Download PDF Summer Bridge on Noise Control Engineering June 15, 2021 Volume 51 Issue 2 What is the role of engineering practice, education, and standards in mitigating human-generated noise? The articles in this issue survey these aspects of the US noise landscape, and offer updates and useful resources. Trains, Planes, and Automobiles: Transportation Noise in the United States Tuesday, June 15, 2021 Author: Gregg G. Fleming Continued research and technical advances are needed to address both persistent and novel concerns in transportation-related noise. Soundscape is the term for the acoustic environment perceived by humans in context. The human-made portion of the outdoor soundscape is largely dominated by transportation. Until transportation vehicles are truly or nearly silent, or a transformative technology emerges that either effectively isolates the noise (e.g., the Hyperloop) or obviates the need for large numbers of vehicles to move people and goods from one place to another, transportation-related noise will remain a major challenge—and a quality of life issue. Introduction Acousticians think of noise in terms of a source-path-receiver challenge, and it is helpful to think about individual modal contributions to the transportation noise landscape in this context. Most professionals agree that the most effective solution to reducing noise is to reduce it at the source, but for a variety of reasons this approach is not equally viable for rail, aviation, and highway noise sources. For one thing, the three transportation modes are not all subject to federal noise standards and regulation. The Federal Railroad Administration (FRA) is responsible for ensuring compliance with noise emission limits for railway equipment. However, these limits are set by the Environmental Protection Agency (EPA; under 40 CFR 201). The Federal Aviation Administration (FAA) is legally responsible for certificating aircraft noise levels (under 14 CFR part 36 and 49 USC 44715). In coordination with the National Aeronautics and Space Administration (NASA), the FAA funds research programs on reduction of noise at the source. There is no comparable legal authority in the Department of Transportation (USDOT) for regulating noise from highway vehicles. Therefore the Federal Highway Administration (FHWA) and state DOTs have strived to reduce highway noise through the construction of noise barriers—a path-based solution to reducing noise. This article reviews research and accomplishments in reducing rail, highway, and aircraft noise in the United States, as well as the roles of various US government agencies. It also considers current research and potential impacts of new technology such as drones, advanced air mobility, high-speed rail, and automated vehicles. Tracking Trends in US Transportation Noise It’s useful to understand the current state of the noise problem before identifying potential solutions, whether they involve source-based fundamental noise research (e.g., aircraft-engine noise reduction), path-based approaches (e.g., highway noise barrier construction), or receiver-centric approaches (e.g., sound insulation in buildings). In 2016 USDOT’s Bureau of Transportation Statistics (BTS), with support from the department’s Volpe National Transportation Systems Center, launched the National Transportation Noise Map initiative to track trends in noise related to multiple transportation modes.[1] The initiative is not intended to support regulatory or rulemaking activity, but rather to provide a repository for tracking and understanding trends in transportation-related noise over time, including consideration of population exposure. The noise maps use simplified noise modeling assumptions to compute the 24-hour equivalent A-weighted sound level (this metric was chosen to distinguish it from USDOT regulatory metrics). Since the initiative was developed in a geospatial environment, it can be adapted for use in a variety of research topics, such as understanding both possible uses of transportation noise to mask noise from other sources and noise-related equity issues. As an example, figure 1 presents the 2018 transportation-related noise map for the New York metropolitan area. Rail Noise “Everybody loves the sound of a train in the distance,” as Paul Simon wrote. Research supports the fact that rail noise is the least annoying of transportation noises (Janssen and Vos 2011). But that’s not to say that the rail sector is without its noise challenges. Train Horns Train horns are the most prominent rail noise challenge. The most effective strategy to address it is to eliminate crossings through grade separation. Second is elimination of horn soundings through the use of quiet zones, where alternate safety measures are implemented (FRA 2013). One FRA-approved alternate safety measure that has been documented to reduce community noise impact is the wayside horn installed at crossings (Hummer and Jafari 2007; Lucke and Raub 2004; Lucke et al. 2012; Multer and Rapoza 1998). While effective in limiting noise exposure to the population in the vicinity of a crossing, wayside horns have not been widely implemented. In 2010 there were 27 crossings with wayside horn installations; 10 years later the number was 55.