each distance range.
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20 Breaking Barriers: Alternative Approaches to Avoiding and Reducing Highway Traffic Noise Impacts Data are also presented for targeted investigations. This includes spectral data plotted for 1⁄3-octave band sound levels to show the frequencies that are being affected by the various strategies (one of these is also included in the summary of results for acoustically soft ground to help explain the noise reduction results)
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21 A summary of on-road design strategies is shown in Table 3-1, including projected noise reduction benefits, approximate costs (on a scale of $–$$$$$) , and context appropriateness.
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22 Breaking Barriers: Alternative Approaches to Avoiding and Reducing Highway Traffic Noise Impacts Strategy Noise Benefit Costs (scale $–$$$$$) Context Appropriateness Quieter bridge decks using diamond grinding or polyester overlays 5 to 10 dB (near source)
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Findings: On-Road Design Strategies 23 Other options to reduce noise are: (1) filling in gaps between the center beams with foam or other flexible material, (2)
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24 Breaking Barriers: Alternative Approaches to Avoiding and Reducing Highway Traffic Noise Impacts Figure 3-3. Example of rubberized asphalt layer (Source: Judith Rochat, Cross-Spectrum Acoustics Inc.)
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25 A summary of highway design strategies is shown in Table 4-1, including projected noise reduction benefits, approximate costs (on a scale of $–$$$$$) , and context appropriateness.
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26 Breaking Barriers: Alternative Approaches to Avoiding and Reducing Highway Traffic Noise Impacts Strategy Noise Benefit Costs (scale $–$$$$$) Context Appropriateness Horizontal alignment shift < 1 dB to 10+ dB depending on project specifics: extent of shift, site topography, and vehicle types ($$$$–$$$$$)
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Findings: Highway Design Strategies 27 noise reduction measures associated with shielding (e.g., solid safety barriers or low berms) , where the effective height of the shielding element is increased.
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28 Breaking Barriers: Alternative Approaches to Avoiding and Reducing Highway Traffic Noise Impacts The amount of noise reduction achieved by replacing guardrail with a solid safety barrier is geometry dependent. Solid safety barriers that block direct line-of-sight to sensitive receptors for the greatest number of highway noise sources achieve substantial noise reduction.
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Findings: Highway Design Strategies 29 both safety and path user confidence. However, the minimum recommended distance between a path and the roadway curb is 1.5 m (5 ft)
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30 Breaking Barriers: Alternative Approaches to Avoiding and Reducing Highway Traffic Noise Impacts details on the investigation methods. Table 4-3 lists the matrix of variables to investigate solid safety barriers alone and then combined with a vertical alignment change (raising the roadway elevation)
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Findings: Highway Design Strategies 31 4.2.3 Summary of Results This section summarizes the results of the solid safety barrier strategy investigation. Solid safety barriers were examined in terms of barrier height and also combined with roadway elevation to help reduce highway traffic noise.
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32 Breaking Barriers: Alternative Approaches to Avoiding and Reducing Highway Traffic Noise Impacts Roadway case a Maximum Solid Safety Barriers effect at two distances; case stated for which this occurs shown in parentheses as (rec ht_gnd_SSB_road elv) b Maximum Solid Safety Barriers effect + Roadway Elevation effect at two distances; case stated for which this occurs shown in parentheses as (rec ht_gnd_SSB_road elv)
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Findings: Highway Design Strategies 33 Roadway case a Maximum Solid Safety Barriers effect at two distances; case stated for which this occurs shown in parentheses as (rec ht_gnd_SSB_road elv) b Maximum Solid Safety Barriers effect + Roadway Elevation effect at two distances; case stated for which this occurs shown in parentheses as (rec ht_gnd_SSB_road elv)
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34 Breaking Barriers: Alternative Approaches to Avoiding and Reducing Highway Traffic Noise Impacts substantially at all distances when the site is hard soil. The SSB contributes less to noise reduction at receivers beyond 46 m (150 ft)
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Findings: Highway Design Strategies 35 type the SSB effect is 1.0 dB or more at distances up to 30 m (100 ft) and over 2 dB at distances up to 15 m (50 ft)
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36 Breaking Barriers: Alternative Approaches to Avoiding and Reducing Highway Traffic Noise Impacts Figure 4-8 shows results for a 4-lane wide freeway with hard soil, 5% HTs, and the 5-ft receiver height. In that scenario the difference in effectiveness related to SSB height is about 3 dB for receivers less than 300 ft from the highway and about 2 dB for receivers at greater distance.
