For a noise barrier to be considered, many steps must be taken. First, we determine if a noise analysis is required, per the Federal Regulation 23 CFR 772. A noise analysis is required when an Iowa DOT highway project is proposed next to an area that may be sensitive to noise (e.g., residential neighborhood) and the project consists of a highway built on a new location, or an existing highway that is being significantly altered by substantially changing the horizontal or vertical characteristics of the road, or the number of through traffic lanes is being increased.
In this case the analysis will determine if a noise sensitive area is “impacted” by noise levels above Federal Highway Administration Noise Abatement Criteria. If the analysis shows that noise levels are above FHWA Noise Abatement Criteria, then we are required to investigate whether noise abatement (i.e. a noise barrier) is feasible and reasonable.
If a noise barrier is determined to be feasible and reasonable, then we will take a poll of homeowners in the area. A simple majority of the residents in the impacted area must indicate that they want the noise barrier for it to be constructed. Noise barriers will not be built if most affected residents do not want them or if there is a lack of interest.
All of this is part of the environmental work that is required for a highway project. The environmental work is summarized in the National Environmental Policy Act document, which includes a summary of the noise analysis.
The linked YouTube video below gives a big picture view of the approach the Iowa DOT takes for noise barriers:
A "noise sensitive receiver" (defined as homes, parks, schools, churches, etc.) is considered impacted by noise if either future noise levels (generally a 20-year projection) approach or exceed the FHWA Noise Abatement Criteria (66 decibel for residences, schools, churches, and parks), or if there is a substantial increase in predicted future noise levels over existing noise levels by 10 decibels pertaining to a proposed major highway project. These are the noise levels experienced at the exterior area of a property (e.g., patio). The LEQ (equivalent sound level) is essentially the average noise level over a period, usually one hour. The standards dictate that a substantial increase is defined as a predicted future noise level 10 decibels or more points over the existing noise level. Impacts such as these require abatement consideration and analysis, which will result in the construction of noise barriers if they are determined to be “feasible and reasonable.”
Feasibility and reasonableness are determined by criteria that are quantifiable, but flexible, and judgments for special and/or unusual circumstances are made on a case-by-case basis.
As a result, noise abatement is not automatically provided where noise impacts have been identified. A barrier is feasible if it can be constructed without major engineering or safety issues and provides a substantial noise reduction of at least five decibels for 50 percent of the impacted receivers.
Reasonableness deals with whether or not the barrier can be constructed in a cost-effective manner, the number of receivers benefitted from the noise barrier, whether or not the noise reduction design goal of 10 decibels is being achieved for at least one receiver, and the desires of the community.
Iowa DOT’s current noise policy is if the cost of a noise barrier is $49,000 per residence or less (based on historical data on 2017 dollars), the noise barrier is deemed cost reasonable. For example, if a noise barrier will benefit 10 residences and the total cost of the noise barrier is $490,000 or less (based on 2017 costs), the noise barrier is deemed to have a reasonable cost.
Additionally, other considerations are part of the determination as to the reasonableness of the noise barrier. Two specific questions are asked, “Was the noise sensitive-development constructed before or after the latest highway capacity or operational improvement (e.g., highway upgrade)? And, “How much noise was added by the highway project?
To accommodate the highway expansion and the resulting drainage needs, it is sometimes necessary to cut trees down to make room for the expansion, drainage and safety needs. As part of Iowa DOT’s beautification program, landscaping is incorporated into the project, which often includes tree replanting.
The amount of noise reduction by trees is usually negligible. An approximate 100 feet of depth of dense and tall trees is needed for trees to reduce noise such that a person can notice the difference. The height, depth, and density of trees needed is very difficult to obtain along the roadside; therefore, use of tree planting as a means of reducing noise is not typically feasible.
To minimize engine brake noise from semi-trucks, known as “jake-braking,” it is possible to have signs posted that prohibit their use in certain areas. The first step would be to have a local (city or county) ordinance in place that provides a statute for law enforcement to reference when writing citation. Once the ordinance is in place, the Iowa DOT places signs where the state highway intersects a city or county line. Signs are not placed at specific locations due to noise complaints.
Due to the nature of the decibel scale, a doubling of traffic will result in an approximately 3-decibel increase in noise level, which in and of itself would not normally be a perceivable noise increase. Traffic would need to increase at least three times to result in a readily perceivable (5-decibel) increase in noise. Using the same reasoning, if a highway is moved half as close to existing homes (i.e., from 200 to 100 feet), the noise level will increase by approximately 3 decibels. Conversely, if a highway is moved twice as far from existing homes, the noise level will decrease by 3 decibels. Noise level increases due to highway projects are usually due to a combination of increased traffic and changes in the roadway alignment.
It is very difficult to substantially reduce traffic noise at your residence. A fence that is long, tall, and dense enough may provide a noticeable decrease in the noise level, but traffic noise will still be heard. This is because traffic noise “bends” around the ends and over the tops of fences, even if the fencing material is dense enough to prevent transmission of sound through the fencing material. Earthen berms four to six feet tall are effective in reducing tire noise but are not very practical for the homeowner to construct.
There can be a substantial effect on traffic noise due to wind and temperature. Wind can essentially “blow” higher noise levels closer to residences. Additionally, drastic temperature changes from day to night can cause louder than normal sound levels. This temperature inversion causes sound to “bend” back toward the earth surface, causing loud sounds to be heard farther from the source than “normal.”
Noise barriers are expensive and with limited funding available the focus of Iowa DOT is keeping up with traffic demands, improving safety, and maintaining Iowa’s current transportation system. It is important to note that noise abatement in the form of barriers is marginally effective when the source to the receiver is a substantial distance from the highway. Noise abatement beyond 250 feet from the source has limited effectiveness. This is because sound “bends” around the top of the wall and around the ends of the wall, plus sound drops off at a rate of 4.5 – 6 decibels for every doubling of distance away from the source. The effect of these physical phenomena results in little realized noise reduction for residences not directly adjacent to the highway.
Rumble strips are a safety enhancement that are utilized throughout the United States to reduce the possibility lane departure and head on collisions. The frequency of driving over the rumble strips is higher in the evening, which can be particularly disturbing to nearby residents who are trying to sleep. Unfortunately, people are more sensitive to traffic noise during sleeping hours. While that noise can be disturbing to nearby residents, the life-saving benefit of having the rumble strips in place makes them beneficial to us all.