[2] The somewhat limited use of wayside horns is due to hurdles similar to those that plague the establishment of quiet zones: the cost of equipment and crossing upgrades, particularly if power is not available at the crossing; and the process of obtaining required approvals. In addition, wayside horns do not eliminate train horn noise entirely and may confer less benefit in terms of noise exposure compared with other quiet zone safety measures. In fact, wayside horns sometimes even increase noise exposure for the population adjacent to the crossing. Their utility must be evaluated on a site-by-site basis. Only about 3 percent of the nation’s 125,600 public at-grade roadway railroad grade crossings are in quiet zones. The GAO (2017) evaluated FRA’s quiet zone rule and recommended that FRA revise its methods for analyzing the safety of quiet zones and develop guidance for quiet zone inspections. Implementation of these recommendations may lead to more widespread use of quiet zones. Other Rail Noise Considerations Other rail noise challenges include wheel squeal on curves, retarder noise in hump yards, engine idling in yards, and rail crossovers, but these are typically site specific and have been studied and mitigated as such (NAE 2010). Trains in the United States, including Amtrak’s Acela Express, operate at speeds up to 150 mph and are compliant with rail noise standards (40 CFR Part 210). They are not yet operational at higher speeds, but plans are moving forward in the Northeast Corridor, Texas, California, and Nevada for high-speed rail (HSR) operations with anticipated speeds up to 220 mph.[3] It is unclear whether these operations will meet existing noise standards or new standards will need to be considered. Highway Noise At highway speeds, noise from vehicles is primarily due to tire/road interaction; engine/exhaust noise tends to be more important at lower vehicle speeds, particularly for trucks. Studies have shown that roadway vehicle noise in the aggregate has varied little over the past 5 decades—automobiles have gotten slightly noisier (likely because of the increase in the number of sport utility vehicles) and trucks slightly quieter at highway speeds (Fleming et al. 1995). As such, there has been very limited research focused on quieting vehicle exterior noise at the source. Quieter Pavement While there has been some limited research on quieting vehicle tires (Ögren et al. 2018), most tire/road noise research has focused on quieter pavements (e.g., Anderson et al. 2013; Donavan and Janello 2018; Lodico and Donavan 2018; McGhee et al. 2016). Quieter pavements have also been a focus for reducing roadway noise in national parks, where preserving the natural soundscape is vitally important (Hastings et al. 2020). Research has shown that some quieter pavements (e.g., open-graded asphaltic concrete) can achieve reductions as large as 10 dB relative to the more common Portland cement concrete and dense graded asphaltic concrete (Donavan and Lodico 2013). Quieter pavements can reduce roadway noise in national parks, where preserving the natural soundscape is vitally important. But movement toward the use of quieter pavements in the United States has been slow, for a number of reasons. Trade-offs—in long-term durability, maintenance, and ultimately costs—are a primary consideration (NAE 2010, chapter 7). Research has shown that the benefits of quieter pavement can deteriorate over time (Anderson et al. 2013; NASEM 2013). The surface can become more compressed, reducing sound-absorbing capacity. The pavement can require additional maintenance and cleaning to retain its sound reduction benefits (PIARC 2013). There are regional dependencies, too; sand and salt used in snowy areas can clog porous sound-absorbing pavements. There have also been practical concerns about establishing quality control/assurance protocols to ensure consistent large-scale installation of quieter pavements (NASEM 2010). States are understandably reluctant to install quieter pavements that may require more frequent maintenance and overlays—and corresponding lifecycle cost increases. Research on quieter pavements is expected to continue to enhance understanding of the long-term trade-offs. But quieter pavements should be in the toolbox for mitigating highway noise. Highway Noise Barriers Given the questions surrounding quieter pavements, most federal/state investment in efforts to reduce highway noise has focused on barriers. Between 1963 and 2016, 3263 linear miles of highway noise barriers were constructed in 48 states and Puerto Rico, at total construction costs of about $7.44 billion—or roughly $34.55/square foot.