Defined as unwanted or excessive sound, noise can be caused by a variety of sources related to expanded infrastructure that supports modern society. As mobility increases, transportation, in particular, can be a key source of noise across modes, from airports to rail to new roads.
Studies have shown that some of the most pervasive sources of noise in our environment are those associated with transportation. Residences and businesses often are faced with increased highway traffic noise, both from newly constructed highways and from highways that are already in place.
The National Environmental Policy Act (NEPA) of 1969 provided broad authority and responsibility for evaluating and mitigating adverse environmental effects including highway traffic noise. NEPA directed the Federal government to use all practical means and measures to promote the general welfare and foster a healthy environment.
Public concern about noise led to federal legislation in 1970 that authorized the use of federal aid highway funds for measures to abate and control highway traffic noise. The Federal Aid Highway Act of 1970 (23 USC 109(h)) mandated that the Federal Highway Administration (FHWA) develop noise standards for mitigating highway traffic noise. The law required promulgation of traffic noise-level criteria for various land use activities. The law further provided that FHWA not approve the plans and specifications for a federally aided highway project unless the project included adequate noise abatement measures to comply with the standards.
The following sections provides an overview of transportation noise issues.
Principles of Highway Noise
When describing effects of highway noise and the types of noise mitigation, it is important to understand the principles of noise. Noise is defined as unwanted or excessive sound. Sound becomes unwanted when it interferes with normal activities, such as sleep, work, speech, or recreation. People react to noise differently, based on many emotional and physical factors, such as hearing sensitivity, the degree to which someone is accustomed to noise, or a person’s ability to sleep with noise.
A decibel (dB) is the unit used to indicate the intensity of a sound wave. Sound (noise) is often measured in decibels using an A–weighted scale (dBA) because this method approximates the way humans hear sound. Table 1 presents typical sound levels at distances measured from the source of the sound.
Indoor and Outdoor Sound Levels
Outdoor Sound Levels
Indoor Sound Levels
|110||Rock Band at 5 m (16 feet)|
|Jet Over–Flight at 300 m
|100||Inside New York Subway Train|
|Gas Lawn Mower at 1m (3 feet)||95|
|90||Food Blender at 1 m (3 feet)|
|Diesel Truck at 15 m (50 feet)||85|
|Noisy Urban Area-Daytime||80||Garbage Disposal at 1 m (3 feet)|
|75||Shouting at 1 m (3 feet)|
|Gas Lawn Mower at 30 m (100 feet)||70||Vacuum Cleaner at 3 m (10 feet)|
|Suburban Commercial Area||65||Normal Speech at 1 m (3 feet)|
|Quiet Urban Area-Daytime||55||Quiet Conversation at 1 m (3 feet)|
|50||Dishwasher in Next Room|
|Quiet Urban Area at Night||45|
|40||Empty Theater or Library|
|Quiet Suburb at Night||35|
|30||Quiet Bedroom at Night|
|Quiet Rural Area at Night||25||Empty Concert Hall|
|15||Broadcast and Recording Studios|
|Reference Pressure Level||0||Threshold of Hearing|
mPA MicroPascals describe pressure. The pressure level is what sound level monitors measure.
dBA A–weighted decibels describe pressure logarithmically with respect to 20 mPa (the reference pressure level).
Source: Highway Noise Fundamentals, Federal Highway Administration, September 1980.
The following general relationships exist between noise levels and human perception:
- A 1 or 2 decibel increase is not perceptible to the average person.
- A 3 decibel increase is just barely perceptible to the human ear.
- A 5 decibel increase is readily perceptible to the human ear.
- A 10 decibel increase is perceived as a doubling in loudness to the average person.
Factors That Affect Sound Levels
There are many factors that affect sound levels. Sound levels are influenced by the distance and the path traveled between the source of the sound and receptor (person). There is a natural reduction of sound levels with increasing distance between the source and the receptor. The type of ground cover between the source and the receptor also affects sound levels by disrupting or deflecting the sound wave. Highway-generated sound levels will decrease with the distance from the roadway. This reduction will vary depending upon the ground type (hard or soft). Hard ground (e.g., pavement, dirt, or water) will decrease sound levels by 3 decibels for each doubling of distance. Soft ground (e.g., grass, trees, or vegetation) will decrease sound levels by 4.5 decibels for each doubling of distance. Obstacles between the source and the receptor, such as buildings, hills, and trees, will result in additional noise reductions depending upon their size, density, and location.
Sound can best be expressed on a logarithmic basis. Consequently, sound levels cannot be added by ordinary arithmetic means. Sound levels from two equal sources will result in a total increase of 3 decibels. For example; two cars, each generating 60 decibels of sound, will result in 63 decibels of sound at a receptor, not 120 decibels.
