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Traffic Noise

What is the difference between sound and noise?

Sound is anything we hear.  Noise is unwanted or undesirable sound. But there is more to it than that...

Sound is created when an object moves and anything that causes small, fast pressure changes will create sound.  For example, we hear a rustle from wind blowing leaves, we hear words when air passes over vocal cords, and we hear music when an electronic stereo signal shakes the cone in a speaker. Movement causes vibrations in air molecules, like ripples on water. When the vibrations reach our ears, we hear sound.

Sound is measured in decibels (dB). To better represent human hearing, very high and very low-pitched sounds are adjusted, or "A-weighted" (dBA).  Traffic noise is reported as dBA.

10 dBA + 10 dBA = 13 dBA.  There are two important things to know about sound energy and traffic noise.  

  1. Sound is measured logrithmically, so 60 dBA + 60 dBA = 63 dBA, not 120 dBA.  In contrast, 60 dBA + 50 dBA = 60 dBA.
  2. The perception of how "loud" a sound is varies by person.  Generally, 3 dBA is considered the minimum audible difference between sound levels.  A 10 dBA change is generally perceived to be 2x or 1/2 as loud depending of whether the sound is increasing or decreasing.

What causes traffic noise?

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, vegetation, and natural and manmade obstacles as a person moves away from a 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.

How are noise impacts determined?

Highway traffic noise is not constant. Noise levels change with the number, type, and speed of the vehicles. For example, traffic noise levels might be lower during rush hour when traffic speeds are reduced compared to times when fewer vehicles are traveling at a higher speed.  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 land uses.

What can be done to reduce traffic noise?

A three-part strategy is used to reduce traffic noise.  Responsibility for the different strategies occur at different levels of government, as described below:

  1. Motor vehicle controls, including tire tread designs - federal government
  2. Highway planning and design - federal, state, and local governments
  3. Land use controls - local governments

1. Motor vehicle controls

Vehicles are the source of traffic noise. There are a number of ways to reduce vehicle noise.  For example, vehicles can be designed with enclosures for the engine, fans that turn off when not needed, and better mufflers. Quieter vehicles would bring about a substantial reduction in traffic noise along roads where no other noise reducing measures are possible. The EPA has issued regulations placing a limit on the noise that new trucks can make. In addition, many local and State governments have passed ordinances or laws requiring existing vehicles to be properly maintained and operated.

Due to limitations in technology, EPA noise regulations for new trucks and State and local regulations for maintenance of vehicles can only partially reduce traffic noise. A 5 to 10 dBA decrease is about the best noise reduction that can be expected from EPA regulations. 

2. Highway Planning and Design

Early in the planning stages of most highway improvements, highway agencies do a noise study.  

  1. For the noise study, the current traffic noise levels are measured and compared to a traffic noise model (TNM) to ensure the accuracy of the model (validate).
  2. Then, the agency used the validated model to predict what the noise levels will be in the future if the project is constructed.
  3. If noise "impacts" (per FHWA criteria) are predicted,  the noise study must evaluate whether noise levels can be reduced in a way that is "feasible" (physically constructible, achieve minimum noise reductions) and "reasonable" (cost-effective).  Feasible and reasonable are defined in the 2011 WSDOT Noise Policy and Procedures (pdf 886 kb).
  4. If noise abatement is recommend, opinions of the affected public are solicited to ensure that abatement is desired.  WSDOT will not construct noise abatement (e.g., noise walls), if they are not desired by the affected residents.  This process is described in Chapter 10 - Public Involvement in WSDOT noise policy.

3. Land Use Control

Sometimes, complaints about highway traffic come from occupants of new homes built adjacent to an existing highway. Many highways were originally constructed through undeveloped lands. There are several hundred thousand miles of existing highways in this country bordered by vacant land which may some day be developed. Prudent land use control can help to prevent many future traffic noise problems in these areas. Such controls need not prohibit development, but rather can require reasonable distances, or "buffers," between noise sensitive buildings and roads.  Soundproofing or other abatement measures can also lessen noise disturbances. WSDOT can work with local governments on this type of "noise compatible planning."

How does WSDOT work to reduce noise on existing roads?

Some noise reduction measures that are possible on existing roads, or roads being rebuilt, include buffer zones, constructing noise walls or earthen berms, installing noise insulation in buildings, and managing traffic.

