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Highway Traffic Noise Analysis and Abatement Policy and Guidance

Highway Traffic Noise Analysis and Documentation

The major objectives of a noise study for new highway construction or a highway improvement are:

Traffic noise studies thus provide useful information, directed primarily to two distinctly different audiences - the government decisionmaker and the lay public. For the government decisionmaker, the study should provide a portion of the data needed for the informed selection of a satisfactory project alternative and appropriate mitigation measures. For the lay public, the study should provide discussion of potential impacts in any areas of concern to the public.

The final product of a highway traffic noise study should be a clear, concise written discussion of the study. There should be a stand-alone discussion, a noise study report, that gives the reader a detailed description of all the elements of the analysis done for the study, including information on noise fundamentals and regulatory requirements. The environmental document for Type I projects, i.e., Categorical Exclusion (CE), Environmental Assessment/Finding of No Significant Impact (EA/FONSI), Environmental Impact Statement (EIS), should contain a brief summary of the important points found in the noise study report. The project development records should fully document the traffic noise analysis level-of-effort expended, strategies considered, adjacent residents' views and opinions, and a final decision on the reasonableness and feasibility of abatement.

The key elements of a highway traffic noise study are as follows:

Definition of Impact Criteria and Identification of Noise-Sensitive Land Uses

The first step in the highway noise study is the definition of criteria for noise impact. With this definition established, the location of noise- sensitive land uses in the vicinity of the various study alternatives can be identified.

A noise impact occurs (1) when the projected highway noise levels approach or exceed the noise abatement criteria in 23 CFR 772 or (2) when the projected highway noise levels substantially exceed existing noise levels in an area. Based upon this, noise-sensitive land uses in the vicinity of each of the study alternatives that may be impacted by future highway noise levels should be identified. Noise-sensitive areas may be identified by individual land uses, or by broad categories of land use for which a single criterion level may apply. In some cases, lands that are undeveloped at the time of the project may be known to be under consideration for development in the future. Depending upon the certainty of development in accordance with Paragraph 772.9b(1) of 23 CFR 772 (see page 11), these lands should be treated as "planned, designed, and programmed," and the severity of highway noise impact should be assessed accordingly.

A brief categorization of land-use types follows:

Example: In this study, all land along the project is considered to fall in activity category B.

Somewhat more detail is provided by the following:

Example: The region is primarily residential, although it is zoned for general business as well. Two apartmentcomplexes and 50 residences are east of Airport Drive, at the south end. The nearest facade of these buildings is approximately 21 meters from the road centerline, and the farthest facade is roughly 122 meters away. The apartments house about 200 families.

Determination of Existing Noise Levels

In general, existing noise levels should be established by field measurements for all developed land uses and activities. Field measurements should be made, since existing background noise is usually a composite from many sources, and noise prediction models are applicable only to noise originating from a specific source. If it is clear that existing noise levels at locations of interest are predominantly due to a highway, then the existing noise levels may be calculated using the FHWA highway traffic noise prediction model.

When making existing noise measurements, a number of factors need to be considered: 1) time of day, e.g., peak hour vs. any other time of day; 2) day of week, e.g., weekend day vs. work day; 3) week of year, e.g., tourist season vs. non-tourist season; and 4) representativeness of the noise. The noise measurement should yield the worst hourly noise level generated from representative noise sources for that area. The period with the highest sound levels may not be at the peak traffic hour but instead, during some period when traffic volumes are lower but the truck mix or vehicle speeds are higher.

Measurements should be made at representative locations - that is, residential neighborhoods, commercial and industrial areas, parks, churches, schools, hospitals, libraries, etc. Measurements are normally restricted to exterior areas of frequent human use; interior measurements are only made when there are no outside activities, such as churches, hospitals, libraries, etc. Measurements are usually taken in one of three exterior locations: (1) at or near the highway right-of-way line; (2) at or near buildings in residential or commercial areas; and (3) at an area between the right-of-way line and the building where frequent human activity occurs, such as a patio or the yard of a home.

Representativeness relates to the noise typically found in a given location. Aircraft noise is usually representative near an airport but not in areas having no airport; the noise from barking dogs is usually representative near kennels but not in a residential neighborhood; and the noise from ambulance or police sirens is usually representative near hospitals or police stations but not in other locations.