Additionally, highway noise levels constantly vary with the number, type, and speed of the vehicles that produce the noise. When conducting measurements of highway noise, it is necessary to account for these varying noise levels. The most common way to account for the time-varying nature of sound is through a measurement known as Leq. Leq averages background sound levels with short-term transient sound levels to provide a uniform method for comparing sound levels over time.
As characterized in the figures below, noise levels from highway traffic are affected by three factors: (1) the volume of the traffic, (2) the speed of the traffic, and (3) the number of trucks in the flow of traffic. Generally, the loudness of traffic noise is increased by heavier traffic volumes, higher vehicle speeds, and greater numbers of trucks. Vehicle noise is a combination of the noise produced by the engine, exhaust, and tires.
Other conditions will also increase traffic noise levels. For example, a steep incline along a roadway will cause heavy laboring of motor vehicle engines.
How Speed Affects Traffic Noise
Traffic at 65 miles per hour sounds twice as loud as traffic at 30 miles per hour.
How Traffic Volume Affects Noise
2000 vehicles per hour sound twice as loud as 200 vehicles per hour.
How Trucks Affect Traffic Noise
One truck at 55 miles per hour sounds as loud as 28 cars at 55 miles per hour.
The time period used for highway noise analyses is typically one hour. Usually, the noisiest hour of the day or night along a highway occurs just before or after the peak hours when the vehicle volumes, speeds, and the truck to auto ratio are in a combined optimum condition to yield the highest hourly noise level. The noisiest hour typically does not occur during the peak traffic hour because the peak hour will have the highest traffic volumes resulting in slower speeds and, therefore, lower highway sound levels. In accordance with FHWA Noise Policy and Guidance, a one-hour Leq is used for assessing highway noise impacts on different land uses.
Methods Used to Reduce Highway Noise
There are several methods to reduce highway noise. These include vehicle noise control, noise compatible planning, traffic management techniques, alteration of a roadway’s horizontal or vertical alignment, acquisition of property or property rights to create buffer zones, vegetation planting, installing noise insulation in public or nonprofit institutional buildings, or the construction of noise barriers.
Vehicle Noise Control
As stated in Highway Traffic Noise in the United States: Problem and Response, “The Noise Control Act of 1972 gives the U.S. Environmental Protection Agency (EPA) the authority to establish noise regulations to control major sources of noise, including transportation vehicles and construction equipment.” In addition, this legislation requires EPA to issue noise emission standards for motor vehicles used in interstate commerce (vehicles used to transport commodities across State boundaries) and requires the Federal Motor Carrier Safety Administration to enforce these noise emission standards.
The EPA has established regulations, which set emission level standards for newly manufactured medium and heavy trucks that have a gross vehicle weight rating (GVWR) of more than 10,000 pounds and are capable of operating on a highway or street. Table 2 shows the maximum noise emission levels allowed by the EPA noise regulations for these vehicles.
Maximum Noise Emission Levels Required by EPA for Newly Manufactured Trucks with GVWR Over 10,000 Pounds
|Effective Date||Maximum Noise Level 50 Feet from Centerline of Travel*|
|January 1, 1988||80 dBA|
|* Using the Society of Automotive Engineers, Inc. (SAE), test procedure for acceleration under 35 mph|
Source: Highway Traffic Noise in the United States: Problem and Response, Federal Highway Administration, 2006
The EPA emission level standards for existing in-use medium and heavy trucks engaged in interstate commerce are shown in Table 3. State or local governments must regulate all other in-use vehicles.
Maximum Noise Emission Levels Required by EPA for In-Use Medium and Heavy Trucks with GVWR Over 10,000 Pounds Engaged in Interstate Commerce
|Effective Date||Speed||Maximum Noise Level 50 Feet from Centerline of Travel|
|January 8, 1986||< 35 mph||83 dBA|
|> 35 mph||87 dBA|
Source: Highway Traffic Noise in the United States: Problem and Response, Federal Highway Administration, 2006
Noise Compatible Planning
Noise-compatible planning encourages State and local governments to use their power to regulate land development in such a way that noise-sensitive land uses are either prohibited from being adjacent to a highway, or that the developments are planned, designed, and constructed in such a way that highway traffic noise impacts are minimized.
To promote noise compatible planning, state transportation agencies coordinate with local officials whose jurisdictions are affected by noise from proposed projects. The following information is typically furnished to the local officials:
- Estimated future noise levels for both developed and undeveloped properties in the immediate vicinity of a proposed project.
- Estimated future noise levels shown at various distances from the roadway to allow the public and local officials to understand where local communities should protect future land development from becoming incompatible with anticipated highway noise levels.
- Eligibility for Federal-aid participation for Type II projects as described in the Type II Projects section of this Web site.
Additional information on noise compatible planning can be obtained at FHWA’s Noise Compatible Planning webpage.
