Noise FAQs

Table of Contents

A. General

• A1. How is Noise Defined?
• A2. How are noise level changes perceived?
• A3. How do changes in traffic or roadway geometry affect noise levels?
• A4. What constitutes a traffic noise impact?
• A5. Does EPA have standards which apply to highway noise?
• A6. Can anything be done about "Jake Brake" use?
• A7. What is the effect of pavement type on noise levels?

B. Noise Analysis

• B1. When is a noise analysis required?
• B2. Is a noise analysis required when the speed limit of a highway is changed?
• B3. Does CDOT analyze noise levels on existing highways?

C. Determination of Noise Levels

• C1. How was the selection of the noise levels in the Noise Abatement Criteria determined?

D. Noise Abatement for Highways

• D1. What is noise abatement?
• D2. What does CDOT consider "feasible and reasonable"?
• D3. What amount of noise reduction should be achieved with a noise wall or berm?
• D4. What are the most common types of noise abatement constructed?
• D5. How does noise abatement work?
• D6. Are noise walls or berms built to protect locations on the upper floors of homes?
• D7. How are noise reflections from buildings and walls considered?
• D8. Will planting vegetation help reduce noise levels?

E. Noise Abatement for Buildings

• E1. Does installing new windows and/or insulation reduce noise inside the building?
• E2. Does installing a noise wall or other structure on private property reduce noise?

A. General

A1. How is Noise Defined?

Noise, usually defined as unwanted or unacceptable sound, is measured in terms of decibels. A decibel is a unit of measurement that quantifies the sound pressure differences in the air that we perceive as sound (or noise) on a scale ranging from zero decibels on up. Zero decibels is the threshold of human hearing, 40 to 50 decibels is normal for a peaceful neighborhood, 70 to 80 decibels is the level adjacent to a busy urban street or 50 feet from a major freeway, and 120 to 140 decibels is a typical level at which sound is painful. For highway traffic noise studies, noise levels are quantified in terms of the equivalent sound level, or Leq. The Leq is essentially the average noise level over period of time, usually one hour. The table below shows a comparison of typical noise levels from common sources to give a sense of the possible levels of traffic noise.

Comparison of Typical Noise Levels from Common Sources

A2. How are noise level changes perceived?

Studies have shown that changes in noise levels of 3 decibels or less are not normally detectable by the average human ear. An increase of 5 decibels is generally readily noticeable by anyone, and a 10-decibel increase is usually felt to be "twice as loud" as before. This short video compares different levels of traffic noise based on decibels.

A3. How do changes in traffic or roadway geometry affect noise levels? 

Due to the nature of the decibel scale, a doubling of traffic will result in a 3-decibel increase in noise levels, which in and of itself would not normally be a perceptible noise increase. Traffic would need to increase at least three times to result in a readily perceptible (5 decibel) increase in noise. Using the same reasoning, if a highway is moved half as close to existing homes as it is now (i.e., from 200 to 100 feet), the noise levels will increase by 3 decibels. Conversely, if a highway is moved double the distance from existing homes, the noise levels 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.

A4.  What constitutes a traffic noise impact?

A "noise sensitive receptor" (e.g., homes, parks, schools, businesses,) is considered impacted by noise if either future noise levels (generally a 20 year projection) approach or exceed the CDOT Noise Abatement Criteria, or if there is a substantial increase in future noise levels over existing noise levels from a proposed CDOT project as described above. These are the noise levels experienced at the commonly used exterior portions of a property on the lowest or ground level for each home or individual unit. For example, for residences, schools, and parks, impact is defined when the Leq is 66 decibels or higher, and for noise sensitive businesses the impact Leq value is 71 decibels. A substantial increase impact occurs when there is a projected 10-decibel increase over existing noise levels. Impacts such as these require mitigation evaluation, which will result in the construction of noise walls or berms if they are determined to be feasible and reasonable.

A5: Does EPA have standards which apply to highway noise?

Not at this time. EPA does have recommended noise levels which are considered goals, but did not recommend those levels as strict standards applicable to highway projects due to factors including cost, engineering feasibility, and geographical characteristics

A6. Can anything be done about "Jake Brake" use?

All commercial vehicles operating on any public roadway in Colorado equipped with a compression or "Jake" brake device are required by law to have mufflers in accordance with Colorado Revised Statute 42-4-225. Failure to do so will result in a $500 fine. Enforcement of this law is the responsibility of local authorities and is typically accomplished through commercial vehicle inspections at port of entry and weigh station facilities. Signs stating "engine brake mufflers required" have been installed around the state to inform motor carriers of this requirement. The presence of these signs alone does not significantly reduce highway noise levels at specific locations.