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Findings: Highway Design Strategies 37 Percent Heavy Trucks Results show that the percent heavy trucks is a contributing factor to the effectiveness of a solid safety barrier (SSB) ; see Figure 4-10 for a representative DOE example, which shows the average reduction over all parameters for a single roadway case (4-lane wide freeway)
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38 Breaking Barriers: Alternative Approaches to Avoiding and Reducing Highway Traffic Noise Impacts Figure 4-11 and Figure 4-12 show examples of the differences in SSB-related noise reduction as a function of all distances tested for a 1.5-m (5-ft) receiver at a lawn site along a 4-lane wide freeway with 0% and 15% heavy trucks.
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Findings: Highway Design Strategies 39 Figure 4-12. Noise reduction as a function of distance for fw4wid; 15% HTs, lawn site, 5-ft receiver, 3.5-ft and 4.8-ft SSB and at-grade and roadway elevated 3 ft.
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40 Breaking Barriers: Alternative Approaches to Avoiding and Reducing Highway Traffic Noise Impacts Figure 4-14. Noise reduction as a function of distance for fw4wid; 5% HTs, hard soil site, 5-ft receiver, 3.5-ft and 4.8-ft SSB and at-grade and roadway elevated 3 ft.
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Findings: Highway Design Strategies 41 Figure 4-16. DOE: Average noise reduction effect at various distances as a function of roadway elevations (data for st4wid, includes all ground types, SSB heights, percentage of heavy trucks, and receiver heights)
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42 Breaking Barriers: Alternative Approaches to Avoiding and Reducing Highway Traffic Noise Impacts Figure 4-18 shows that increasing roadway elevation increases SSB effectiveness slightly at a 5-ft receiver on a hard soil site within 75 ft of a 4-lane wide street with 15% heavy trucks when the SSB is 3.5 ft high and somewhat at all distances less than 500 ft when the SSB is 4.8 ft high. Also, comparing Figure 4-17 to Figure 4-18 shows that the overall SSB effectiveness related to roadway elevation increases is far less for 15% heavy trucks, as shown in Figure 4-16.
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Findings: Highway Design Strategies 43 raised 0.91 m (3 ft)
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44 Breaking Barriers: Alternative Approaches to Avoiding and Reducing Highway Traffic Noise Impacts Figure 4-19. Comparative spectra for st4wid, 5% HT, hard soil, with no SSB + roadway at-grade, 107-cm (3.5-ft)
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Findings: Highway Design Strategies 45 Figure 4-20. Comparative spectra for st4wid, 5% HT, lawn site, with no SSB + roadway at-grade, 107-cm (3.5-ft)
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46 Breaking Barriers: Alternative Approaches to Avoiding and Reducing Highway Traffic Noise Impacts a freeway/highway scenario (assumes a 3-m or 10-ft shoulder) (Wijnant 2020)
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Findings: Highway Design Strategies 47 SPL(1/3-octave) @100 ft Barrier height = 3.5[ft]
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48 Breaking Barriers: Alternative Approaches to Avoiding and Reducing Highway Traf c Noise Impacts SPL(1/3-octave)
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Findings: Highway Design Strategies 49 SPL(1/3-octave)
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50 A summary of right-of-way design strategies is shown in Table 5-1, including projected noise reduction benefits, approximate costs (on a scale of $–$$$$$) , and context appropriateness.
