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SHAs must evaluate noise barriers for impacted land uses. Communities with residences, schools, and parks that meet the criteria for noise abatement are eligible to receive some form of noise control as part of the project. In the United States, the criteria for noise abatement are state specific and follow federal regulations. In addition to noise walls, follow- ing federal regulations, SHAs may also consider these other noise-reducing measures:
• Traffic management measures. These include factors such as modified speed limits and vehicle restrictions. These typically do not provide the required noise reductions for the measures to be both feasible and reasonable.
• Alteration of horizontal and vertical alignments. These measures are generally not possible for widening projects, although such shifts could provide the needed noise reduc- tions for projects on new alignments.
• Acquisition of property to serve as a buffer zone. Although SHAs are allowed to acquire property to serve as a buffer, they typically do not do this.
• Noise insulation for land uses such as places of worship and schools. Sound insulation could include air condition- ing and new windows and doors. SHAs in some states do not have the authority to insulate buildings, so insulation cannot always be included as part of a highway project.
Although not permitted under federal regulations for use as traffic noise abatement for projects receiving federal aid, qui- eter pavements can effectively reduce noise. For cases where noise barriers do not meet all the necessary requirements to be built and/or where there are elevated hillside receivers, quieter pavements can provide some reduction in noise. In addition to reducing tire-pavement noise, some pavements can also absorb sound, thereby reducing propagated noise. To maintain their noise-reducing capabilities, quieter pave- ments require periodic examination to determine the poten- tial need to repave, redo the surface treatment, or clean.
Current Challenges
There are many challenges associated with evaluating high- way traffic noise. Some of these are discussed briefly below.
Changing Noise Sources
As noise sources change over time, so must noise predic- tions. The FHWA TNM currently incorporates a compre- hensive vehicle noise emission database collected in the early to mid-1990s. As vehicles get quieter, tires evolve, and there is more understanding of pavement effects, this presents a challenge in updating the TNM database. Obtaining new
data is both cost and time intensive, and it is currently be- ing discussed by the Transportation Research Board (TRB) Transportation-Related Noise and Vibration Committee, the FHWA, and state agencies as to how and when this can be accomplished.
Quieter Pavements
From the 1990s to the present, many research studies have focused on the effects of various pavement types on ve- hicle and traffic noise. Several US states have shifted away from the use of louder pavements (e.g., transversely tined concrete) and/or toward the use of quieter pavements (e.g., open-graded or porous asphalt) as common pavement con- struction and maintenance practice, based on current find- ings and understanding. Future research would need to fo- cus on acoustic durability of pavements in order for quieter pavements to be used as a noise abatement measure.
Quieter Rumble Strips
Rumble strips are placed along roadways for safety reasons to warn drivers about unintended lane departure, thus pre- venting accidents. Unfortunately, due to poor placement and/or traditionally loud designs, numerous accidental rumble strip incursions adversely affect nearby communi- ties. There are only a few high-quality targeted studies that quantify noise emanating from rumble strip incursions. Al- though some have focused on quieter rumble strip designs, the challenge remains in verification and widespread appli- cation of these designs.
Noise-Compatible Land Use Planning
The FHWA advocates that local governments use their regu- latory authority to prohibit incompatible development adja- cent to highways or require planning, design, and construc- tion of developments that minimize highway traffic noise impacts (FHWA, 2011). Unfortunately, the development of noise-sensitive land uses adjacent to existing heavily trav- eled roadways continues to be a pervasive problem. SHAs cannot use federal funds to construct noise abatement mea- sures for these new uses, unless changes to the roadway trig- ger a Type I project that would include a noise impact analy- sis and noise barrier consideration.
Alternative Project Delivery Methods
Project delivery methods such as design build have become more common and can create additional issues for projects that involve noise abatement measures. Commitments made for noise abatement during the environmental process may be based on preliminary project plans. These plans could
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