Q. What causes traffic noise?
A. The level of highway traffic noise depends on three things:
1. Traffic volumes - Roads with more vehicles are generally louder.
2. Traffic speeds - traffic is louder at higher speeds.
3. Percent of heavy trucks on the road -- Heavy trucks (e.g. semi-trucks).
Traffic noise is a combination of the noises produced by vehicle engines, exhaust, and tires. Traffic noise is also increased by defective mufflers or other faulty equipment. Conditions, like a steep incline, that causes heavy laboring of vehicle engines will also increase traffic noise levels. Other factors also complicate the loudness of traffic noise. For example, traffic noise levels are reduced by distance, terrain, dense vegetation, and natural and manmade obstacles as a person moves away from the highway.
Traffic noise is not usually a serious problem for people who live more than 500 feet from heavily traveled freeways or more than 100-200 feet from lightly traveled roads.
Q. Why does WSDOT build noise barriers?
A. In 1976, the U.S. Congress passed legislation requiring the states to provide abatement 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. Noise barriers are free standing barriers built parallel to a highway. They are usually made of concrete and are found near public areas (such as parks) and residential homes. The walls range in height from 6 to 20 feet, but normally they are 12 to 15 feet tall.
Most noise barriers are included as part of large construction projects that add new lanes (thus adding vehicle capacity) to highways, shift the highway closer to residents, build a new highway , or alter roadside topography. Long before construction begins, acoustical engineers evaluate sources and patterns of noise in neighborhoods near the project limits. The findings are used to determine if noise walls would be appropriate and cost-effective.
Noise evaluations take into account many factors, only one of which is actual highway noise. Among other things, acoustical engineers look at
- Area topography
- Population density
- Expected levels of noise reduction a wall would provide
If, for example homes near a project are widely-spaced or built high on a hill, we often do not build noise barriers because the:
- Cost the cost to reduce noise for each resident is usually quite high.
- Noise does not noticeable decrease.
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.
Sometimes we build noise walls in high-noise neighborhoods that were built before the freeway. These walls, known as "retrofit" or Type II walls. Type II walls are rare because funds for these walls compete with other important programs, like safety improvements and pedestrian accommodations. To be fair to everyone, retrofit noise walls are ranked and built according to a statewide priority list. We build, on average, one retrofit wall every two years. Even if your neighborhood qualifies for one, it may be several years before it is actually built.
Q. What is an impacted location?
A. Any sensitive outdoor human use activity area that is predicted to have a future design year traffic noise level of 66 decibels (A-weighted for human hearing) or greater.
To be conservative in our estimates of traffic noise levels, WSDOT typically models traffic noise levels with rush hour traffic traveling at the posted speed limit.
The most common statistical descriptor for traffic noise is Leq. Leq is a time-weighted average. A 1-hour Leq is usually used by the Federal Highway Administration (FHWA) to determine traffic noise impacts. FHWA has established different noise impact criteria for different land uses. In Washington State, a 66 dBA (1-hr Leq) is considered the threshold for traffic noise impacts to residential homes.
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 noise impacts are then considered for noise abatement. 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 abatement and at least 7 decibel reduction at one residence. Feasible refers to whether the barrier can provide a substantial (at least five 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.
Barriers do have limitations. For a noise barrier to work, it must be high enough and long enough to block the view of the road. Noise barriers do very little good for homes on a hillside overlooking a road or for buildings which rise above the barrier. Openings in noise walls for driveway connections or intersecting streets destroy the effectiveness of barriers. In some areas, homes are scattered too far apart to permit noise barriers to be built at a reasonable cost.
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.
Vegetation can decrease highway traffic noise levels, if it is high enough, wide enough, and dense enough (cannot be seen through). It is often impractical to plant this much vegetation but if dense vegetation already exists, it could be saved. Vegetation is not used as abatement by WSDOT because it is not approved for this use by FHWA.
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. Air conditioning is usually necessary once the windows are sealed, because open windows allow sound to enter the building. Sometimes, noise-absorbing material can be placed in the walls of new buildings during construction, but this acoustic insulation is very expensive. Unfortunately, WSDOT does not have funds available for such a program. WSDOT considers interior noise reductions for certain types of structures defined as Category D in the 2011 WSDOT noise policy.
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.
Tips to Lower Noise around Your Home:
- To reduce noise levels inside your home, consider reinsulating walls and ceilings and replacing single pane windows with insulated double-pane windows. Less expensive change like sealing door and window cracks can also noticeably reduce noise levels. Use indoor fans, rather than open windows to provide ventilation around the home and to reduce unwanted noise.
- Outside, use interesting landscaping to obscure the roadway. Even though plants do not effectively reduce noise levels, they give a sense of privacy and serenity. Some plants help mask traffic noise by rustling in the wind. Talk to a landscape architect for ideas.
- Enclose a favorite garden spot, deck or patio with transparent plastic or other barrier. Or, create a quiet spot in an area facing away from traffic.
- Consider building a "do-it-yourself" noise barrier. If it is properly built with appropriate materials, you can get significant noise reductions around your home. Talk to an acoustical consultant first.
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.
Q. Does WSDOT consider 'quieter pavement' as a noise abatement?
A. Pavement can affect traffic noise. Noise levels vary with changes in pavements. For example, a new asphalt pavement is generally quieter than asphalt that is 10 years old. this is especially true in Washington State where tire studs and chains damage roadways and make pavements louder at a faster rate than pavements in other states.
FHWA has strict requirements about using different pavement types as traffic noise abatement and Washington State is not allowed to consider pavement design as abatement. However, WSDOT has some of the most comprehensive data in the world on quieter pavement performance and we continue to explore new "quieter" pavement designs.