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WESTERN WAKE FREEWAY TRAFFIC NOISE REPORT

TRAFFIC NOISE REPORT (Due to the graphics and maps included in this report, this file is over 20mb and may take a few minutes to load.)

Below is an excert from Section 2.0 of Traffic Noise Report prepared in June of 2007 for the Western Wake Freeway project. To view the report in its entirety, click on the link above. 

2.0 Characteristics of Noise
Noise is defined as unwanted sound. It is emitted from many sources, including airplanes, factories, railroads, power-generating plants, and highway vehicles. Highway noise, or traffic noise, is a composite of noises from engine exhaust, drive train, and tire-roadway interaction. The magnitude of noise is described by its sound pressure. A logarithmic scale is used to relate sound pressures to a standard reference level. Sound pressure levels are often defined in terms of frequency-weighted scales (A, B, C, or D). The weighted-A scale is used on environmental studies because it best approximates the frequency response of the human ear. Sound levels measured using A-weighting are expressed as dBA. Throughout this report, references are made to dBA, which means an Aweighted decibel level.
 
Since sound pressure is continuously varying, the equivalent noise level, Leq, is used. The Leq is the equivalent steady-state sound level, which in a defined period of time contains the same amount of acoustic energy as a time-varying sound level during the same period of time. In other words, the fluctuating sound levels of traffic noise are represented in terms of a steady noise level with the same energy content. The Leq is an energy summation integration and, as such, it can adequately consider single-event noises and does not rely on statistical parameters. This report utilizes the Leq noise descriptor.

PROPOSED NOISE WALLS FOR WESTERN WAKE FREEWAY.

NOISE WALLS

Anytime there is a change in your surroundings, noise levels change. A new or expanded road will change the noise level with increased traffic. There are laws that define how much “new noise” is allowable and these laws determine how to evaluate the potential “new noise” and what steps can be taken to limit this. These laws also define when federal tax dollars can’t be spent to limit “new noise”. NCDOT must follow all these laws.

When are Noise Walls Considered?
Whenever a highway project uses state or federal funds, the potential for increase noise – and how to decrease it – must be evaluated.

Potential noise increases are evaluated for any building built before the “Date of Public Knowledge”.

What is “Date of Public Knowledge”?
This is the date that the public (and local government) is notified of future path of the road.
If a road project was approved BEFORE September 2004 – this date is based on either the approval of the Final Environmental Document or the Design Public Hearing (whichever is later).
If a road was approved AFTER September 2004 – this date is the same as the Final Environmental Document.

How does NCDOT decide which communities get noise walls and which do not?

Evaluation:
NCDOT experts go to homes, churches, businesses, etc. (built before the date of public knowledge) and use special equipment to monitor noise.
Using complex computer modeling, they determine how much MORE noise will reach these locations once the road is built and traffic increases.
If the anticipated noise increase is MORE than the level defined by Federal guidelines, NCDOT begins to consider how to decrease the noise.

Considerations:
Several very important questions must then be answered.
Will a noise wall decrease the noise? Sometimes, a noise wall will not decrease the noise.
Is a noise wall technically realistic? Every road is different – is the geography conducive to building a wall?
How many people will hear a difference in noise? Is that number high enough to justify the cost? Sometimes, the state just can’t afford to build a wall because the cost vs. benefit is too high.
Do more than 50% of property owners want (and vote for) a noise wall?

Noise Decrease Measures:
There are many ways to decrease noise. Noise walls are one tool, but berms, road design and the type of materials used to build the road can help decrease noise as well.

Sometimes, noise walls will NOT decrease the noise because of the location of the road. Each new road must be examined individually to determine what measures can be taken.

What about buildings built AFTER the “Date of Public Knowledge”?
Federal law will not allow federal funds to be used to build noise walls for any buildings built AFTER the date of public knowledge.

In these situations, NCDOT will work with local governments if they want to build a noise wall to ensure a noise wall meets all the federal and state standards.

When do noise walls work?
Sound travels very much like water or light. Sound travels the easiest path over, under, and around things in its path. The further away from the source of the sound, the lower the noise.

Noise walls do not work if:
- The source of the noise can be seen. The sound will simply travel through that opening. Much like water will flow through a crack in a damn.
- If a building is located higher than a noise wall, the sound will flow over the wall to the building.

How can engineers estimate traffic noise?
The type of vehicle and the speed play a significant role in estimating sound. Large trucks create more sound than cars. The faster the speed on the road, the louder the sound.

For example:
- Traffic at 65 mph is TWICE as loud as traffic 30 mph
- One truck at 55mph creates the same noise as 28 cars traveling 55 mph
- 2000 vehicles an hour is twice as loud as 200 vehicles per hour

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Noise Reduction Measures | Noise Studies | Noise Walls / Bridge Aesthetics