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Findings: Right-of-Way Design Strategies 51 Strategy Noise Benefit Costs (scale $–$$$$$) Context Appropriateness Standard low-height berms (up to 6 ft high)
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52 Breaking Barriers: Alternative Approaches to Avoiding and Reducing Highway Traffic Noise Impacts Strategy Noise Benefit Costs (scale $–$$$$$) Context Appropriateness Thick vegetation belts [> 20 m (65 ft)
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Findings: Right-of-Way Design Strategies 53 Strategy Noise Benefit Costs (scale $–$$$$$) Context Appropriateness 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)
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54 Breaking Barriers: Alternative Approaches to Avoiding and Reducing Highway Traffic Noise Impacts a side, and unusual shapes. The berm placement, slope (without supporting structures)
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Findings: Right-of-Way Design Strategies 55 vegetative screens reduce noise by reflection and scattering from the surfaces of leaves, branches and trunks (Dobson and Ryan 2000)
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56 Breaking Barriers: Alternative Approaches to Avoiding and Reducing Highway Traffic Noise Impacts Vegetated swales/ditches or retention basins, like those shown in Figure 5-4, are constructed to reduce or store storm water run-off. It is possible for these features to provide some reduction in highway traffic noise for adjacent communities.
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Findings: Right-of-Way Design Strategies 57 affected most by the ease with which air can move in and out of the ground surface. This is indicated by the flow resistivity (high values make it difficult for the air to flow, and low values allow easy flow; acoustically soft ground has lower values than hard)
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58 Breaking Barriers: Alternative Approaches to Avoiding and Reducing Highway Traffic Noise Impacts 0.3 m (1 ft) deep.
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Findings: Right-of-Way Design Strategies 59 5.2 Detailed Investigations – Low Berms 5.2.1 Description Low berms were examined further to determine potential noise reduction for a matrix of scenarios, as described in the Investigation Method (Section 2.2 for the general method and Section 5.2.2 for specifics)
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60 Breaking Barriers: Alternative Approaches to Avoiding and Reducing Highway Traffic Noise Impacts heavy truck percentage options) included only an at-grade road with two, four, or eight lanes, and a line of receivers at heights of 1.5 and 4.6 m (5 and 15 ft)
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Findings: Right-of-Way Design Strategies 61 • Changing the roadway elevation for cases with depressed roads. • Deleting receivers, as needed, that were within the footprint of each berm to avoid affecting TNM's ground model.
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62 Breaking Barriers: Alternative Approaches to Avoiding and Reducing Highway Traffic Noise Impacts 5.2.3 Summary of Results This section summarizes the results of the low berm strategy alone and combined with roadway depression. Highlights of the findings are as follows: • For roadways with moderate heavy truck volumes (approximately 5% of total traffic volume)
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Findings: Right-of-Way Design Strategies 63 Roadway case Maximum Noise Reduction at Two Distances (berm only) ; Case Stated for Which this Occurs Shown in Parentheses Maximum Noise Reduction at Two Distances (berm + depressed road)
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64 Breaking Barriers: Alternative Approaches to Avoiding and Reducing Highway Traffic Noise Impacts Table 5-5. Low berm investigation; adjustments to maximum noise reduction for receivers within 76 m (250 ft)
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Findings: Right-of-Way Design Strategies 65 Receiver Height Results were computed for receiver heights of 1.5 m (5 ft)
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66 Breaking Barriers: Alternative Approaches to Avoiding and Reducing Highway Traffic Noise Impacts 1.5-m (5-ft)
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Figure 5-12. Lines of sight from pavement-height sub-source to 1.5-m (5-ft)
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Figure 5-13. Lines of sight from pavement-height sub-source to 1.5-m (5-ft)
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Findings: Right-of-Way Design Strategies 69 Figure 5-14. DOE; Average noise reduction as a function of distance and berm height (5-ft receiver height, all roadway and berm geometries, 5% heavy trucks)
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70 Breaking Barriers: Alternative Approaches to Avoiding and Reducing Highway Traffic Noise Impacts Figure 5-17. Noise reduction as a function of distance and berm height (15-ft receiver height, 8-lane narrow freeway, 4:1 soft near berm, 2-ft top, hard soil site, 0-ft depression, 5% heavy trucks)
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Findings: Right-of-Way Design Strategies 71 Figure 5-18 and Figure 5-19 are similar to Figure 5-16 and Figure 5-17, except that the results provided are for a default lawn site, rather than the default hard soil site shown in the previous figures. Figure 5-18 provides results for 1.5-m (5-ft receivers)
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72 Breaking Barriers: Alternative Approaches to Avoiding and Reducing Highway Traffic Noise Impacts by different roadway depths, when combined with a 1.8-m (6-ft) high berm.