Buffer zones are undeveloped open spaces which border a highway.  WSDOT can purchase land, or development rights to prevent future dwellings from being constructed near the highway where traffic noise impacts may occur in the future.  Buffer zones can also improve the roadside appearance. Buffer are used infrequently because of the large amount of land that must be purchased to effective and dwellings already border many existing roads.

WSDOT can use federal funds to purchase buffer zone for noise abatement.

Noise barriers are walls or earthen berms constructed between homes and a roadway to reduce traffic noise levels.  Effective noise barriers can reduce noise levels by 10 to 15 decibels, cutting the loudness of traffic noise in half. Earth berms have a natural appearance but can require quite a lot of land if it is very high. Walls take less space and are usually limited to 25 feet tall for structural and aesthetic reasons. Noise walls can be built from wood, stucco, concrete, masonry, metal, and other materials.

Barriers do have limitations. For a noise barrier to work, it must be high enough and long enough to block the view of a 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.

Noise barriers are the most common form of traffic noise abatement used by WSDOT.

Vegetation can decrease highway traffic noise levels if it is high enough, wide enough, and dense enough (cannot be seen through). A general rule is that it takes about 200-feet of dense vegetation to audibly reduce traffic noise. It is often impractical to plant this much vegetation but if dense vegetation already exists, it could be saved. Roadside vegetation can provide psychological relief from traffic noise, even when it does not actually reduce traffic noise levels.

Vegetation is not used as abatement by WSDOT because it is not approved for this use by FHWA.

Insulating buildings can reduce highway traffic noise, especially when windows are sealed and cracks and other openings are filled. 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. 

WSDOT considers interior noise reductions for certain types of structures defined as Category D in the 2011 WSDOT noise policy.

Some types of traffic management can reduce noise levels. For example, trucks can be prohibited from certain streets and roads, or they can be permitted to use certain streets and roads only during daylight hours. Traffic lights can be changed to smooth out the flow of traffic and to eliminate the need for frequent stops and starts. Speed limits can be reduced.  A 10 mile-per-hour reduction in speed is usually necessary for a noticeable decrease in noise levels.

Traffic management is an FHWA-approved form of traffic noise abatement that WSDOT has used to reduce traffic noise levels.

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 pavements 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.

How does WSDOT work to reduce noise on new roads?

All of the measures used on existing roads can also be used on new roads. There are also additional measures which may be possible to use for new roads only.

  • Locate road in undeveloped areas or away from noise-sensitive areas, such as schools or hospitals, and placed near nonsensitive areas, such as businesses or industrial plants.
  • Construct new roads below ground level. The noise from vehicles on this type of road is deflected into the air by embankments on the side of the road. These embankments function in much the same way as noise barriers.
  • Design roads to be as level as possible. The elimination of steep inclines helps to reduce traffic noise because motor vehicle engines, especially multi-geared truck engines, do not have to work as hard.  Avoiding steep declines will reduce noise from braking, especially the use of truck compression brakes.

All of the many noise reduction measures have limitations.  In some case, none of these noise reduction measures can be used either because of physical or cost constraints.

What is the role of Federal Highway Administration (FHWA) for traffic noise?

FHWA administers the Federal-aid highway program that allocates federal funds to the individual States for highway improvement projects.  The projects must be approved by FHWA, which requires compliance with Federal statutes and regulations. One of these regulations (23 CFR 772) requires that a noise study are done to determine if noise impacts will result from the proposed project and what (if any) noise reductions measures will be pursued.

If the State highway agency and FHWA determine abatement to be practicable, reasonable, and acceptable to the public, it must be incorporated into the project. The costs of the noise-reduction measures are included with the other costs of the project and are eligible for Federal funding in the same proportion.

State highway agencies may also use Federal highway grants for noise-reduction projects on existing roads on the Federal-aid system.  These are called Type 2, or "retrofit," projects.  Federal money spent on retrofits are deducted from funds which would otherwise be available for highway construction.

Federal funds may be used to construct noise barriers, for land acquisition to build, and to purchase undeveloped land as a preemptive buffer zone. Traffic operational measures, such as truck routes and restriction of hours of operation, and speed limit adjustments, are also eligible for Federal funding.  Other FHWA-approved (and FHWA funding eligible) methods of traffic noise abatement are described in the 2011 WSDOT Noise Policy and Procedures (pdf 886 kb).