Traffic Management Techniques
Controlling traffic can sometimes reduce highway traffic noise problems. Some possible ways of achieving this is are:
- Prohibiting certain vehicle types (usually trucks) from particular streets and roadways. This could be accomplished by way of traffic control devices and signing. The prohibition of trucks from a major roadway can produce up to an 8 dBA to 10 dBA noise reduction.
- Permitting certain vehicle types (again, typically trucks) to use certain streets and roads only during certain noise-sensitive periods, such as daylight hours.
- Timing traffic lights to achieve smooth traffic flow and to eliminate the need for frequent acceleration and deceleration.
- Reducing speed limits. About a 20 mile-per-hour reduction in speed is necessary for a readily perceptible decrease in noise levels.
- Separating noisier vehicles from other vehicles and placing them farther from the receivers (i.e., exclusive lane designation).
Alteration of a Roadway’s Horizontal or Vertical Alignment
Alteration of the horizontal or vertical alignment is usually limited to new roadways or major reconstruction of existing roadways. Since noise is reduced from 3 to 4.5 dBA per doubling of distance between the source and receiver, shifting the horizontal alignment away from an affected area often is an effective method to reduce noise. Vertical alignment shifts, such as depressing the highway to form a vertical cut section, can also be quite effective since the top of cut acts as a barrier between the source and receiver. Occasionally, the roadway can be placed so that attenuation is provided by natural barriers, such as hills and wooded areas.
Acquisition of Property or Property Rights to Create Buffer Zones
Buffer zones can be created by purchasing unimproved property or property rights adjacent to an existing or proposed highway so as to preclude future highway traffic noise impacts where development has not yet occurred. An additional benefit of buffer zones is that they often improve the roadside appearance. Buffer zones can only be used in Type I projects.
Creating buffer zones is often not possible because of the large amount of land that must be purchased and because, in many cases, residences already border existing or proposed roadways.
Vegetation, if it is high enough, wide enough, and dense enough that it cannot be seen through, can decrease highway traffic noise. A 200-foot width of dense vegetation can reduce noise by 10 decibels, which cuts in half the loudness of traffic noise. It is usually impossible, however, to plant enough vegetation along a road to achieve such reductions.
The planting of trees and shrubs, however, may provide psychological benefits, if not an actual lessening of highway traffic noise levels, and may be provided for visual, privacy, or aesthetic treatment. Vegetation planting is not a mitigation measure that is eligible for Federal-aid highway funding.
Noise Insulation in Public or Nonprofit Institutional Buildings
FHWA allows federal funds to be used for noise insulation of public use or nonprofit institutional structures, such as churches, schools, hospitals, and libraries. Insulating buildings can greatly reduce interior noise levels, especially when windows are sealed and cracks and other openings are filled. However, insulation can be costly because air conditioning is usually necessary once the windows are sealed. The installation of air conditioning has two benefits. First, windows can be kept shut in warm weather to reduce noise from exterior sources. Second, it creates a steady background noise that masks exterior and interior noises by removing rapid noise level fluctuations. Where possible, using double-paned windows and reducing window area can also be effective.
The most common measure to mitigate noise impacts is construction of noise barriers. Noise barriers are solid obstructions built between the highway and noise-sensitive receptors such as homes, schools, or churches. Effective noise barriers reduce noise levels by 7 decibels to 10 decibels, which reduces the perceived loudness of traffic noise by more than half.
Noise barriers can be formed from high vertical walls, from earth mounds (usually called earth berms), or a combination of the two. Earth berms have a natural appearance and are usually attractive. However, an earth berm can require a large amount of land to attain the desired height. For example, a 16-foot high earth berm would require 32 feet of space at the base. Noise walls require substantially less space.
For a noise barrier to be effective, it must be high enough and long enough to block the view of the road. Noise barriers are ineffective in situations where there are numerous intersecting streets or where openings for access to driveways must be provided.
Noise walls are often limited to 25 feet in height for structural and aesthetic reasons and are commonly 12 feet to 18 feet high. Noise barrier walls can be built of brick, concrete, plastic, Plexiglas™, metal, recycled materials, or wood.
Highway Noise Analysis Process
FHWA has developed a noise policy and has issued regulations for mitigation of highway traffic noise in the planning and design of Federally aided highways. These regulations, titled Procedures for Abatement of Highway Traffic and Construction Noise, establish standards for abating highway traffic noise. The regulations are found in Title 23, Part 772 of the Code of Federal Regulations. Compliance with the noise regulations is a prerequisite for the granting of Federal-aid highway funds for construction or reconstruction of a highway.
The noise regulations include the following requirements during the planning and design of a highway project: (1) identification of highway traffic noise impacts; (2) examination of potential abatement measures; (3) the incorporation of reasonable and feasible highway traffic noise abatement measures into the highway project; (4) coordination with local officials to provide helpful information on compatible land use planning and control; and (5) identification and incorporation of necessary measures to abate construction noise.
