Physical Environmental Impact Evaluation

Highway Noise

While vehicles provide the means for people to move from one place to another, vehicles also generate noise. When a home is close enough to a highway, the highway- related noise level at home may become unacceptable and unhealthy for its residents. To protect residents from harmful noise, Congress passed the Noise Control Act, which directed the FFIWA to establish standards for roadway noise control.

The FHWA developed and codified the “Procedures for Abatement of Flighway Traffic Noise and Construction Noise” in 23 CFR Part 772. Table 5-1 lists highway noise abatement criteria established in the aforementioned Federal regulation.

Noise abatement criteria for different locations vary. The type of activity performed (Category А, В, C) at a site and whether the activity is exterior or interior orientated determine the applicable noise abatement criterion.

23 CFR Part 772 is only applicable to Federal-funded new alignment highway projects or capacity improvement projects (roadway widening projects) where additional through lanes are added.

State governments establish state-specific regulations in dealing with other scenarios (e.g., existing highway noise impact) related to highway noise abatement.

As highways are widened, roads become closer to homes. Therefore, highway noise impact analysis and abatement have become increasingly challenging for highway agencies. State DOTs have developed and adopted a wide range of noise abatement measures, including the highly visible noise abatement walls, as illustrated in Figure 5-5a. Highway noise analysts must understand not only the science

Table 5.1 Noise Abatement Criteria Per 23CFR772 (Hourly A-Weighted Sound Levela decibels)



Activity Criteriab



Activity Description







Lands on which serenity and quiet are of extraordinary significance and serve an important public need and where the preservation of those qualities is essential if the area is to continue to serve its intended purpose.










Active sport areas, amphitheaters, auditoriums, campgrounds, cemeteries, day care centers, hospitals, libraries, medical facilities, parks, picnic areas, places of worship, playgrounds, public meeting rooms, public or nonprofit institutional structures, radio studios, recording studios, recreation areas, Section 4(f) sites, schools, television studios, trails, and trail crossings





Auditoriums, day care centers, hospitals, libraries, medical facilities, places of worship, public meeting rooms, public or nonprofit institutional structures, radio studios, recording studios, schools, and television studios





Hotels, motels, offices, restaurants/ bars, and other developed lands, properties or activities not included in A-D or F.


Table 5.1 (Continued) Noise Abatement Criteria Per 23CFR772 (Hourly А-Weighted Sound Level1 decibels)



Activity Criteriab



Activity Description




Agriculture, airports, bus yards, emergency services, industrial, logging, maintenance facilities, manufacturing, mining, rail yards, retail facilities, shipyards, utilities (water resources, water treatment, electrical), and warehousing





Undeveloped lands that are not permitted

Sources: 23 CFR Part 772,

a Either Leq(h) or LI0(h) (but not both) may be used on a project. b The Leq(h) and L10(h) Activity Criteria values are for impact determination only, and are not design standards for noise abatement measures. c Includes undeveloped lands permitted for this activity category.

a Illustrations of noise abatement walls

Figure 5.5a Illustrations of noise abatement walls.

of noise modeling and measurement, but also relevant policy, regulations, and laws. Additionally, analysts must be able to communicate effectively with the public both orally and in writing.

Basic Noise Science

Vibrations cause changes in air pressure. These pressure changes create a series of pressure waves. When a pressure wave travels through air and reaches people’s eardrums, people sense the pressure change and describe it as sound. A sound is a composite of different pressure waves. Undesired and annoyance sound is termed noise. Sound Frequency (/)

Frequency (/) is a measure of the number of vibrations per second and has a unit of hertz (Hz). One Hz is equal to one event cycle per second.

Human hearing perceives sound frequency as pitch (e.g., a high-pitch violin or a low-pitch trombone). Humans can decipher frequencies between approximately 20 Hz and 20,000 Hz. As people age, the ability to hear high-frequency sound tends to decline. Sound Speed (c)

The speed at which sound travels (c) is primarily affected by the density and the compressibility of its traveled medium. For highway-related noise, the travel medium is air. The sound speed in the air is approximately 1,130 feet per second. Sound Wavelength (2)

Wavelength (2) is the distance traveled by a sound wave during one cycle. Sound waves audible to humans range from approximately 1 inch to 55 feet in length. Relationship

Frequency, wavelength, and speed of sound waves are linked by the equation c =fx 2. Sound speed is the mathematical product of its frequency and wavelength. Octave Bands

An octave band divides the entire frequency range into smaller segments called octaves. The method for this division is that the higher-frequency octave segment is twice that of the lowest frequency. It quantifies effective frequencies without looking at each frequency one at a time. One of the most common octave frequency bands is 31 Hz, 63Hz, 125Hz, 250Hz, 500Hz, 1 kHz, 2kHz, 4kHz, 8kHz, and 16kHz. Sound Energy and Pressure

Sound power is the amount of energy per unit of time that is emitted from a source in the form of sound waves. Linking sound power and sound pressure is critical in building modeling tools to predict noise levels under various conditions.

