Noise

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.

Source: http://www.fhwa.dot.gov/environment/noise/index.cfm

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.

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.

Table 2

Maximum Noise Emission Levels Required by EPA for Newly Manufactured Trucks with GVWR Over 10,000 Pounds

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.

Table 3

Maximum Noise Emission Levels Required by EPA for In-Use Medium and Heavy Trucks with GVWR Over 10,000 Pounds Engaged in Interstate Commerce

Source: Highway Traffic Noise in the United States: Problem and Response, Federal Highway Administration, 2006

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.

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 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.

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.

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.

FHWA has developed a noise policy titled Highway Traffic Noise Analysis and Abatement: Policy and Guidance 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.

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.

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:

1. A basis for comparing existing noise conditions to the effects of noise abatement procedures and/or forecast changes in airport activity.

2. 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.