Noise Impacts and Noise Abatement Criteria
To help protect public health and welfare from excessive vehicle traffic noise, FHWA established Noise Abatement Criteria (NAC) based upon various land use activities and upon noise levels associated with the interference of speech.
A highway traffic noise impact occurs when the measured or predicted existing or future highway traffic noise levels approach or exceed the NAC or when predicted future highway traffic noise levels substantially exceed existing highway traffic noise level, even though the predicted levels may not exceed the NAC. Typically, state transportation agencies define approaching the criteria as meaning that noise levels are within 1 dBA of the appropriate FHWA noise abatement criterion. State transportation agencies typically define a substantial noise increase as one in which noise levels increase by 10 dBA or greater.
FHWA regulations require highway agencies to make every reasonable and feasible effort to provide noise mitigation when the criteria are approached or exceeded.
For more information, see FHWA’s Highway Traffic Noise website.
The impact of construction noise does not appear to be serious in most instances and calculation of construction noise levels is usually not necessary for traffic noise analyses. Potential impacts of highway construction noise are usually addressed generally (i.e., qualitatively rather quantitatively) and the temporary nature of the impacts are noted. An indication of the types of construction activities that can be anticipated and the noise levels typically associated with these activities can be obtained from existing literature and presented in a noise analysis.
A common-sense approach should be used to identify measures to mitigate potential highway construction noise impacts. Low-cost, easy-to-implement measures, such as work-hour limits, equipment muffler requirements, location of haul roads, elimination of “tailgate banging,” reduction of backing up for equipment with alarms, community complaint systems, and complaint mechanisms can be incorporated into the special provisions to a project’s construction specifications, as appropriate. These options can then be applied during the construction of the project by the contractor. Additional information on issues associated with highway construction noise is included in the FHWA Highway Construction Noise Handbook.
In certain instances, construction noise impacts are anticipated at a particular sensitive receiver. Also, public comment may indicate that construction noise impacts are of substantial concern. In these cases, construction noise levels can be predicted through use of FHWA’s Roadway Construction Noise Model. FHWA’s distribution memorandum and the model itself are available from FHWA’s Construction Noise Web site.
Noise from Other Transportation Modes
Aircraft noise is the second largest source of transportation noise after highway traffic noise and is a significant concern for approximately 40 square miles surrounding most major airports. While commercial aviation is responsible for the majority of total aircraft noise, private aviation and military operations also play a role.
Take-off of aircraft may lead to a noise level of more than 100 dBA at the ground, while approach and landing creating lower levels. Since aircraft landing in inner-city airports are often lower than 200 feet above roof level, a noise level above 100 dBA can be realized.
The Federal Aviation Administration’s Part 150 Airport Noise Compatibility Planning guidelines provide:
- A basis for comparing existing noise conditions to the effects of noise abatement procedures and/or forecast changes in airport activity.
- A quantitative basis for identifying potential noise impacts.
Both of these functions require the application of objective criteria for evaluating noise impacts. Part 150 provides the FAA’s recommended guidelines for noise-land use compatibility evaluation. These guidelines indicate that all uses normally are compatible with aircraft noise at exposure levels below an Ldn of 65 dBA. The Ldn describes a receiver’s cumulative noise exposure from all events over a full 24 hours, with events between 10:00 PM and 7:00 AM increased by 10 decibels to account for greater nighttime sensitivity to noise.
For more information, link to FAA’s Airport Noise website.
Rail noise is considered one of the most important environmental issues that railways must manage. Basically, rolling noise in railways is created by rough wheels and tracks.
The Federal Railroad Administration document, titled High-Speed Ground Transportation Noise and Vibration Impact Assessment, dated October 2005, is for use in planning high-speed passenger train projects. This manual contains criteria and procedures for use in analyzing potential noise and vibration impacts from various types of high-speed fixed guideway transportation systems, such as high-speed trains using traditional steel-wheel on steel-rail technology and magnetically levitated (Maglev) systems. A Freight Noise and Vibration Model was developed for the Chicago Region Environmental and Transportation Efficiency Program (CREATE) using the FTA procedures.
Experience has shown that noise and vibration are among the major concerns with regard to the effects of a transit project on the surrounding community. A transit system is of necessity placed near population centers and often causes significant noise and vibration at nearby residences and other sensitive types of land use.
Transit noise and vibration issues are discussed in an FTA report titled Transit Noise and Vibration Impact Assessment, dated May 2006. This document presents procedures for predicting and assessing noise and vibration impacts of all types of proposed mass transit projects (i.e., bus and rail projects). This document also contains noise and vibration impact criteria that are used to assess the magnitude of predicted impacts and describes a range of mitigation measures for dealing with adverse noise and vibration impacts.