Sound Barriers Guidelines - Highway Traffic Noise

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Sound is created when an object moves. This movement causes vibrations or waves in air molecules, like ripples of water. When the vibrations reach our ears, we hear sound.

Noise is unwanted sound. It is a pollutant and a hazard to human health and hearing. Noise levels are measured in decibels. The decibel (dB) is a unit which expresses the ratio of the sound pressure level being measured to a standard reference level. The higher the decibel level, the louder the noise. Sounds louder than 80 decibels are considered potentially harmful to the human ear. The noise chart below gives an idea of average decibel levels for everyday sounds around you.


Outdoor Noise Sources
Noise Level(Decibels)
Indoor Noise Sources
Rock Band
Jet Flyover at 1,000 Feet
Inside Subway Train (NY)
Gas Lawn Mower at 3 Feet
Diesel Truck at 50 Feet
Food Blender at 3 Feet
Noisy Urban Daytime
Garbage Disposal at 3 Feet, Shouting at 3 Feet
Gas Lawn Mower at 100 Feet
Vacuum Cleaner at 10 Feet
Commercial Area
Normal Speech at 3 Feet
Large Business Office
Quiet Urban Daytime
Dishwasher, Next Room
Quiet Urban Nighttime
Small Theater, Large Conference Room (Background)
Quiet Suburban Nighttime
Quiet Rural Nighttime
Bedroom at Night, Concert Hall (Background)
Broadcast and Recording Studio
Threshold of hearing

Adapted from Guide on Evaluation and Attenuation of Traffic Noise. AASHTO. 1974.


Highway traffic noise has been of increasing concern to both the public and the government over the years. Fortunately, modern acoustical technology has been providing better ways to lessen the adverse impacts of highway traffic noise.


The following affect highway traffic noise:

  1. Traffic Volume
    Highway Vehicles
    2000 vehicles per hour sounds twice as loud as
    Highway Vehicles
    200 vehicles per hour.

  2. Vehicle Speed
    Highway Vehicles
    Traffic at 65 miles per hour sounds twice as loud as

    Highway Vehicles
    traffic at 30 miles per hour.

  3. Trucks
    Highway Vehicles
    One truck at 55 miles per hour sounds as loud as
    Highway Vehicles
    Highway Vehicles
    10 cars at 55 miles per hour.

Please note the following:
  • The loudness of traffic noise is generally increased by a closer distance to the highway, heavier traffic volumes, higher speeds, and greater numbers of trucks.

  • Vehicle noise is a combination of the noise from the engine, exhaust, and tires.

  • Defective mufflers or other faulty vehicle parts can also increase the loudness of traffic noise.

  • Any condition such as a steep incline that causes heavy laboring of motor vehicle engines will also increase traffic noise levels.

Source: FHWA Website on Highway Traffic Noise

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In general, an increase in traffic volume will cause increased noise levels. However, the amount of increase in noise will depend on the relative increase in traffic volume as shown in the chart below:

Graph showing Effect of Traffic Volume Changes on Noise

  • The average person can just begin to perceive a change in noise level when there is an increase of at least 3 decibels.

  • In each instance in the chart above, it is shown that for an increase of 3 dBA in the overall noise level to occur, the number of vehicles must be doubled (that is increased by 100%).

  • This doubling must occur regardless of the actual volume of traffic. For example, if a highway carrying 5,000 vehicles per hour produces an overall noise level of 80 dBA, the traffic volume must increase to 10,000 vehicles per hour for the overall noise level to increase by 3 to 83 dBA.

In conclusion, as the total number of vehicles on a roadway increases, it requires more and more additional vehicles to cause a noticeable change in the overall noise level. This condition is often manifested in the results of noise impact studies for projects to widen existing highways, especially those with already substantial traffic volumes.


Source: SHA Travel Forecasting

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The State Highway Administration (SHA) aims to help reduce noise levels for communities through:
  1. Land Use Control

    Sometimes, concerns about highway traffic noise come from occupants of new homes built adjacent to an existing highway. Many of these highways were originally constructed through undeveloped lands. Prudent land use control can help to prevent many future traffic noise problems in these areas.

    The SHA will work with local jurisdiction throughout Maryland to develop policies to regulate land use planning and control through legislative statutes that control the building of noise sensitive receptors like homes, offices, and churches adjacent to existing highways.

  2. Highway Planning and Design

    Early in the planning stages of most highway improvements, the SHA performs a noise study. The purpose of this study is to determine if the project will create any noise problems. First, the existing noise levels of a highway are measured or computed by models. Then, there is a prediction of future noise levels if the project is constructed. If the predicted noise levels are above State noise criteria, the State considers measures that can be taken to lessen these adverse noise impacts.