[4] Research in the 1970s to reduce aircraft fuel burn also led to a marked reduction in aircraft noise. To comply with 23 CFR 772, FHWA’s Traffic Noise Model (TNM) must be used on all federal-aid highway projects. It has become the de facto standard for predicting noise and designing noise barriers in the vicinity of highways (and can account for some benefits of quieter pavements). FHWA has committed to the advancement of TNM to include the latest science and analytical capabilities, such as improved mapping and integration of a construction noise module in the near future. The FHWA website provides the latest version of TNM (3.0) along with a wealth of related information on highway noise.[5] Aircraft Noise The challenges of commercial aircraft noise date back to at least the early 1950s, with the first flight of the Comet and the dawning of the jet age. With aircraft, it was recognized early on that the traditional approach to analyzing noise probably wasn’t adequate. Research by Boeing, the New York Port Authority, and others led to the development of the tone corrective perceived noise level and eventually the effective perceived noise level (EPNL; Kryter 1960), which is still used for aircraft noise certification. After the 1970s spike in oil prices, a substantial amount of research was undertaken to reduce aircraft fuel burn. A major outcome of that research, the high-bypass ratio turbofan engine, also resulted in a marked reduction in aircraft noise. As mentioned above, interdependencies (in this case between noise and fuel burn) are a critical factor in understanding transportation noise. Transport aircraft are certificated according to noise stages; the current one is Stage 5.[6] Modern jet aircraft have achieved a reduction of approximately 15 dB in the average measured EPNL at the three individual certification measurement locations (directly beneath the aircraft on approach, and during takeoff directly beneath as well as off to the side of the aircraft) compared to the first-generation jet aircraft, the Boeing 727s and Douglas DC8s. The corresponding cumulative reduction in certificated EPNL of 40–50 dB represents a cumulative reduction in level at three measurement locations. Figure 2 shows the progress made over the years, and FAA and NASA goals for further noise reduction. Population Exposure Another way of looking at progress in reducing aviation noise is in terms of population exposure. In 1975 there were roughly 200 million enplanements; in 2019 there were more than four times that amount—about 935 million. Yet from 1975 to 2019 the population impacted by aircraft noise (measured in terms of the commonly accepted threshold of 65 dB DNL[7]) dropped from 7 million people to about 440,000. So there’s no longer a problem with aircraft noise, right? Wrong! Some challenges are technical, but others are more nuanced. The American public is more educated on the topic of aircraft noise, and for those who are adversely affected it is a quality of life issue. Recent FAA research has shown that people are significantly more sensitive to aircraft noise in general, compared to prior studies.[8] For example, the FAA survey found that, at a noise exposure level of 65 dB DNL, 60–70 percent of people were highly annoyed by aircraft noise, compared with just 12.3 percent 3 decades ago (FICON 1992). There is also the question of whether the equal-energy principle applies regardless of the level and frequency of the associated events. In other words, would 10 very loud aircraft events result in the same level of public response as 100 moderately loud events, assuming that their cumulative noise energy was equivalent? One of the more recent technical challenges in aircraft noise is the impact of advanced navigation technology. Precision-based navigation (PBN) and area navigation of aircraft enabled by geographic positioning systems (GPS) have many benefits, including, importantly, improved safety through reductions in pilot and air traffic controller workload. But some of the other benefits of PBN—fuel savings, time savings, and congestion reduction—entail trade-offs. Figure 3 shows aircraft flight tracks at Boston’s Logan International Airport before and after the implementation of PBN. It does not take an acoustician to conclude that the noise impacts on the ground