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Noise Analysis

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Noise analysis FAQs

 

 

 

 

Why does MnDOT build noise barriers?

Noise barriers generally prove to be quite effective in providing noise reduction at receptors near transportation facilities.


In 1972, the U.S. Congress passed legislation requiring the states to provide mitigation for highway noise (considered an environmental impact) at impacted locations where it was found to be reasonable and feasible as a part of all Type I Federal Aid projects. 

 

MnDOT Metro District also has a Highway Noise Abatement Program.  This program is entirely state funded (without Federal-aid funds), therefore no FHWA review or approval is required.  Metro noise barrier locations are ranked in priority order and included in MnDOT’s Metro District Highway Noise Abatement Study, directed by the Minnesota Legislature in 1996 and updated in 2002, 2007, and again in 2011.  (For further information see MnDOT Noise Policy, 2011.)

 

 

What is a Type I project?

A Type 1 project is a proposed Federal or Federal-aid highway project for the construction of a highway meeting one or more of the following conditions:

    1. The construction of a highway on a new location; or,
    2. The physical alteration of an existing highway where there is either:
      1. Substantial Horizontal Alteration:  A project that halves the distance between the traffic noise source and the closest receptor between the existing condition to the future build condition; or,
      2. Substantial Vertical Alteration:  A project that removes shielding, therefore, exposing the line-of-sight between the receptor and the traffic noise source.  This is done by either altering the vertical alignment of the highway or by altering the topography (not including the addition or removal of vegetation) between the highway traffic noise source and the receptor; or
    3. Bridge replacement projects that satisfy item 2. above.
    4. The addition of a through-traffic lane(s).  This includes the addition of a through-traffic lane that functions as a HOV lane, contraflow lane, High-Occupancy Toll (HOT) lane, bus lane, or truck climbing lane; or,
    5. The addition of an auxiliary lane, except for when the auxiliary lane is a turn lane.
    6. The addition or relocation of interchange lanes or ramps added to a quadrant to complete an existing partial interchange; or,
    7. Restriping existing pavement for the purpose of adding a through-traffic lane or an auxiliary lane; or
    8. The addition of a new or substantial alteration of a weigh station, rest stop, ride-share lot or toll plaza.

 

If a project is determined to be a Type I project as defined above, then the entire project area as defined in the environmental document is a Type I project.

 

 

What is an impacted location?

A “noise sensitive receiver” (defined as homes, parks, schools, businesses, etc.) is considered impacted by noise if either the future (generally a 20-year traffic projection) noise levels exceed the State Noise Rules and/or FHWA Noise Abatement Criteria, or if there is a substantial increase (greater than or equal to 5 db(A)) in future noise levels over existing noise levels from a proposed MnDOT project as described above.  These are the noise levels that are experienced at commonly used exterior portions of the property.

 

Table 1 lists Minnesota's noise level standards.  Table 2 lists the FHWA's Noise Abatement Criteria.

 

Table 1

Minnesota Pollution Control Agency
State Noise Standards

Land Use

Code

Day (7:00 a.m. - 10:00 p.m.)
dBA

Night (10:00 p.m. - 7:00 a.m.)
dBA

Residential

NAC-1

L10 of 65

L50 of 60

L10 of 55

L50 of 50

Commercial

NAC-2

L10 of 70

L50 of 65

L10 of 70

L50 of 65

Industrial

NAC-3

L10 of 80

L50 of 75

L10 of 80

L50 of 75

Notes

  1. NAC-1 includes household units, transient lodging and hotels, educational, religious, cultural entertainment, camping and picnicking land uses.
  2. NAC-2 includes retail and restaurants, transportation terminals, professional offices, parks, recreational and amusement land uses.
  3. NAC-3 includes industrial, manufacturing, transportation facilities (except terminals), and utilities land uses.
  4. From Minnesota Pollution Control Agency, Minn. Rules sec. 7030.0040

 

 

Table 2

Federal Highway Administration Noise Abatement Criteria

Activity Category

Activity Criteria (1,2)

L10(h), dBA

 

Evaluation Location

Activity Description

A

60

Exterior

Lands on which serenity and quiet are of extraordinary significance and serve an important public need and where the preservation of those qualities is essential if the area is to continue to serve its intended purpose.

B(3)

70

Exterior

Residential

C(3)

70

Exterior

Active sport areas, amphitheaters, auditoriums, campgrounds, cemeteries, day care centers, hospitals, libraries, medical facilities, parks, picnic areas, places of worship, playgrounds, public meeting rooms, public or nonprofit institutional structures, radio studios, recording studios, recreation areas, Section 4(f) sites, schools, television studios, trails, and trail crossings

D

55

Interior

Auditoriums, day care centers, hospitals, libraries, medical facilities, places of worship, public meeting rooms, public or nonprofit institutional structures, radio studios, recording studios, schools, and television studios

E(3)

75

Exterior

Hotels, motels, offices, restaurants/bars, and other developed lands, properties or activities not included in A-D or F

F

- - Agriculture, airports, bus yards, emergency services, industrial, logging, maintenance facilities, manufacturing, mining, rail yards, retail facilities, shipyards, utilities (water resources, water treatment, electrical), and warehousing
G - - Undeveloped lands that are not permitted

Notes
(1)  L10(h) shall be used for impact assessment.
(2)  The L10(h) Activity Criteria values are for impact determination only, and are not design  standards for noise abatement measures.

(3)  Includes undeveloped lands permitted for this activity category.