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Findings: Right-of-Way Design Strategies 73 Figure 5-21 shows the additional benefit provided by different depths of roadway depression when combined with a lower 0.9-m (3-ft) high berm.
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74 Breaking Barriers: Alternative Approaches to Avoiding and Reducing Highway Traffic Noise Impacts Figure 5-22. DOE; Average noise reduction effect as a function of distance and default ground type (all data, 5-ft receiver height)
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Findings: Right-of-Way Design Strategies 75 Figure 5-24. Noise reduction as a function of distance and berm height (5-ft receiver height, 2-lane narrow street, 4:1 soft near berm, lawn site, 5% heavy trucks)
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76 Breaking Barriers: Alternative Approaches to Avoiding and Reducing Highway Traffic Noise Impacts Berm Ground Type Berm ground types included hard soil and "soft." (Although "soft" was modeled as "lawn," a similar EFR could be obtained with a non-vegetative surface.) In all cases, the soft berm option provided greater noise reduction than the hard soil option.
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Findings: Right-of-Way Design Strategies 77 Berm Shape Berm shapes considered included 2:1 and 4:1 slopes on both the roadway and receiver sides (2:½:1 and 4:¼:1) , 6:1 slopes on only the roadway side with 2:1 and 4:1 slopes on the receiver side (6:½:1 and 6:¼:1)
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78 Breaking Barriers: Alternative Approaches to Avoiding and Reducing Highway Traffic Noise Impacts As shown in Figure 5-31, for sites with default lawn ground type, on average, soft berms with 2:1 slopes provided approximately 1 dB higher noise reduction than soft berms with 4:1. Unlike the situation with a hard soil site, use of a soft berm at a default lawn site does not introduce additional soft ground.
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Findings: Right-of-Way Design Strategies 79 For freeways with "narrow" medians, noise reductions were slightly greater with the 8-lane case than with the 4-lane case. For freeways with "wide" medians, the number of lanes made essentially no difference.
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80 Breaking Barriers: Alternative Approaches to Avoiding and Reducing Highway Traffic Noise Impacts For "far" placement, the toe of slope on the receiver side was located at the right-of-way line. For modeling purposes, all streets were assumed to have a total right-of-way width of 46 m (150 ft)
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Findings: Right-of-Way Design Strategies 81 Percent Heavy Trucks The majority of the low berm cases were modeled using 5% heavy trucks. An additional sensitivity analysis comparing 0%, 5% and 15% heavy trucks was conducted to determine the effect of this parameter.
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82 Breaking Barriers: Alternative Approaches to Avoiding and Reducing Highway Traffic Noise Impacts Figure 5-36. DOE; Average noise reduction effect as a function of distance and % heavy trucks (3-ft berms; 0, 5, and 15% HTs)
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Findings: Right-of-Way Design Strategies 83 0.9-, 1.4-, and 1.8-m (3-, 4.5-, and 6-ft) berms, respectively.