have changed: fewer people are impacted since implementation of PBN, but those beneath these superhighways in the sky probably experience an increase in noise from the concentrated overflights. These heavily trafficked skyways may also raise equity issues. Trying to assess the impacts of PBN-related changes is a challenge for both airports and the FAA. Federal Initiatives The FAA, NASA, and their partners support efforts in the following areas to better understand and improve the aviation noise soundscape: (i) better modeling capabilities; (ii) studies of noise and its impacts on public annoyance, sleep, learning, and health; (iii) community outreach, including education tools; (iv) research supporting noise reduction at the source; and (v) noise mitigation through operational procedures and sound insulation at sensitive receptors. Initiatives include the continued advancement of FAA’s Aviation Environmental Design Tool (AEDT),[9] Center of Excellence for Alternative Jet Fuels and Environment (ASCENT),[10] and Continuous Lower Energy, Emissions and Noise (CLEEN) Program[11]; and NASA’s Aeronautics Research Program.[12] AEDT is the de facto standard for modeling aircraft noise in the vicinity of airports. Its development is supported in part by research initiatives of ASCENT, which is an FAA partnership with NASA, EPA, the Air Force Research Lab, Department of Agriculture, Department of Energy, Defense Logistics Agency, Transport Canada, a consortium of seven universities, and a range of industry partners. CLEEN is a public-private partnership closely coordinated with NASA and including a number of aerospace industry contributors. Because aviation noise is a challenge for a number of federal agencies, the Federal Interagency Committee on Aviation Noise[13] was established in 1993 to “assist agencies in providing adequate forums for discussion of public and private sector proposals, identifying needed research, and in encouraging the conduct of research and development in these areas.” Helicopter Noise Helicopter noise is particularly prevalent in urban areas such as New York, Los Angeles, Chicago, and Washington (GAO 2021) as well as cities near the Gulf of Mexico (associated with oil rig access). It is also a concern in national parks (along with noise from other small, tour aircraft). Community engagement on the subject of helicopter noise often involves noise levels lower than those of fixed-wing aircraft, but concerns are due to the different nature of the sound. Rotary-wing noise has therefore been a focus of the FAA, NASA, and Department of Defense, particularly in research to reduce source noise. The Helicopter Association International has launched the Fly Neighborly initiative, “a voluntary noise reduction program that seeks to create better relationships between communities and helicopter operators by establishing noise mitigation techniques and increasing effective communication.”[14] Looking Forward It is unclear how the covid-19 pandemic will affect the US and global transportation landscape. Will the demand for business air travel be permanently reduced through new platforms like video conferencing? Will rail transit return to prepandemic levels? How will the shift to a more remote workforce impact highway congestion, and transportation demand more broadly? No one knows the answers to these questions and speculation is well beyond the scope of this article. Independent of the impacts of the covid-19 pandemic, following are major anticipated trends in transportation noise: Promotion and implementation of quiet zones, including train horn noise mitigation strategies, will continue in the vicinity of rail-grade crossings. Also, once operational in the United States, HSR is expected to lead to new noise challenges that will require research and new policies and regulations. Research into quieter pavements will continue, to complement highway noise reductions achieved by noise barriers. In addition, substantial investment in vehicle charging stations across the country is expected to facilitate greater use of electric vehicles.