Measurements are made to represent an hourly equivalent sound level, Leq(h). For statistical accuracy, a minimum of approximately eight minutes of measurements must be made. Most SHAs have automated measurement equipment and typically measure 15-minute time periods to represent the Leq(h). This is acceptable if nothing unusual is expected to occur during the noisiest hour. Measurements along low-volume highways may require longer measurement periods (e.g., 30-60 minutes) to attain desirable statistical accuracy. If information is not available to identify the noisiest hour of the day or if there is public controversy at a specific location, 24-hour measurements are sometimes taken.

Measurements should be made with noise meters of sufficient accuracy to yield valid data for the particular project (ANSI S1.4-1983, TYPE II or better). Procedures should be adopted and followed so that measurements will have consistent and supportable validity. Traffic conditions, climatic conditions, and land uses at the time of measurement should be noted.

The following excerpt from an environmental impact statement shows how existing noise levels can be documented.

Example: Figure __ is a plan map of the study area and shows the location of the noise measurement sites. The microphone was located 1.5 meters above the ground. Measurement Site Nos. l, 2, and 4 are along the existing Airport Drive and near the apartment buildings closest to the project roadway. These locations were chosen to document existing noise levels and traffic conditions at the residential area where the potential for noise impacts due to the project exists. Sites 3 and 5 are located in residential areas near the location of the proposed extension of Airport Drive. In these areas, existing noise levels are expected to be the lowest in the project corridor. Sites 6 and 7 are near the other roadways in the study area that carry substantial traffic and connect to the proposed project.

The existing noise measurements were made during midday hours on June 12 and 13, 1988. The temperature varied from 18 degrees C to 27 degrees C, and winds were light and variable, having little effect on sound propagation over moderate distances.

Noise measurements were obtained with the BBN Model 614 portable Noise Monitor, set to compute sound level distributions on a minute-by-minute basis. During each minute of analysis, the ambient noise sources were noted and local traffic counts were made. The duration of each measurement period was between 20 and 35 minutes.

Prediction of Future Traffic Noise Levels

The next step involved in the highway noise study is to analyze the noise levels expected to occur as a result of the proposed highway or highway extension. Noise levels should be estimated for each of the potential project alternatives, including the "do-nothing" case. The method used to predict traffic noise levels and traffic data for the various alternatives should be well documented.

Example: Prediction of the Future Traffic Noise Levels For each of the seven alternatives under consideration, traffic noise at each receptor for the year 2000 was predicted using the FHWA Highway Traffic Noise Prediction Model, STAMINA 2.0. This model uses the number and type of vehicles on the planned roadway, their speeds, and the physical characteristics of the road, e.g, curves, hills, depressed, elevated, etc. In this regard it is to be noted that only preliminary alignment and roadway elevation characteristics were available for use in this noise analysis. Each alternative was modeled assuming no special noise abatement measures would be incorporated. Only those existing natural or man-made barriers were included. The roadway sections were assumed to be at-grade, except where grade separation of intersections was necessary. Thus, the analysis represents "worst-case" topographic conditions. The traffic volumes used in the projections were obtained from the Metropolitan Council Regional Traffic Assignment Model. The noise predictions made in this report are highway- related noise predictions for the traffic conditions during the design year. It was assumed that the peak-hour volumes and corresponding speeds for trucks and automobiles result in the noisiest conditions. During all other time periods, the noise levels will be less than those indicated in this report.

Identification of Traffic Noise Impacts

The next step in the noise study involves a comparison of the predicted noise levels for each project alternative with the noise abatement criteria and existing noise levels. This comparison identifies the traffic noise impacts associated with each alternative in terms of the change in existing levels and the amount by which criteria may be approached or exceeded. The main purpose of this comparison is to contrast the noise impacts that are expected to occur as a result of the highway project, for each active alternative, with the existing noise impacts.

The noise abatement criteria from 23 CFR 772 are listed in Table 5 (see page 8). Abatement must be considered when future noise levels approach or exceed these criteria. Traffic noise analyses should recognize and consider absolute noise levels as well as incremental increases in noise levels when identifying traffic noise impacts and when considering noise abatement measures.