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84 Breaking Barriers: Alternative Approaches to Avoiding and Reducing Highway Traffic Noise Impacts At 76 m (250 ft) from the near lane, the same berm provides noise reductions of about 6.5 dB, 4.5 dB, and 3.5 dB with 0%, 5%, and 15% heavy trucks, respectively.
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Findings: Right-of-Way Design Strategies 85 Figure 5-41. Comparative spectra for fw8nar with 5% heavy trucks at 100 ft; hard soil default site (top)
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86 Breaking Barriers: Alternative Approaches to Avoiding and Reducing Highway Traffic Noise Impacts Figure 5-42. Comparative spectra for fw8nar with 5% heavy trucks at 250 ft; hard soil default site (top)
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Findings: Right-of-Way Design Strategies 87 In each of the figures, the upper plot shows results with a hard soil default site and the lower figure shows results with a lawn default site. Figure 5-41 provides predicted 1⁄3-octave band data 30 m (100 ft)
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88 Breaking Barriers: Alternative Approaches to Avoiding and Reducing Highway Traffic Noise Impacts 5.3 Detailed Investigations – Acoustically Soft Ground 5.3.1 Description Acoustically soft ground adjacent to transportation systems can help reduce noise in nearby communities. The effectiveness of soft ground surfaces for highways is dependent on the soft ground placement, ground type/material, number/placement of traffic lanes, and vehicle mix.
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Findings: Right-of-Way Design Strategies 89 Using a subset of runs, it was determined that increasing the ground strip sound absorption by applying an EFR value of 1 cgs rayls compared to 10 cgs rayls resulted in negligible differences, so only 10 cgs rayls was applied to the remaining cases. Also, 1⁄3-octave band results were examined only where necessary, to help understand broadband sound level results and to show the noise reduction effects as a function of frequency.
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90 Breaking Barriers: Alternative Approaches to Avoiding and Reducing Highway Traffic Noise Impacts • 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.
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(15 ft_hard_50 ft) (15 ft_hard_50 ft)
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92 Breaking Barriers: Alternative Approaches to Avoiding and Reducing Highway Traffic Noise Impacts Parametera Adjustments to maximum noise reductionsa Distance - Greatest effect ≤ 200 ft from road for hard ground site - Less effect farther from road for hard ground site down to < 1 dB - Greatest effect generally < 100 ft for soft ground site, although with complex variation near the road and little drop off with distance with combined strategy HT% - Decreasing effect with increasing %, although only tenths of a dB broadband (0% HTs shows greatest reduction) Default ground type - Hard ground site: decreases to < 1 dB farther from road - Soft ground site: minimal effects from strip; controlled by QP Receiver height - More reduction at lower receiver closer to the road (~1 dB)
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Findings: Right-of-Way Design Strategies 93 average reduction over all parameters for a single roadway case (4-lane wide freeway/highway)
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94 Breaking Barriers: Alternative Approaches to Avoiding and Reducing Highway Traffic Noise Impacts Figure 5-45. Noise reduction as a function of distance for fw4wid; soft ground site, 0% HTs, with and without quieter pavement; 5-ft receiver (top)
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Findings: Right-of-Way Design Strategies 95 about 1 dB more at the high receiver. In general, for soft ground, the noise reduction can vary between the two receiver heights by about 1 dB near the road, but results are similar farther from the road.
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96 Breaking Barriers: Alternative Approaches to Avoiding and Reducing Highway Traffic Noise Impacts Figure 5-48. Noise reduction as a function of distance for st4nar; hard ground (hard soil)
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Findings: Right-of-Way Design Strategies 97 Figure 5-49. Noise reduction as a function of distance for st4nar; soft ground (lawn)
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98 Breaking Barriers: Alternative Approaches to Avoiding and Reducing Highway Traffic Noise Impacts parameters. For the hard ground site, it can be seen that the noise reduction reaches about 5.5 dB for the widest strip (50 ft)
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Findings: Right-of-Way Design Strategies 99 Figure 5-51. 1⁄3-octave band sound levels as a function of frequency and EFR value of strip (st2nar, at a distance of 75 ft, hard ground site, 15% HT, TNM average pavement)
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100 Breaking Barriers: Alternative Approaches to Avoiding and Reducing Highway Traffic Noise Impacts more benefit closer to the road than farther from it for hard ground sites and about the same benefit across all distances for soft ground sites. Further discussion of quieter pavement contributions can be found in Section 5.3.4, where spectral data are examined.