 

In addition, Minnesota Statute 116.07 states ”No standards adopted by any state agency for limiting levels of noise in terms of sound pressure which may occur in the outdoor atmosphere shall apply to (1) segments of trunk highways constructed with federal interstate substitution money, provided that all reasonably available noise mitigation measures are employed to abate noise; (2) an existing or newly constructed segment of a highway, provided that all reasonably available noise mitigation measures, as approved by the commissioners of the Department of Transportation and Pollution Control Agency, are employed to abate noise; and (3) except for the cities of Minneapolis and St. Paul, an existing or newly constructed segment of a road, street, or highway under the jurisdiction of a road authority of a town, statutory or home rule charter city, or county, except for roadways for which full control of access has been acquired. . .”

 

The FHWA interprets its noise abatement criteria as guideposts for the need of noise abatement, rather than absolute limits.  Minnesota's noise level standards are regarded as absolute limits which carry the weight of law.  Due to the guidepost interpretation of its criteria, the FHWA requires that states give numerical meaning to the phrase "approach the criterion."  MnDOT defines a level as "approaching" the criterion level when it is 1 dBA, or less, below the criterion level.

 

 

Why was the Minnesota daytime residential standard set at an L10 of 65 dBA and an L50 of 60dBA?

These standards are based on the present body of knowledge for the preservation of public health and welfare.  These standards are consistent with speech intelligibility and annoyance requirements for receivers within the impacted areas.

 

 

How does MnDOT determine where to place noise barriers?

A computerized noise model program is used 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, feasible and cost effectiveness criteria.  For barriers built under MnDOT’s Metro District Highway Noise Abatement Program, areas are identified by their ranking in the Highway Noise Abatement Study.  Noise barriers are not typically placed along highways that have uncontrolled access points due to the loss of effectiveness from numerous openings in the barriers.

 

 

How does MnDOT determine if a noise barrier is feasible, reasonable or cost effective?

Some of the factors that MnDOT uses to determine if a noise barrier can be placed are:

 

 

How is the height of the barrier determined?

Noise barriers are generally designed to provide noise reductions of 8 dBA or more.  However, a minimum reduction of at least 5 dBA (with at least one receptor getting a 7 dBA reduction) is required in order for the noise barrier to be considered minimally effective.  The goal is a 10 dBA reduction of the average traffic noise levels for the majority of the first row of residences directly abutting the highway right-of-way.  The reasonableness criteria places a practical cost limitation on the height of any noise barrier.  The maximum allowed average height for a noise barrier is 20 feet.

 

 

Why not plant trees instead of putting up a barrier?

When highway noise mitigation strategies were first being researched there was some thought that dense plantings of vegetation might provide effective blocking of sounds from the highway.  However, to be effective at blocking sound there must be complete blockage of the line of sight from the receiver to all noise sources and a great enough mass density to stop the transmission of sound.  If the height of the trees extends at least 15 feet above the line of sight, and the vegetation has a depth of 100 feet, then a 5 dBA attenuation is allowed.  Most vegetative plantings near highways have not been found to meet these prerequisites.

 

 

How effective are noise barriers?

Generally the effectiveness of a noise barrier depends on (1) the distance between the listener and the noise source, (2) the distance between the listener and the noise barrier and (3) the height of the noise barrier above the line-of-sight between the listener and the noise source.  Typically, the benefit due to the noise reduction by a noise barrier will be greatest for the listeners nearest the noise barrier.  For residences located directly behind a 20 foot noise barrier, a reduction of about 10 dBA would be typical.  This benefit decreases as the listener moves farther away from the barrier and is barely perceptible at distances greater than 300 feet.

 

 

How much do concrete post/wood plank noise walls cost?

Current construction costs on an average are $20.00 per square foot.  This translates into a 20-foot high wall costing approximately $2.11 million dollars per mile.

 

 

Does MnDOT have a program to provide insulation of private residences?

Adding sound insulation (windows and sound deadening materials or acoustical insulation) achieves interior noise attenuation only and does not address exterior receptor areas where Minnesota’s noise standards apply.  The costs for acoustical insulation of typical single residences are considered disproportionate for the amount of attenuation achieved and the number of receptors affected.  There may be cases involving schools, hospitals, nursing or convalescent homes, or other public buildings where the number of receptors and the noise sensitivity of the receptors may justify the expense of insulating for noise mitigation.

 

 

Who pays for the noise barriers?

For Type I projects, noise barriers are built alongside Trunk Highways and Interstates, in which case the federal government pays the majority, and the State of Minnesota pays the remainder.

 

 

What types of barriers are there?

Noise barrier materials used by MnDOT include earth berms, concrete and wood.  Earthen berms work the best and are the least expensive, but a lack of available right-of-way usually makes noise walls the most practical solution.

 

 

Does the public have any input?

The design and planning offices will work closely with the impacted residents and city staff through the public involvement process to ensure that reasonable requests regarding the design of the barriers are considered for the project plan.  Any noise barrier that meets the feasible and reasonable criteria (including cost-effectiveness) will be included in the project unless more than fifty percent of the total eligible points for the benefited receivers are not in favor of the barrier.  (See MnDOT Noise Policy, 2011 for further information regarding solicitation of benefited receptors.)

 

 

Can a municipality build it's own noise barrier?

If a municipality wishes to construct a barrier on MnDOT’s right of way, they must submit plans certified by a registered engineer or landscape architect to be reviewed by MnDOT before constructing.  Per State Statute 429.021 subdivision 1, the municipalities are given the power to make these improvements.

 

 

How many miles of noise walls have been constructed?

As of December 2011, MnDOT has constructed approximately 114 miles of noise walls and berms located throughout the state.