[15] Hybrid technology will also likely become more pervasive (the path to hydrogen-powered vehicles is less clear). These technologies will help reduce noise on roadways, but mostly at lower speeds where tire/road noise is less pronounced. Furthermore, these technologies can have unintended consequences for safety, particularly for the visually impaired at lower speeds, and these must be considered (Garay-Vega et al. 2010). Continued research in aviation-related source noise reduction, including (i) traditional large-passenger tube-and-wing designs; (ii) advanced concept airframes and propulsion systems for large cargo and passenger aircraft (e.g., blended-wing body, electric, and potentially hydrogen); and (iii) drones, advanced air mobility (AAM), helicopters, and commercial space vehicles. While safety must remain the priority, reductions in aviation noise will likely need to be commensurate with significant reductions in fuel burn, given concerns about climate change and the substantial cost of aircraft fuel. Electric aircraft offer important potential benefits in terms of noise and energy use, but their mission capabilities are expected to be very limited. There are widely differing views on the future of hydrogen propulsion technology for aircraft. It is expected that there will be a resurgence in commercial supersonic flight, but for at least the next decade such flights will likely be limited to overwater routes. NASA, in cooperation with the FAA and others, is designing and building a low-boom supersonic demonstrator to assess its acceptability in overland flight.[16] While vehicle automation[17] will provide societal benefits, care must be taken to mitigate unintended consequences such as the potential for increased noise due to more freely flowing highway traffic and expected growth in drone and AAM operations.[18] Workshops on drone and AAM noise in 2018 and 2020 (INCE-USA 2020) highlighted the many noise-related considerations associated with the rapidly increasing demand for these new technologies. On the positive side, roadway vehicle automation could eventually yield noise benefits such as omission of the need for train horns. Likewise, cargo delivery drones may reduce the use of ground-based vehicles, potentially resulting in noise benefits, or maybe just changing the nature of the noise experience for the public. In addition, there needs to be more consideration of transportation noise in the aggregate—rail, highway, and aircraft and both traditional and autonomous vehicles. The BTS-led transportation noise mapping initiative is an excellent step in that direction. Also, more consolidation of noise prediction models (e.g., the planned integration of a construction noise module in TNM) will help. And with the anticipated growth of HSR in the United States, why not a rail noise module in TNM? But updating noise models to consider multimodal contributions is not enough. It does not make sense to design noise barriers along an interstate near a runway without considering the aircraft noise, or to make residential insulation decisions about aviation noise impact for a community adjacent to an interstate without considering the noise from the latter. Electric vehicles and hybrid technology will help reduce noise on roadways, but mostly at lower speeds. Integrated models will help inform approaches in these situations, but it is also necessary to consider the adequacy of modally based transportation-related noise policies and land-use planning guidance. What impact will FAA’s 2021 survey have on aviation noise policy? Might it affect the noise policies of other modes? Time will tell! Conclusions There has been much progress in reducing transportation-related noise. But there is still more to do. Some challenges are technical, but in many cases they are related to willingness, awareness, and trade-offs. What’s needed is sustained, coordinated, and arguably greater attention to transportation-related noise in the United States. All the tools in the proverbial toolbox must be used to ensure that—with expected population growth and related increased demand for transportation as well as new modes of movement (e.g., drones, autonomous vehicles)—the transportation noise landscape at least maintains the status quo and, ideally, improves. Acknowledgments I thank the following individuals for their review, assistance, and invaluable suggestions for this article: Don Scata of the FAA; Cecilia Ho of FHWA; and Christopher Roof, Aaron Hastings, Cynthia Lee, Amanda Rapoza, and Cindy Sabin, all of the USDOT/Volpe Center. References Anderson K, Uhlmeyer JS, Sexton T, Russell M, Weston J. 2013. Evaluation of Long-Term Pavement Performance and Noise Characteristics of Open-Graded Friction Courses Project 3 – Final Report. Olympia: Washington State Department of Transportation. Donavan P, Janello C. 2018. Arizona Quiet Pavement Pilot Program: Comprehensive Report. Phoenix: Arizona Department of Transportation. Donavan PR, Lodico DM. 2013. Influence of quieter pavement and absorptive barriers on US-101 in Marin County, California. Transportation Research Record 2362(1):25–34. FICON [Federal Interagency Committee on Noise]. 1992. Federal Agency Review of Selected Airport Noise Analysis Issues. Washington. Fleming GG, Rapoza AS, Lee CSY. 1995. Development of National Reference Energy Mean Emission Levels for the FHWA Traffic Noise Model (FHWA TNM), version 1.0. McLean VA: Federal Highway Administration. FRA [Federal Railroad Administration]. 