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Findings: Right-of-Way Design Strategies 101 Figure 5-53. Comparative spectra for fw4wid, hard ground site, at 100 ft; 0% HTs (top)
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102 Breaking Barriers: Alternative Approaches to Avoiding and Reducing Highway Traffic Noise Impacts Figure 5-54. Comparative spectra for fw4wid, hard ground site, at 250 ft; 0% HTs (top)
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Findings: Right-of-Way Design Strategies 103 Figure 5-55. Comparative spectra for fw4wid, soft ground site, at 100 ft; 0% HTs (top)
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104 Breaking Barriers: Alternative Approaches to Avoiding and Reducing Highway Traffic Noise Impacts Figure 5-56. Comparative spectra for fw4wid, soft ground site, at 250 ft; 0% HTs (top)
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Findings: Right-of-Way Design Strategies 105 For soft ground sites (Figure 5-55 and Figure 5-56) , the following is observed: • At 100 ft – A gravel strip provides low frequency (about 400 Hz and below)
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Figure 5-58. Predicted distance ranges of influence at two heights (5 ft, 15 ft)
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Findings: Right-of-Way Design Strategies 107 regions of influence for the same cases for lane 1 and lane 2, respectively (with lane 1 being the outermost lane closest to the strip)
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108 Breaking Barriers: Alternative Approaches to Avoiding and Reducing Highway Traffic Noise Impacts Figure 5-59. Predicted distance ranges of influence at two heights (5 ft, 15 ft)
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Figure 5-60. Noise reduction as a function of distance for st2nar; hard ground site, 5% HTs, with and without quieter pavement; 5-ft receiver (top)
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110 Breaking Barriers: Alternative Approaches to Avoiding and Reducing Highway Traffic Noise Impacts The change in regions going from 5-ft receivers to 15-ft receivers moves the influence region farther from the road. The shift helps to explain why the peaks in the reduction as a function of distance plots are at greater distances at 15 ft compared to 5 ft (see Figure 5-60 as an example, which shows the peak shifting from 23 to 38 m or 75 to 125 ft for st2nar)
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111 A summary of operations management strategies is shown in Table 6-1, including projected noise reduction benefits, approximate costs (on a scale of $–$$$$$) , and context appropriateness.
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112 Breaking Barriers: Alternative Approaches to Avoiding and Reducing Highway Traffic Noise Impacts by roughly 10 to 15 dBA, thereby reducing the potential for annoyance, speech interference, and sleep disturbance. In some cases, practical obstacles exist to implementing truck restrictions.
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113 A summary of strategies implemented by receptors or local governments is shown in Table 7-1, including projected noise reduction benefits, approximate costs (on a scale of $–$$$$$) , and context appropriateness.
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114 Breaking Barriers: Alternative Approaches to Avoiding and Reducing Highway Traffic Noise Impacts Strategy Noise Benefit Costs (scale $–$$$$$) Context Appropriateness Site Planning Up to 3 dB when distance to the roadway is doubled.
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115 A summary of sound-absorptive treatment strategies is shown in Table 8-1, including projected noise reduction benefits, approximate costs (on a scale of $–$$$$$) , and context appropriateness.
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116 Breaking Barriers: Alternative Approaches to Avoiding and Reducing Highway Traffic Noise Impacts Sound-absorptive walls or treatment should be installed if there are noise-sensitive receptors opposite a noise wall or retaining wall. There is guidance in the NCHRP Project 25-44 as to when a receptor would qualify: if the distance from the barrier to the receptors is less than 20 times the barrier height (Bowlby et al.
