Highway Traffic Noise - Abatement Evaluation

This is the last of a three-part series presenting information on highway traffic noise.  This third module presents information on noise abatement as it pertains to highway traffic noise.  Specifically, it will cover noise abatement in Illinois, noise abatement approaches, noise barrier evaluations, and noise barrier design.

Noise Abatement in Illinois

Through 2001, more than 67 miles of noise walls totaling more than $88 million have been constructed in Illinois.  Illinois ranks tenth among state highway departments in miles constructed and dollars spent for noise barrier construction.

Noise Levels Warranting Abatement Evaluation

In Illinois, traffic noise impacts are determined to occur under two conditions.  The first condition is when design year traffic noise levels are predicted to approach, meet or exceed the Noise Abatement Criteria  (NAC).  IDOT considers noise levels within one dBA as approaching the NAC.  For example, a traffic noise level of 66 dBA would be considered a residential impact as it is within one dBA of the residential NAC of 67 dBA.  The second type of traffic noise impact occurs when design year traffic noise levels are predicted to be more than 14 dBA over existing noise levels.  For example, a new road in a rural area may increase traffic noise levels from 50 dBA to 65 dBA.  While this does not approach the NAC, it is a substantial noise increase and considered an impact by IDOT.

Noise Abatement Approaches

Once a noise impact is identified, IDOT will evaluate feasible and reasonable noise abatement methods to reduce traffic noise impacts.  Traffic noise can potentially be reduced by addressing the noise source, the noise path, or the noise receiver.   

Noise Barriers are an Abatement Option

When a noise impact has been identified, and after all noise source options and noise path abatement options have been evaluated, IDOT typically uses noise barriers due to:

• Cost Effectiveness

• Maintenance Issues

• Abatement within Right-of-Way

 Maintenance Issues

Maintenance issues include maintaining the barrier appearance (i.e. graffiti) and repair (i.e. collision damage).

     Type I Noise Barriers

•  IDOT maintains the roadway side of the noise barrier.

• The other side of the noise barrier is maintained by the local agency when there is insufficient right-of-way available to adequately maintain the noise barrier.

     Type II Noise Barriers

•  The local sponsor will be responsible for maintaining the noise barrier.

 Local Considerations

• Noise barriers may be implemented by IDOT when the noise barrier is determined to be feasible and reasonable according to the noise abatement analysis.

• Comments from the public, including local government entities, are considered during the Public Involvement process.

 Noise Wall Design Considerations

• Utilities - Conflicts with existing and proposed utilities such as natural gas lines, water mains, and electric lines will need to be resolved.

• Drainage - Stormwater drainage will need to be maintained to avoid flood issues.

• Right-of-Way - Noise barriers need to be within IDOT right-of-way for maintenance purposes.  Coordination with local agencies may allow exceptions.

•  Sight Distance - Noise barriers cannot obstruct drivers view of roads and intersections.

• Clear Zones - Noise barriers cannot be constructed directly adjacent to the roadway without special design elements, i.e., guard rails.  A clear zone adjacent  to the roadway is maintained for safety reasons.

 Traffic Noise Reduction Attainability

*IDOT uses an 8-dBA traffic noise reduction goal.

Source: USDOT, Highway Traffic Noise in the United States, Problem and Response, April 2000.

 Noise Barrier Evaluation

• Conducted using FHWA approved noise model

• Computer model evaluates barrier variations

• Barrier height

• Barrier length

• Barrier location

• Barrier elevation

Noise Abatement Goals

Noise impacts trigger a noise abatement evaluation.  The noise abatement design must meet IDOTs feasible and reasonable criteria.  The noise abatement option is determined to be feasible if it achieves at least an 8-dBA traffic noise reduction.  A noise abatement option must also be economically reasonable. This means that the noise abatement design does not cost more than $24,000 per benefited receptor.  For example, a noise wall benefiting 10 residences at a cost of $240,000 is considered economically reasonable.

Noise Barrier Materials

IDOT currently accepts three types of materials for noise barrier construction based on testing and research results. 

• Concrete

• Composite Materials

• Earth Berm

Noise Wall Effectiveness

Several elements affect the level of traffic noise reduction achieved.  Effectiveness is reduced in the linked scenarios.

Parallel Walls

A noise wall is designed to achieve an 8-dBA traffic noise reduction for an area.  When a second noise wall is located across the roadway, noise may reflect off the wall and back toward the original wall. 

IDOT evaluates these conditions to consider measures needed to achieve an 8-dBA traffic noise reduction..

Reflective vs. Absorptive Noise Walls

Reflective Noise Walls

•  Typical noise walls of concrete or composite materials reflect noise.

Absorptive Noise Walls

• Considered by IDOT where parallel walls occur and the ratio of the distance between walls to the height of the wall is less than 20:1.

• A sound absorbing noise wall material may be considered by IDOT to reduce reflected noise.

 IDOT Noise Wall Funding Sources

          Type I Projects

        Federal and/or State funds.

          Type II Projects

        State/Local Funds
Note: IDOT does not currently have a separate funding program for Type II noise walls. It is the local sponsoring agencies responsibility to secure funds for their noise wall project. If state funds are being sought for the project, it will have to compete against all other eligible state funded projects in the Department's Multi-Year Highway Program.