    Most times, sound barriers are used as a noise abatement measure.

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A sound barrier is a solid obstruction built between the highway and the homes along the highway to help reduce the overall loudness of highway traffic noise. These barriers can be constructed from earth mounds or berms, concrete, masonry, wood, metal, and other materials. The material used is based on aesthetics, durability, maintenance, cost, and the desires of the public.

Sound barriers:
  • considerably reduce noise levels for people living next to highways.
  • can reduce the loudness of traffic by as much as half.
  • do not totally block all traffic noise.
  • are most effective within 200 feet of a highway.
  • can be effective, regardless of the material used.


The Maryland Department of Transportation, State Highway Administration (SHA) Sound Barrier Policy provides guidance for the evaluation of traffic noise effects and noise abatement opportunities for communities adjacent to state highways. This policy was adopted with the approval of the Federal Highway Administration (FHWA) as meeting the intent of the provisions of the Federal Regulations (23 CFR 772) which require states to adopt their own policies on the abatement of highway traffic noise. The Maryland State Highway Administration’s policy is applicable to projects funded with Federal and/or State funds, and projects funded by third parties intended to be accepted by the State for maintenance and functional or aesthetic enhancements.

In the state of Maryland, sound barriers are evaluated in two separate categories:
  • Type I Program

    Sound barrier construction is considered when a new highway is being constructed or an existing state-maintained highway is expanded.

  • Type II Program

    This is a voluntary program under which sound barrier construction is considered for existing highways not being expanded where the majority of the impacted development was built prior to the original construction, or approval of the highway.

    A program to implement Type II projects is an optional decision by the State, as the development and implementation of Type II projects are not mandatory requirements of Federal law or regulation.

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Sound barriers reduce the noise which enters a community from a highway by absorbing, reflecting, or forcing the sound to take a longer path over and around the barrier. Sound is energy that decreases in intensity as it travels away from its point of origin. Sound waves travel out in all directions from vehicles on the roadways. When the sound waves encounter an obstacle or barrier, some of the sound will bounce off the barrier's surface (reflective surface). If the surface is porous, where there are small cavities or holes that extend into the interior of the barrier material, a portion of the sound waves will travel inside the cavities when they reach the surface. The waves will bounce around and eventually expend all their energy. This process is called absorption.

Reflective Wall Absorptive Wall

A noise barrier can achieve a 5 decibel (dBA) noise level reduction, when it is tall enough to break the line-of-sight from the highway to the home or receiver. After it breaks the line-of-sight, it can achieve approximately 1.5 decibels of additional noise level reduction for each meter (appr. 3 feet) of barrier height. To effectively reduce the noise coming around its ends, a barrier should be at least eight times as long as the distance from the home or receiver to the barrier.

Sound Barrier Noise Reduction

Source: FHWA Website

Maryland SHA determines the height of proposed barriers using acoustic profiles (noise measurement equipment is used to collect noise data in such a way that it can be displayed, mapped, and then worked with) so that a 7 to 10 decibel noise level reduction can be achieved. There are no standard barrier heights since each project site has different topography that needs to be accounted for.

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Sound barriers do have limitations and are not always an effective noise abatement method. To determine if the construction of a sound barrier will provide a reasonable noise level reduction, both the distance of the impacted community from the roadway and the topography of the area are considered.

Role of Distance

Typically, the primary impacted residences (which we also refer to as first row residences) are within 100 to 200 feet of the roadway. Second row residences, which are also quite often impacted, are usually a next door neighbor or located across the street from a first row residence. As noise impacts and potential noise abatement methods are evaluated past second row residences, it becomes increasingly difficult to provide effective abatement. The construction of a sound barrier is not likely to reduce noise levels for residents who live far away from the roadway.

Role of Topography

To work effectively, the barrier must be high enough and long enough to block the view of the road from the area that is to be protected. Sound barriers do very little for homes on a hillside overlooking a road.

Shadow Effect 

Source: FHWA Website

As seen above, the house at the bottom of the hill is protected by the sound barrier, but the one on top of the hill (overlooking the roadway) is not.

In addition, buildings higher than barriers, homes scattered too far apart, and openings in noise barriers for driveway connections or intersecting streets are not good areas for sound barriers. In some cases, SHA can offer alternatives to help reduce noise levels. These alternatives are evaluated on a case-by-case basis consistent with Federal guidelines.


Please go over the information for our Type I and Type II programs, to learn more about our policy on noise abatement. To submit a highway traffic noise inquiry, please complete the online form. Your inquiry will be investigated and you will receive a response.

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For more information
Additional questions? Contact the Office of Highway Development(OHD) at 1-888-228-5003.