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Part I Noise Fundamentals

Part II Noise Analysis

Part III Noise Abatement

Frequently Asked Questions

Glossary and Acronyms


Highway Traffic Noise - NOISE ABATEMENT


In Illinois, traffic noise impacts are determined to occur in the following situations:

  • Design year build traffic noise levels (typically projected 20 years into the future) are
    predicted to approach, meet, or exceed the Noise Abatement Criteria (NAC).

    - OR -
  • Design year build traffic noise levels are predicted to substantially increase (greater
    than 14 dB(A)) over existing noise levels.


Once a noise impact is identified, IDOT will evaluate feasible and reasonable noise abatement
measures to reduce traffic noise impacts. Traffic noise can potentially be reduced by addressing
one of the following:

  • Noise Source
  • Noise Path
  • Noise Receiver


Traffic noise levels can potentially be reduced by source modification, such as:

  • Vehicle Noise Emission Standards
  • Pavement Materials
  • Traffic Restrictions
  • Speed Limitations
  • Engine Braking Restrictions


Noise abatement can be accomplished by interrupting the noise path between the
source and the receiver. Abatement measures include:

  • Construction of noise barriers.
  • Alteration of horizontal highway alignment.
  • Alteration of vertical highway alignment.


  • Vegetation can provide aesthetic value and psychological relief from traffic noise.
  • Vegetation is not used by FHWA or IDOT for traffic noise reduction because:
    • It would need to be at least 200 ft wide and 18 ft high to reduce noise levels by 10 dB(A).
    • In most cases, available right-of-way cannot accommodate this width.


When a noise impact has been identified, IDOT typically evaluates noise barriers for abatement due to:

  • Cost Effectiveness
  • Maintenance Issues
  • No Additional Right-of-Way Required

When considering noise barriers, utilities, drainage, right-of-way, sight distance, and clear zones must be considered.


A noise barrier that just breaks the line of sight between a noise source and a receiver will
reduce noise levels by 5-dB(A). Noise reductions beyond 5-dB(A) become increasingly harder to achieve.


  • Conducted using FHWA approved traffic noise model.
  • Computer model evaluates barrier variations.


  • A barrier that just breaks line of sight between the noise source and receiver reduces noise by 5-dB(A).
  • Each additional two feet in noise barrier height reduces the traffic noise level one dB(A).



  • Barriers are most effective closest to the noise source or closest to the receiver.


  • The ground elevation of the area between a noise source and receiver affects the height of the noise barrier needed. In the example below, Wall A must exceed the height of the noise source to break the noise path to the receiver. Wall B, located at a higher ground elevation, can be much shorter to achieve the same noise reduction as Wall A.


The noise abatement evaluation is triggered by the determination of traffic noise impacts. The evaluation is to determine if abatement will meet IDOT’s Feasibility and Reasonableness Policy:




To determine whether a noise abatement option is economically reasonable, three factors may be
considered to adjust the base allowable cost of $24,000 per benefited receptor. These factors include:

  • Absolute Noise Level
  • Increase in Noise
  • Build Order of Roadway and receptor

The consideration of these adjustment factors can potentially raise the allowable cost per benefited receptor from $24,000 to a maximum of $37,000.


Any receptor afforded a 5-dB(A) or greater traffic noise reduction.


  1. Why aren’t noise barriers proposed in some cases?
    A noise barrier may be proposed when a noise impact occurs and the noise barrier will be feasible and economically reasonable. A noise barrier is determined to be feasible if it achieves at least a 5-dBA traffic noise reduction for one impacted receptor. Issues, such as driveway access and elevation of the receptor, may prevent achievement of a 5-dBA reduction.

    A noise barrier must also be economically reasonable, which includes three criteria:

    1. Achieve at least an 8 dBA traffic noise reduction for at least one benefited receptor.
    2. Estimated build cost per benefited receptor is less than the allowable cost per benefited receptor.
    3. The viewpoints of benefited receptors must be considered.

    If a noise barrier is not considered feasible and economically reasonable for an area, the noise barrier abatement measure may not be implemented as part of the project.

    Other feasibility factors that influence if a noise barrier will be proposed include whether or not sufficient right-of-way is available for the safe placement of the barrier, impacts to the line-of-sight of approaching vehicles in the vicinity of on-ramps, off-ramps, and intersecting streets and/or interference with utilities and/or drainage design elements.

  2. Would a berm be as effective as a noise wall in reducing noise levels?
    Earth berms are just as effective as noise walls. Studies have shown that earth berms actually reduce noise levels to a greater extent than noise walls due to absorption and edge effects. However, the use of berms depends on available space. For instance, since IDOT requires at least a 3:1 slope for maintenance purposes, a 12-foot berm with a 3:1 slope would be approximately 72 feet wide at the base. The available area for abatement would need to accommodate this base width.

  3. What is the cost of a noise wall?
    The average unit cost of noise wall construction used for the barrier evaluation is $25 per square foot (including materials and installation). This cost is based on Illinois construction costs and walls built. In areas where there are utilities or drainage issues that may need to be addressed, additional costs may be incurred. Typical noise walls cost $1,500,000 per mile. The unit cost is re-evaluated by IDOT at least every five years and is based on actual construction costs.

Additional information regarding noise issues is available in our "Highway Traffic Noise Assessment Manual."

This completes the third and final part of our series on Highway Noise.

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