Q. Why does WSDOT build noise barriers?
A. In 1976, the U.S. Congress passed legislation requiring the states to provide mitigation for highway noise (considered an environmental impact) as a part of all Type I Federal Aid projects at impacted locations where it is reasonable and feasible.
Q. What is a Type I project?
A. A proposed project for the construction of a highway at a new location or the physical alteration of an existing highway that significantly changes either the horizontal or the vertical alignment or increases the number of traffic through-lanes.
Q. What is an impacted location?
A. Any sensitive outdoor human use activity area that is predicted to have a design year traffic noise level of 66 decibels (A-weighted for human hearing) or greater.
Q. Why was the impact level set at 66 dBA?
A. 66 dBA was chosen as the impact threshold because researchers have shown that above this level conversation between two people standing three feet apart and speaking in a normal voice is impaired.
Q. How does WSDOT determine where to place noise barriers?
A. Computerized noise models are developed for all Type I projects to predict design year (usually twenty years in the future) traffic noise levels. All locations that are predicted to have impacts are then considered for noise mitigation. Analysts make every attempt to qualify these impacted locations for noise barriers based on the reasonable and feasible criteria.
Q. What does reasonable and feasible mean?
A. Reasonable refers to the maximum barrier square footage or cost per residence benefiting from the noise mitigation. Feasible refers to whether the barrier can provide a substantial (at least seven decibels) reduction in noise and other constructability issues.
Q. How is the height of the barrier determined?
A. Noise walls are designed to provide a minimum of five decibels of reduction in average background traffic noise for the majority of first row of residences located directly behind the wall, with a 7 decibel reduction at least one location. The design goal for a sound barrier to a 10 decibel reduction. The reasonableness criteria places a practical limitation on the height of any noise barrier.
Q. Why not plant trees instead of putting up a wall?
A. Trees provide a visual shield and some psychological benefit, but are not nearly as effective at reducing noise levels as a solid barrier. It would take at least 100 feet of dense vegetation to provide the same acoustical benefit as our smallest feasible noise wall.
Q. How effective are noise walls?
A. This depends on the distance between the listener and the wall. For residences located directly behind the wall, the perceived noise level will be cut in half. This benefit decreases as a listener moves farther away from the wall and is negligible at distances greater than 500 feet.
Q. How much do noise walls cost?
A. Current construction costs are averaging $53 per square foot. This translates into a fourteen-foot high wall costing about 3.9 million dollars per mile. Construction costs for rural barriers may be lower and urban barriers may be much higher. The higher urban costs are associated with the existence of other infrastructure (like retaining wall, water pipes, etc.) that may need to be retrofitted or moved to allow the placement of barrier.
Q. Does WSDOT have a program to provide noise insulation of private residences?
A. Installation of double pane windows, hanging of heavy draperies and sealing of cracks can result in a noticeable reduction in traffic noise inside the home and reduce heating costs as well. Unfortunately, WSDOT does not have funds available for such a program.
Q. Who pays for noise barriers?
A. Many noise walls are built alongside interstate highways, in which case the federal government pays a partial amount, and the state of Washington pays the remainder. The cost of walls built on other state routes is paid entirely by the state of Washington or by local jurisdictions depending on who is sponsoring the project. In special cases, if a local community would like to enhance the barrier with aesthetic treatments or to make the barrier longer or taller than recommended, the community may provide additional funding if the proposed improvements meet WSDOT safety, maintenance and future right of way needs.
Q. What types of barriers are there?
A. Noise barriers materials used by WSDOT include earth, concrete, wood, and masonry block. Earthen berms work the best and are the least expensive, but a lack of available right of way usually makes concrete walls the most practical solution.
WSDOT also uses temporary noise shields made from wood and closed cell foam, or other materials, to reduce noise from some types of construction equipment.
Q. Does the public have any input?
A. The WSDOT design office will work closely with the impacted community to ensure that reasonable requests regarding the design of the wall are included in the project plans.
Q. Can I build my own noise wall?
A. Many residents elect to fashion their own noise barrier by constructing a wooden fence at the property line. To prevent a substantial amount of noise transmission, whatever material is used must weigh at least 4 pounds per square foot and have no gaps. Blocking the line of sight to the noise source will usually result in a 5-decibel reduction. Each additional foot in height of the fence will likely reduce the noise by an additional half decibel, but there may be a point where there are diminishing returns of noise reduction when the fence is really high. The fence must either be long enough to prevent noise from coming around the ends, or it must continue down the property line enclose the area to be protected.
Q. What is noise masking?
A. Many people have reported some success with a technique called noise masking. Noise masking involves the use of white noise, such as a fan or waterfall, to drown out the traffic noise and make it less noticeable. While this technique may be appealing for some individuals, it is not a practical mitigation tool on a community scale.
Additional Questions? Contact the Washington State Department of Transportation Acoustics Program at (206) 440-4000.