While it is not possible to measure acoustic energy directly, pressure changes derived from vibrations can be detected easily and precisely. Acoustic energy is proportional to sound pressure squared. The sound pressure threshold for hearing is 20 micro-pascals (1,000 Hz). Sound Measuring Unit - Decibels

Decibels (dB) is the unit used to describe the intensity of sound. The mathematical equation is:

P2: pressure in pascals (Pa)

P0: reference pressure 0.00002 Pa (20 micro-pascals)

Decibel is on a logarithmic scale. A small change in the dB level means a large change of energy. When P2 is at its lowest human decipherable pressure of 0.00002 Pa, the noise level is equal to zero. Loudness of Sound

Loudness is how human ears sense sound. Sound loudness depends on both the acoustic pressure and frequency. Three internationally standardized sound frequency weighting methods known as A, C, and Z frequency weightings are used to characterize sound loudness.

Lowest human decipherable sound pressure is approximately 20 micro-pascals.

А-weighting (dBA) deemphasizes low-frequency waves. Human ears are less sensitive to both low and very high frequencies. А-weighting is commonly used for environmental noise measurement.

C-weighting (dBC) simulates human ears’ frequency sensitivity at the very high noise level. The C-weighting scale is very flat as compared with А-weighting because it includes a greater range of low-frequency sounds than the A scale.

Both A and C weightings are attempts to simulate how human ears react to sound.

Z-weighting using the same weight for all frequencies, and it is also referred to as no frequency weighting.


Human ears react differently to different sound frequencies. Weighting is an attempt to simulate human ears' reaction to a composite sound wave.

Highway-related noise criteria shown in Table 5-1 are based on based on A-weighting.

Noise measuring equipment for highway field noise monitoring should be equipped with the А-scale weighting function. Leq and L10 Sound Levels

Leq is an equivalent sound level for a specific period. A 1-hour Leq is an equivalent (average) constant sound level for a whole hour.

L10 sound level is the sound level exceeding 10% of the time during a specific period. And 1-hour L10 is the sounding level exceeding 10% of the time during a whole hour. Insertion Loss

Insertion Loss associated with highway noise analysis refers to noise level reduction resulting from a noise wall, a berm, or other installed barriers or devices. For example, with a project, it is determined that a 12.6 feet high concrete noise abatement wall can reduce 8.2 dBA of noise. The 8.2 dBA reduction is called “Insertion Loss.” Sound Transmission in the Field

Sound or noise impact analysis is the assessment of sound propagation from a source to a receiver. A receiver is a location representing an outdoor area where there are human activities. The most commonly identified outdoor activity area in a highway setting is a home’s backyard illustrated in Figure 5-5b. The centroid (лг, у, z) of the backyard is typically used to represent the impact location. The sound level at the centroid site is compared to the noise impact criteria listed in Table 5-L

Figure 5-5b illustrates the geospatial layout among sound source (road), terrain, sound wall, and a receiver.

As vehicle noise propagates toward a house, the sound is reduced from (a) geometric attenuation, (b) atmospheric absorption, (c) terrain ground surface bouncing, and (d) barrier blockage if one is in existence.

Sound geometric attenuation refers to the reduction in sound pressure caused by the spread of the radiated sound energy over a sphere of increasing area (4лг2). The area of spherical wave front increases in proportion to the square of the distance r (radius of the sphere) from the source. The resulting sound pressure level decreases

b Noise transmission from source to receiver illustration

Figure 5.5b Noise transmission from source to receiver illustration.

at a rate inversely proportional to r. This inversed squared relationship is why distance itself is a very effective measure to mitigate noise impact.

Atmospheric absorption attenuation is the conversion of molecular oscillation energy to heat due to the viscous frictional movement of air molecules.

Terrain ground surface bouncing covers both ground absorption and reflection of sound waves. This ground refection leads to sound wave direction changes and a reduction of energy level.

