Frequently Asked Questions - Noise Analysis

A noise analysis is required for state or federal highway construction or reconstruction projects that include:

            1.         A new roadway where one did not exist before, or

            2.         The physical alteration of existing highway alignment which:

–significantly changes the horizontal or vertical alignment, or

–increases the number of through-traffic lanes.

These projects are called Type I projects and generally include changes that have the potential to increase traffic noise.

Is wind/weather accounted for in measuring noise levels?

Weather conditions have some effect on noise measurement readings.  Noise measurements should not be taken if the wind speed exceeds 12 m.p.h.  A wind screen on the noise monitor is used at all times to reduce wind effects.  The conditions during monitoring are always recorded for comparison and review purposes.  In the computer traffic noise model, the default weather used for analyses are 50% relative humidity and 20°C (68°F).

Why don’t we use noise monitoring results instead of modeling results?

Monitored noise levels represent a snapshot of existing conditions.  This means the monitored noise levels reflect weather and traffic conditions for that time period only.  In addition, noise monitoring detects all noise sources present at the monitoring location, which may artificially inflate the traffic noise levels.

Noise levels for impact analysis are peak-hour conditions and may vary from the noise monitoring results.  The computer model is used to consistently predict future noise levels at peak traffic which is a worst-case condition. Traffic noise impacts are determined from future traffic conditions, which can only be predicted using the computer noise model.

Were noise levels predicted for my house?

The first step in conducting a noise analysis is to select representative receptors.  Each receptor represents an area called a noise sensitive area (NSA) with the following characteristics:

            a)  Similar land use

            b)  Similar distance to roadway

            c)  Same basic topography

If your house was not selected as the representative receptor, it was included in the NSA, and noise levels at your home can be expected to be similar to those predicted for the representative receptor.

Why doesn’t IDOT analyze noise for every house?

Every house in close proximity to the roadway is considered in the noise analysis, either directly or indirectly by representation in an area.  Noise receptors are used to represent an area with similar land use, proximity to roadway, and basic topography.  Predicting noise levels at every house is not necessary when similar characteristics would provide similar noise levels.  The selected representative receptor generally represents the worst-case (i.e. it is the closest to the roadway) of all receptors included in the area and noise levels can be expected to be similar for all receptors within the group.

What factors are used as inputs into the computer model to predict noise?

There are 10 inputs needed to estimate noise using the model:

• Traffic Volumes

• Traffic Composition (% of cars, med. trucks, heavy trucks)

• Traffic Speed

• Receptor Location and Elevation

• Roadway Layout

• Terrain Lines

• Ground Zones

• Building Rows

• Tree Zones

• Traffic Controls

Where did IDOT get the traffic data used in the computer model?

There are two types of traffic data that can be used in traffic noise modeling:

•    Peak Hourly Traffic

•   Average Daily Traffic (ADT) - The total traffic volume during a given period divided by the number of days in that period.  Current ADT volumes can be determined by either continuous or periodic traffic counts.

Existing volumes are typically generated from actual traffic counts.  Future volumes are typically projected by the highway department or a metropolitan planning organization,  based on typical traffic growth rates, planned development and projected growth for the area.

What about noise levels in my second floor bedroom?

Noise Abatement Criteria (NAC) are developed for activities occurring outdoors at ground level.  IDOT’s primary consideration is given to exterior areas where frequent activity occurs.  Any floor beyond the first floor is not considered in the noise analysis.  Noise abatement will only be designed to reduce noise levels on the first floor of a residence, apartment building, or other structure.

When does a traffic noise impact occur?

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

• Design year (typically projected 20 years into the future) traffic noise levels are predicted to approach, meet, or exceed the noise abatement criteria (NAC); or,
• Design year traffic noise levels are predicted to substantially increase (greater than 14 dBA) over existing traffic-generated noise levels.

How does IDOT address construction noise?

Construction noise is an inevitable result of project construction but IDOT considers ways to eliminate and/or minimize noise.  IDOT will evaluate construction noise to see:

• if there is sufficient need for recommending early construction of proposed noise barriers.
• if provisions should be made for any of the following special construction measures:

–  work hour limits

–  equipment muffler requirements

–  location of haul roads

–  elimination of “tail gate banging,” reduction of backing up for equipment with alarms

–  use of “sound curtains”

–  placing materials stockpiles to form temporary noise barriers

–  positioning equipment as far as practical from sensitive areas

• if the duration of contract period should be limited (calendar date of completion).
• if construction during special events, such as outdoor concerts and athletic events, should be limited.

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