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Findings: Sound-Absorptive Treatment Strategies 117 ray theory. In the case of multiple reflections between a wall and vehicle or opposing wall, noise sources can be the original source or a reflected source.
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118 Breaking Barriers: Alternative Approaches to Avoiding and Reducing Highway Traffic Noise Impacts 8.1.3 Tunnels Quieter choices for sound-absorptive treatment in tunnels: higher NRC materials, optimized placement. Sound-absorptive treatment can also be applied to tunnel surfaces.
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119 This chapter discusses the process in choosing a strategy and also provides a brief description of the practitioner's handbook. 9.1 Choosing a Strategy Choosing a strategy starts with the consideration of context appropriateness.
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120 Breaking Barriers: Alternative Approaches to Avoiding and Reducing Highway Traffic Noise Impacts change may eliminate ground interaction) ; and upward vertical alignment shifts may not be effective if decreasing the distance between the sound source and an elevated receptor.
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121 10.1 Conclusions Results of this NCHRP project research are intended to provide stakeholders with information about alternative strategies to reduce highway traffic noise. Opportunities to apply alternative strategies include: (1)
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122 Breaking Barriers: Alternative Approaches to Avoiding and Reducing Highway Traffic Noise Impacts (0.9 m or 3 ft) can increase reduction by about 1 dB.
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Conclusions and Suggested Research 123 implementation of a chosen noise-reduction strategy. The practitioner's handbook is briefly discussed in Section 9.2 and full text is shown in Part II.
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124 Breaking Barriers: Alternative Approaches to Avoiding and Reducing Highway Traffic Noise Impacts To help get some idea of what actual noise reduction may be, additional research is needed to investigate the effects of solar panel arrays. Objectives: Review literature and summarize the latest information regarding solar panel array noise reduction and potential right-of-way configurations.
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Conclusions and Suggested Research 125 10.2.4 Effectiveness of Vegetated Screens to Reduce Highway Traffic Noise and When to Include Effects in Modeling Background: Vegetated screens can effectively reduce noise, with results being dependent on the width of the vegetated belt adjacent to a highway and the type and density of vegetation. Limited studies have measured noise reduction associated with vegetated screens, and there is limited regional guidance as to associated parameters necessary to achieve meaningful noise reduction.
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126 Breaking Barriers: Alternative Approaches to Avoiding and Reducing Highway Traffic Noise Impacts Objectives: Review literature and summarize the latest information regarding roadway shoulder or right-of-way in-ground treatments and potentially drivable structures that can reduce noise. Also, identify promising in-ground treatment structures (could consider lattice structures or other structures that absorb sound or beneficially interfere with ground reflections)
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Conclusions and Suggested Research 127 sound levels. With the noise wall, the effect of absorptive treatment on the bridge understructure could be substantial, possibly more than 5 dB reduction.
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128 Abbas, A., R
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Bibliography 129 Burge, P., J Crawford, and P
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130 Breaking Barriers: Alternative Approaches to Avoiding and Reducing Highway Traffic Noise Impacts Donavan, P., and B Rymer.
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Bibliography 131 Hendriks, R
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132 Breaking Barriers: Alternative Approaches to Avoiding and Reducing Highway Traffic Noise Impacts Lester, T., V Dravitzki, P
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Bibliography 133 Rochat, J
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134 Breaking Barriers: Alternative Approaches to Avoiding and Reducing Highway Traffic Noise Impacts Van Renterghem, T., and D Botteldooren.
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135 Appendices A through E are available on TRB's website at www.trb.org by searching on "NCHRP Research Report 984." The appendices are as follows. Appendix A: Terminology; Appendix B: Summary of Noise-Reducing Strategies; Appendix C: Low Berms (LB)
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Key Terms
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