However, even with proper use of mufflers, engine braking still produces a distinct sound.  Because they are a safety device, the use of engine brakes is not prohibited on state highways unless explicitly forbidden by local ordinance. Local law enforcement officials can provide additional information regarding local noise ordinances and their enforcement in the area.

A7. What is the effect of pavement type on noise levels?

Pavement type is often cited as a possible means to reduce highway traffic noise.  The majority of noise emitted from a highway is due to the tire-pavement interaction.  Research on this issue has been ongoing for the past 30 years. The effect of different pavements over long periods, 20 years or more, has still not been clearly established. Studies have indicated that open-graded asphalt pavements, when first placed, can produce a benefit of 2- to 5 dBA of noise level reduction. However, after 6 months to 2 years, aggregate becomes polished and voids in the pavement fill, so noise reduction benefits are lost. Concrete pavement, while perhaps louder than asphalt when it is initially placed, will become quieter over time. Longitudinal tining or diamond grinding of the concrete, where possible, does result in reduced noise levels compared to smooth concrete surfaces. Transverse tining, or tining of the concrete perpendicular to the direction of travel, creates an annoying high-pitched whine and should not be used. 

But pavement type cannot be used as a substitute for noise mitigation. CDOT's present policy for pavement type selection is made based on a life‐cycle cost analysis, which at this time does not consider noise as a primary factor. See CDOT’s brochure on the effect of pavement types on highway noise.

B. Noise Analysis

B1. When is a noise analysis required?

A noise analysis is required for a proposed CDOT project if that project consists of: A new highway built on a new location, interchange modifications, or an existing highway that is significantly altered by substantially changing the horizontal or vertical characteristics of the road, or the number of through traffic lanes being increased or auxiliary lanes added. More details of which type of projects require noise analysis are provided in 23 CFR 772.5 (see “Type I Project”). Noise analysis is also required if there is an addition of a new, or a substantial alteration of an existing, weigh station, rest stop, ride-share lot, or toll plaza. Minor projects, such as normal roadway resurfacings (without adding new lanes), do not require a noise analysis.

B2. Is a noise analysis required when the speed limit of a highway is changed?

No. Under the current regulations, a speed limit increase does not qualify as a project in which a noise analysis is required. CDOT does not have legal enforcement authority on the highways and cannot enforce lower speeds; enforcement of the traffic laws are the responsibility of local law enforcement.

B3. Does CDOT analyze noise levels on existing highways?

Yes, for certain types of highway projects as described in question B1, CDOT does perform noise studies, which may lead to mitigating noise for existing highways.

C. Determination of Noise Levels

C1. How was the selection of the noise levels in the Noise Abatement Criteria determined?

CDOT's selection of the noise abatement criteria levels were based on guidance from FHWA, and is consistent with the criteria used by all state DOT's. FHWA used numerous approaches in establishing the noise abatement criteria including hearing impairment, annoyance, sleep interference, and speech communication interference. The main challenge in establishing the criteria was to balance noise levels which are desirable with those that are achievable. As a result, speech impairment was usefully applied as the condition that best met that balance.

D. Noise Abatement for Highways

D1. What is Noise Abatement?

Noise barriers (in the form of walls or berms) are the most commonly used form of noise abatement and are the only form of noise abatement required for consideration on Federal or Federal-aid projects. Noise walls or berms are solid obstructions built between the roadway and the receivers along the roadway.

Noise insulation can be used as a mitigation measure but can only be considered by CDOT for public use or nonprofit institutional structures (e.g. places of worship, schools, hospitals, libraries). Noise insulation can greatly reduce highway traffic noise but can be costly.

D2. What does CDOT consider "feasible and reasonable"?

A noise wall or berm must be both feasible and reasonable if it is to be constructed with a highway project. Feasibility and reasonableness are determined by criteria that are quantifiable. As a result, noise mitigation is not automatically provided where noise impacts have been identified. A wall or berm is feasible if it can provide, at a height of 20 feet or less, a noise reduction of at least 5 dBA and if it can be constructed without major engineering or safety issues. Reasonableness deals with whether the wall or berm can be designed to achieve a noise reduction of at least 7 decibels, constructed in a cost-efficient manner, and if the community wants the wall or berm. All 3 of these criteria must be met for a wall or berm to be considered reasonable to build.

D3. What amount of noise reduction should be achieved with a noise wall or berm?

