Highway Traffic Noise - Analysis

The Illinois Department of Transportation maintains more than 17,000 miles of roadway per year.  This includes 2,050 miles of interstate roads.

Noise issues are a consideration from initial design through the lifetime of the roadway.

Projects requiring highway noise analysis fall within two categories:

• Type I

• Type II

Type I and Type II projects are defined in the US Code of Federal Regulations (23 CFR Part 772) by the Federal Highway Administration (FHWA).

Type I Before Construction

Type I After Construction

A Type II project includes studying noise abatement for:

• Existing Fully controlled-access state highways (Freeways/Interstates).

• State highways in urban areas.

•  Noise abatement projects sponsored by a state or local government sponsor.

Federal Regulations

For different land uses, there are different noise criteria for impacts. 

A traffic noise impact occurs when noise levels approach, meet or exceed the Noise Abatement Criteria (NAC) warranting a noise abatement analysis.

IDOT Policy

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

Design year (typically 20 years into the future) traffic noise levels are predicted to approach, meet, or exceed the NAC, with approach defined as 1 dBA from NAC.

                                       OR

Design year (typically 20 years into the future) traffic noise levels are predicted to substantially increase (greater than 14 dBA) over existing traffic-generated noise levels.

 Noise Monitoring

Noise monitoring is conducted at selected locations to measure existing noise levels. 

A typical noise meter is about the size of a large camera and is placed on a tripod approximately 5 feet high. 

Each monitoring period is approximately 10 minutes.

Noise Monitoring Location

Typical Noise Meter

During the noise monitoring, automobile and truck traffic volumes are counted and other conditions such as weather, wind speed, and other background noises are recorded on field sheets.  Noise monitoring is conducted at times when there is peak traffic traveling at peak speeds.  Noise monitoring provides an instantaneous snapshot of existing highway noise levels including other common background sources (i.e. lawn mowers, barking dogs, and playground areas).

Noise Monitoring versus Computer Noise Modeling

Noise monitoring is a tool that only provides information for existing conditions.  Computer noise modeling is used to predict traffic noise levels for existing and future conditions and is the tool used to predict potential traffic noise impacts.

Computer Noise Modeling

The FHWA approved computer noise model is used to predict noise levels.

1. Select representative noise sensitive areas
2. Determine model inputs
3. Compare existing and future noise levels predicted by the model to Illinois  policy for purposes of determining noise impacts

 Scenarios evaluated in the model include:

The first step in conducting a noise analysis is to select representative receptors that characterize Noise Sensitive Areas (NSA) with:

1. Similar land use

2. Similar distance to roadway

3. Same basic topography

IDOT defines a receptor as a sensitive land use where frequent human use occurs and where a low noise level would be of benefit.  Receptors typically include homes, schools, hospitals, nursing homes, parks, motels, and may include business locations.

There are 10 inputs needed to estimate noise by model:

• Traffic Volumes

• Traffic Composition

• Traffic Speed

• Receptor Location and Elevation

• Roadway Layout

• Terrain Lines

• Ground Zones

• Building Rows

• Tree Zones

• Traffic Controls

Predicted noise levels for each receptor are compared to the Noise Abatement Criteria (NAC).  For example, (as shown on the chart below) the noise level at Receptor R1 in the Existing and Future Without Improvement conditions, meet the NAC.  The Future With Improvement would raise the noise level above the NAC.  Therefore, a noise abatement evaluation would be required. 

For Receptor R2 all noise levels are below the NAC of 67 dBA.  Therefore, no further evaluation is required.

In Part II, several aspects of noise analysis were reviewed including when a noise analysis is required, federal regulations regarding noise, how noise monitoring is conducted, and considerations and inputs into the computer noise model.

Part III focuses on Noise Abatement Evaluation.