Barrier attenuation is noise reduction due to a solid barrier located between a source and a receiver. A barrier such as a wall interrupts the direct sound propagation path. To achieve the maximum effectiveness, a barrier should be designed such that it interrupts the direct line of sight between the source and the receiver. Sound Propagation Modeling

To estimate the highway noise impact, computer-based noise modeling programs are developed. These models include practical empirical engineering models, semiempirical mechanistic models, and numerical mechanistic models.

With the practical empirical engineering approach, methods rely on correlating actual field measurements of sound levels associated with a wide range of sound sources, terrain types (e.g., flat, hilly, grassy, paved), environmental conditions (e.g., wind speed, humidity, temperature), distances between sources and receivers, and other parameters. Vehicle speed, acceleration, deceleration, vehicle types, tailpipe location and height, tire types, pavement types are also tested and correlated parameters. Regression equations and lookup tables are often deployed to relate sound levels to these parameters.

These practical empirical engineering models are computationally simple and easy to use. However, the simplifications associated with such models make them less accurate and sometimes difficult to apply to unique situations.

Empirical semi-mechanistic approaches are partially based on analytical solutions of wave equations. However, their reliance on empirical data is still a critical component in model development. Semi-mechanistic approaches often can model more complicated scenarios than pure empirical engineering models. The FHWA’s Traffic Noise Modeling (TNM) program is an example of a semi-mechanistic modeling software.

The last modeling approach is the mechanistic numerical approach. While this type of model provides great flexibility in modeling sound propagation, its input requirements are substantial. Its sound level estimates are often less accurate than those of the other methods. The advantage of such mechanistic models is that their applications are less constrained by scenario conditions.

Noise Impact Analysis Practice

Noise impact analysis as related to highway projects follows the sequence of:

  • 1. identifying potentially noise-sensitive sites (receivers),
  • 2. determining actual receiver location coordinates (x, у, г) for these potential noise-sensitive sites,
  • 3- coding geospatial relationship among roadways and receivers (x, y, z),
  • 4. obtaining traffic data (volume, class, and speed) from the Design Traffic report covering the current-year, opening-year, and design-year traffic data,
  • 5. entering the above data into the FHWA TNM model and run the model, and
  • 6. interpreting TNM outputs.

If a projected noise level approaches 67 dBA (the common interpretation of approaching 67 dBA is when the actual numerical number exceeds 65 dBA), then appropriate noise abatement measures, including the feasibility of a noise wall, are analyzed.

For any noise wall modeling, a potential wall’s spatial location in the same geo- referenced system (x, y, z) used for roadway and receivers should be used. A noise wall’s horizontal alignment typically follows the right of way line. Its vertical alignment follows the topography along the right of way. Analysts should systemically model a wall by adjusting its height (г) sequentially until the noise level at receivers is below the 67 dBA (65 dBA) predicted by the TNM.

Whether a noise wall should be built or not depends on a host of additional conditions such as those listed below. [1] [2]

3- Desires of impacted residents - If an impacted resident does not want to have a wall, the state may not force the resident to have one.


  • • geometric attenuation,
  • • atmospheric absorption,
  • • terrain ground surface bouncing, and
  • • barrier refection and absorption.
  • 4. Effectiveness of a wall - A wall must be truly effective in providing noise reduction for an impacted resident. If a wall with its dimensions and locations cannot provide adequate insertion loss (e.g., 5 dBA), then the wall is deemed ineffective and should not be built.
  • 5. Other site conditions (e.g., constructability) and limitations preventing a wall’s feasibility.
  • 6. Antiquity consideration - State DOTs may also consider a wall’s feasibility based on the rationale of “who is there first.” If a highway was there before a receiver site, the site might not qualify for a noise wall.

Highway noise impact is very controversial. Impacted citizens are often very emotional. Project engineers, project managers, and noise analysts must pay close attention to this issue. Alternative measures such as planting evergreen bushes and trees (no noise abatement capability) to soften the visual impact of a highway are often offered to sites that do not qualify for noise abatement walls.

  • [1] Cost - State DOTs have limitations on the maximum amount of money thatcan be invested for a given receiver. If the cost exceeds the limit, a wall isconsidered not feasible.
  • [2] Wall height — State DOTs may have limitations on what the maximum heightof a wall may be. In addition to increased cost, a too-tall wall is often considered undesirable by residents.
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