Sound Barriers Guidelines - Highway Traffic Noise Analysis


The National Environmental Policy Act (NEPA) of 1969 and the Federal-Aid Highway Act of 1970 control the evaluation of highway traffic noise impacts. They require environmental evaluation of federal or federal-aid highway projects, and reasonable and feasible mitigation of identified impacts. Also, on May 14, 1976, the Federal Highway Administration (FHWA) publicly stated that local governments must help control noise impacts through noise-compatible land-use planning and zoning.

The Federal Highway Administration (FHWA) highway traffic noise regulation 23 CFR 772 requires the following during the planning and design of a highway project:

  • Identification of highway traffic noise impacts.
  • Examination of potential abatement measures.
  • The incorporation of reasonable and feasible highway traffic noise abatement measures into the highway project.
  • Coordination with local officials to provide helpful information on compatible land use planning and control.
  • Identification and incorporation of necessary measures to abate construction noise.

The regulation requires that FHWA make every feasible and reasonable effort to provide substantial noise reduction when highway traffic noise impacts occur. Compliance with 23 CFR 772 is a State prerequisite for receiving Federal-aid highway funds for construction or reconstruction of a highway.


The Maryland Department of Transportation State Highway Administration State Highway Administration (MDOT SHA) conducts noise analysis for Type I projects as part of the NEPA process. A typical traffic noise analysis includes:

  1. Identification of Existing Activities and Land Use

    MDOT SHA does all existing land uses and future planned development such as residential houses, schools, houses of worship, and parks in the vicinity of a proposed highway project. It is at these locations that existing noise levels are determined, future noise levels predicted, and the degree and extent of noise impact determined.

  2. Determination of Existing Noise Levels

    MDOT SHA does a noise measurement to determine the existing noise level in an area. It is important to determine the source of the noise, as there may be non-highway noise sources, especially in commercial areas. The goal is to establish a baseline for assessing the impact of highway traffic noise. In assessing traffic noise impact, the relationship of existing noise to predicted future noise levels is a factor.

  3. Prediction of Future Noise Levels

    Based on the proposed highway alignment, MDOT SHA uses a computer program developed by the Federal Highway Administration (FHWA) to forecast the level of noise in the design year, which is typically about 20 years in the future. The computer program uses predicted loudest-hour (worst-case) traffic volumes, the percentage of trucks, and travel speeds to forecast noise levels.

    Contrary to popular belief, the worst source of highway noise is not typically a traffic jam. When there is a traffic jam, vehicles do not move as fast as they normally would if there was no jam, and therefore do not make as much noise. The level of service (LOS) of a roadway describes the operating conditions on the facility in terms of traffic performance measures related to speed and travel time, freedom to maneuver, traffic interruptions, comfort and convenience. Levels of service range from LOS A (least congested) to LOS F (most congested) as shown below:

    Levels of ServiceAdapted from A Policy on Geometric Design of Highways and Streets. AASHTO. 2001

    Highway traffic noise is worst in LOS D — this is when the roadway is nearing capacity. Although vehicles may not be moving quickly, they are all moving and are doing so at a steady speed. This is the point at which any more vehicle additions will result in an unstable flow of traffic (stop-and-go traffic).

    The computer program also examines distance of the receptors from the proposed highway and physical features such as hills, valleys, buildings, and obstructions that affect how noise would travel to the receptors.

    The most recent computer program developed by the FHWA is TNM (Traffic Noise Model) Version 2.5, available through the McTrans Center at the University of Florida.

  4. Determination of Noise Impact

    'Sensitive noise receptors' — residences, schools, houses of worship, and historic sites — are strongly affected by traffic noise if:

    • future noise levels are forecasted to reach or exceed the threshold of 66 dBA, or

    • a highway project will increase future noise levels by 10 dBA or more over existing measured levels

  5. Evaluation of Noise Abatement Alternatives

    If noise impacts are identified, MDOT SHA considers noise abatement to minimize or eliminate impacts from the project. This impact alone does not determine that abatement will be implemented. All criteria, as set forth in the MDOT SHA Sound Barrier Policy, must be met before abatement is considered reasonable and feasible.

    Abatement may or may not be in the form of a noise barrier. MDOT SHA determines the specific form of noise abatement on a site-by-site basis, and can be influenced by the use or activity that is affected.

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Why are noise measurements taken?

MDOT SHA takes noise measurements for the following reasons:

  • To evaluate existing highway traffic noise impacts.

  • To analyze potential impacts from a planned highway project and to determine noise abatement measures if needed.

  • To validate noise prediction computer models.

How are noise measurements taken?

  • MDOT SHA staff measure highway traffic noise using a sound level meter.

    Noise meter

  • Each meter is fitted with a microphone, and is calibrated before and after each monitoring session using a standard acoustic calibrator.

    Noise measurement equipment

  • Statistical descriptors describe varying traffic noise levels. The most common statistical descriptors used for traffic noise are the L10 and Leq.

    • L10 is the sound level that is exceeded 10 percent of the time in a given period.

    • Leq is an average of the total sound energy measured in a given period. The Leq is the noise level descriptor commonly used in the State of Maryland.

  • Following standard practice in environmental noise measurement, MDOT SHA uses an A-weighted frequency response dB(A), which is a weighting of high- and low-pitched sounds, to approximate how an average person hears sounds. The A-weighted decibel scale begins at zero. This represents the faintest sound that can be heard by humans.

  • In the State of Maryland, highway noise measurement studies may involve short-term measurements of 15 - 20 minutes or continuous monitoring for a minimum of 24 hours. The duration of the measurement is based on the needs of the study.

  • Before noise measurements can be taken at residences, MDOT SHA must receive permission from the residents.

When can noise measurements be taken?

Environmental noise measurements can only be taken under favorable weather conditions.

  • Winds cannot exceed 10 - 12 mph (calm winds or winds below 5 mph are preferable; up to 10 mph is acceptable). Excessive winds create additional noise in a microphone; in some cases they break up the signal and destroy data.
  • Measurements are not conducted if it is raining or snowing. Under such conditions, absorption or scattering of sound waves can result in a falsely high or low reading.

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For more information

Additional questions? Contact the Office of Highway Development(OHD) at 1-888-228-5003. ​