2013. Guide to the quiet zone establishment process. Washington. GAO [Government Accountability Office]. 2017. Railroad Safety: Quiet Zone Analyses and Inspections Could Be Improved (GAO-18-97). Washington. GAO. 2021. Aircraft Noise: Better Information Sharing Could Improve Responses to Washington, DC Area Helicopter Noise Concerns (GAO-21-200). Washington. Garay-Vega L, Hastings A, Pollard JK, Zuschlag M, Stearns MD. 2010. Quieter Cars and the Safety of Blind Pedestrians: Phase 1. Washington: National Highway Traffic Safety Administration. Hastings AL, Rapoza AS, Kaye S, Flynn D. 2020. Quieter Pavement Project – Death Valley National Park: Interim report for one-year post-treatment conditions. Ft Collins CO: National Park Service. Hummer JE, Jafari MR. 2007. Railroad Crossing Wayside Horn Evaluation. Raleigh: North Carolina Department of Transportation. INCE-USA [Institute of Noise Control Engineering of the USA]. 2017. Engineering a Quieter America: Commercial Aviation: A New Era. Reston VA. INCE-USA. 2020. Engineering a Quieter America: UAS and UAV (Drone) Noise Emissions and Noise Control Engineering Technology. Reston VA. Janssen SA, Vos H. 2011. Dose-response relationship between DNL and aircraft noise annoyance: Contribution of TNO (DOT/FAA/AEE/2011-04). The Hague: Netherlands Organisation for Applied Scientific Research. Kryter KD. 1960. The meaning and measurement of perceived noise level. Noise Control 6(12). Lodico D, Donavan P. 2018. Quieter Pavement: Acoustic Measurement and Performance. Sacramento: California Department of Transportation. Lucke RE, Raub RA. 2004. Use of automated wayside horns for improving highway-rail grade crossing safety. Proceedings, ITE Annual Mtg and Exhibit, Aug 1–4, Lake Buena Vista FL. Lucke RE, Raub RA, Thunder TE. 2012. Improving road safety and residential quality of life. Applied Health Economics and Health Policy 3:71–78. McGhee KK, De León Izeppi ED, Flintsch GW, Mogrovejo DE. 2016. Virginia quieter pavement demonstration program. Transportation Research Record 2571(1):49–58. Multer J, Rapoza A. 1998. Field Evaluation of a Wayside Horn at a Highway-Railroad Grade Crossing. Washington: Federal Railroad Administration. NAE [National Academy of Engineering]. 2010. Technology for a Quieter America. Washington: National Academies Press. NASEM [National Academies of Sciences, Engineering, and Medicine]. 2010. 2008 Survey of European Composite Pavements. Washington: National Academies Press. NASEM. 2013. Evaluating Pavement Strategies and Barriers for Noise Mitigation. Washington: National Academies Press. Ögren M, Molnár P, Barregard L. 2018. Road traffic noise abatement scenarios in Gothenburg 2015–2035. Environmental Research 164:516–21. PIARC [World Road Association]. 2013. Quiet Pavement Technologies (2013R10EN). Paris. [1] The map is available at https://data.bts.gov/stories/s/National-Transportation- Noise-Map/ri89-bhxh. [2] FRA Railroad Crossing Inventory Dashboard, https://railroads.dot.gov/crossing-and-inventory-data/ grade-crossing-inventory/crossing-inventory [3] Information about US HSR initiatives is available at https://railroads.dot.gov/passenger-rail/high-speed-rail/ high-speed-rail-timeline. [4] FHWA, Summary of Noise Barriers Constructed by December 31, 2016, https://www.fhwa.dot.gov/environment/noise/noise_barriers/ inventory/ [5] FHWA, Highway Traffic Noise, https://www.fhwa.dot.gov/environment/noise/ [6] FAA Stage 5 Airplane Noise Standards, https://www.federalregister.gov/documents/2017/10/ 04/2017-21092/stage-5-airplane-noise-standards 7] DNL, the day-night average sound level, is the total sound energy over a 24-hour period. It is the principal metric of airport noise. [8] FAA, Neighborhood Environmental Survey (as of Jan 12, 2021), https://www.faa.gov/regulations_policies/policy_guidance/ noise/survey/ [9] https://aedt.faa.gov/ [10] https://ascent.aero/topic/noise/ [11] https://www.faa.gov/about/office_org/headquarters_offices/ apl/research/aircraft_technology/cleen/ [12https://www.nasa.gov/aeroresearch [13] https://fican.org [14] https://www.rotor.org/initiatives/fly-neighborly [15] FAA Alternative Fuel Corridors, https://www.fhwa.dot.gov/environment/alternative_fuel_ corridors/ [16] NASA low-boom flight demonstration, https://www.nasa.gov/X59 [17] USDOT Intelligent Transportation Systems Joint Program Office: ITS Research Automation, https://www.its.dot.gov/research_areas/automation.htm [18] FAA, Unmanned Aircraft Systems, https://www.faa.gov/uas About the Author:Gregg Fleming is director of policy, planning, and environment at the US DOT/Volpe National Transportation Systems Center in Cambridge, MA.