The following example illustrates a discussion of impact in an EIS:

Example: A noise analysis has been conducted for the proposed actions. The greatest noise impact will be felt at residential sites which are near the proposed loop location. Table No. 7 shows the results of this analysis. The average impact on the selected noise sites is +12 dBA which will seem about 2½ times as loud as the existing noise environment. The largest impacts (up to +25 dBA) will be felt at rural residences that are now on the less traveled backroads and will be close to the proposed highway.

For the recommended Alternate 3, 52 single-family residences,12 multiple-family residences and 2 churches equal or exceed the noise abatement criteria. Fifty-two single-family residences, 28 multiple-family residences, 2 businesses, and 2 churches will experience a substantial increase in existing noise levels, that is, an increase of 10 dBA or more.

Identification and Consideration of Abatement

The next step in the noise study is identification and evaluation of various noise abatement measures that could mitigate the adverse impacts predicted for the proposed highway project. For example, traffic management measures such as the following should be included in the evaluation:

Additional noise abatement measures are discussed in detail in the Section V. For each abatement measure, the following information should be presented:

Examples: The most likely method available to lessen the noise levels and thus alleviate noise impact from Airport Drive is to incorporate noise control into the highway design stage. Since the alignment and grade of Airport Drive have already been established, noise barriers beside the roadway are probably the most acceptable means of noise control.

. . . The first location for which a noise control barrier has been designed is along Airport Drive at the East Avenue-Fair Oaks apartment complex. The proposed barrier is located 3.6 meters from the edge of Airport Drive, is about 540 meters long, and runs from a point about 45 meters north of the edge of Niners Road at the Airport Drive intersection to about 21 meters north of the northernmost apartment building. If the top of the barrier is 3 meters above grade level, it will provide 9-11 dB reduction in the noise levels at the nearest building, first floor elevation (1.5 meters above ground). This will reduce the predicted exterior Leq noise levels near these buildings from 73-74 dB to 62-65 dB.

. . . The cost of noise barriers depends directly on the material used to build it. Depending upon material selection, barrier costs including installation may be as little as $50 per lineal meter or as great as $250 per lineal meter. If wooden barriers are erected along Airport Drive, the cost ofhe barrier for the apartments would be about $85,000, and the cost of the barrier for the three homes would be about $35,000.

Table 9: Example of Abatement Information for an EIS
Existing and Future Exterior Noise Levels (Leq in dBA)
Land Use
Numbers by
Activity (1)
Distance to
Noise Level
Future Noise Levels by Project Alternative
Without and With Abatement (2)
1 (No-Build) 2 3 4
1 B 3 MF 300' 67 55 63 66/58 68/60 68/60
2 B 7 SF 170' 67 58 58 70/60 72/61 73/65
3 C 2 B 260' 72 54 55 67/60 69/60 70/63
4 B 11 SF, 7 MF 100' 67 56 62 73/65 75/65 75/69
5 B 16 MF 150' 67 52 52 62/59 66/61 67/64
6 B 14 SF 170' 67 52 54 75/66 77/69 77/71
7 B 12 SF, 1 MF 200' 67 53 56 66/62 69/67 69/66
8 B 2 CH 180' 67 53 54 69/61 73/62 73/69
9 C 3 B 150' 72 62 67 69/- 69/- 70/-
10 B 7 SF, 1 MF 230' 67 57 61 69/66 69/64 70/64
(1) SF = Single-Family Residence
MF = Multiple-Family Residence
B = Business
CH = Chruch
(2) 66/58: 66 = Noise Level Without Abatement
58 = Noise Level With Abatement
- = Abatement Not Considered

Construction Noise Analysis

The consideration of construction noise must be addressed in an environmental document. The following example illustrates a construction noise discussion from an EIS:

Example: It is difficult to predict reliable levels of construction noise at a particular receptor or group of receptors. Heavy machinery, the major source of noise in construction, is constantly moving in unpredictable patterns. Daily construction normally occurs during daylight hours when occasional loud noises are more tolerable. No one receptor is expected to be exposed to construction noise of long duration; therefore, extended disruption of normal activities is not anticipated. However, provisions will be included in the plans and specifications requiring the contractor to make every reasonable effort to minimize construction noise through abatement measures such as work-hour controls and maintenance of muffler systems.

Coordination with Local Government Officials

The final part of the noise study is coordination with local officials whose jurisdictions are affected. The primary purpose of this coordination is to promote compatibility between land development and highways.

The highway agency should furnish the following information to appropriate local officials:

Updated: 8/24/2017
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