A noise wall or berm must provide at least a readily perceptible decrease in noise levels to adjacent receivers to be feasible. This is defined as a noise decrease of at least 5 decibels. As noise level changes of 3 decibels or less are not generally perceivable, it is not prudent to construct a noise wall or berm that gives only a 1 or 2 decibel benefit to adjacent properties.

D4. What are the most common types of noise abatement constructed?

Noise barriers are commonly constructed as walls, earthen berms, or a combination of the two. Walls are most common, and are usually constructed out of dense materials such as concrete or masonry block. Earth berms are a natural alternative to walls, but require much more land to construct. Walls can be constructed on top of berms in order to raise the overall height of the barrier.

D5. How does noise abatement work?

Noise walls or berms reduce noise by blocking the direct travel of sound waves from a source (such as a highway) to adjacent homes or businesses, forcing the waves over the top or around the wall or berm. The wall or berm must be high enough and long enough to block the view (line of sight) of the highway. This is the phenomenon that allows a noise barrier to provide a perceivable noise reduction. Noise walls or berms are not effective for homes on a hillside overlooking a road or for buildings which rise above the barrier. Openings or gaps in walls for driveway connections or street intersections reduce the effectiveness of the mitigation. Noise walls or berms are most effective for the first one or two rows of homes at distances up to 200 to 300 feet from the wall or berm. As noise levels decrease with distance, there is a point away from the highway at which noise mitigation is no longer effective. It is important to note that walls or berms are not designed to eliminate or block all noise.

D6. Are noise walls or berms built to protect locations on the upper floors of homes?

Noise walls or berms may, under certain geographic conditions, be able to be designed to protect upper levels of multi-family structures, where each unit is a separate residence. For single-family homes, the primary consideration is the outdoor, ground-floor areas of human activity. Walls or berms built for the second floor would have to be tall enough to provide a substantial noise reduction for those areas, and while CDOT can build walls up to 20 feet high, the safety of a wall is an important factor. If a wall would cause an excessive reduction in sight distance, this would be considered a fatal flaw. 20 feet is the maximum wall height allowed so as not to compromise structural integrity under typical construction design specifications.

D7. How are noise reflections from buildings and walls considered?

Highway traffic noise levels are not substantially increased by construction of a wall or berm or the presence of a building on the opposite side of a highway from sensitive properties. This is because the theoretical maximum noise increase from a source is limited to 3 decibels, which corresponds to a doubling of energy from the noise source. In practice, not all of the sound energy is reflected back to the receiver. Some of the sound is diffracted over the wall or berm, some is reflected to points other than the affected property, some is scattered and/or absorbed by ground coverings and other terrain, and some is blocked by the presence of other vehicles on the highway. The overall noise increase is normally limited to 1-2 decibels at the most. In general, this is not a perceptible increase, but the character of the noise may seem to change, which is what is usually noticed.

In the case of parallel walls, however, studies have shown that if two walls are constructed very close together, there is a potential for multiple reflections that may perceptibly increase noise levels. Generally, this is not normally a problem for walls greater than 200 feet apart or where the width-to-height ratio is more than 10:1 (walls 10 feet high should be at least 100 feet apart).

D8. Will planting vegetation help reduce noise levels?

Vegetation is not effective for reducing noise levels. It is not feasible to plant enough vegetation along a highway to achieve reduction, although planting trees or shrubs can provide aesthetic benefit and visual screening. Studies have shown that vegetation must be at least 100 feet thick, at least 20 feet high, and dense enough (100% opacity year round) to provide a 5 dBA noise reduction.

E. Noise Abatement for Buildings

E1. Does installing new windows and/or insulation reduce noise inside the building?

Installing new windows with thicker glass and/or double panes will help reduce noise inside a building. For new windows, increased airspace width between the glass panes, increased glass thickness, proper use of sealing, slightly dissimilar thickness of the panes and slightly non-parallel panes increases their performance.

E2. Does installing a noise wall or other structure on private property reduce noise?

Installing a wall or other structure (e.g., large shed, garage) on private property can reduce noise, depending on the materials used (for walls) and where the structure is placed; it must be placed between the noise source and the area where noise should be reduced. A fence can help reduce noise, but only if it is solid (e.g., no gaps) and dense. The more dense the wall or fence (e.g., stone, brick, concrete), the more noise will be reduced, because sound waves are reflected by dense objects. A wall or fence that does not reach to the ground will allow noise to go under. Likewise, a low structure will allow more sound waves to go over the top. If the noise source can be seen, it will be heard.