Our Air Quality section is currently being updated.
New Hampshire has been evaluating the public impact of increased highway traffic noise since the development of the National Environmental Policy Act of 1969 (NEPA). The act required, among other aspects, evaluation of the impacts associated with improved and newly constructed roadways. The first noise abatement structure in New Hampshire was built in 1977 along Interstate 93 in Manchester. Since then numerous additional structures have been added throughout the state.
Noise abatement in New Hampshire is generally in the form of a sound barrier or berm. The locations of these barriers are identified using a specific set of criteria set forth by the NH Department of Transportation (NH DOT) and the Federal Highway Administration (FHWA). An area is not considered eligible for noise abatement unless it meets ALL of the criteria set forth in the Department's Noise Policy. The following are highlights of those criteria.
- Type I Project - NH DOT considers an area eligible for the evaluation of noise abatement when impacts are associated with a Type I project. Type I projects are generally those which involve the addition of a new roadway, or the physical alteration of an exiting roadway. Type II projects are those designed solely for the purpose of traffic noise abatement in areas where no highway improvements are scheduled. NH DOT does not have a Type II noise policy and therefore does not usually evaluate areas where projects are not proposed. As such, in order for an area to be considered eligible for noise abatement, there MUST be a Type I project scheduled in its immediate vicinity.
- Decibel Level - Residential properties are considered for noise abatement when traffic noise levels reach or exceed 66 decibels (dBA). Commercial properties are considered for noise abatement when traffic noise levels reach or exceed 71 dBA.
- Feasibility - A minimum of a 5 dBA insertion loss must be expected for at least 1 impacted receptor (home or business) in order for an area to be considered eligible for noise abatement. Despite this criterion NH DOT generally makes every reasonable effort to maximize the number of benefited receptors and to obtain a 10 dBA (or greater) decrease in sound levels after construction is completed. For safety and stability reasons, the structural portions of a barrier should not be in excess of 25 feet in height. Impacts to wetlands, endangered species, historic resources, recreational facilities and other area resources can also effect the feasibility of a particular abatement measure.
In order for an abatement measure to be considered reasonable, it must meet one or both of the following effectiveness criteria.
- Cost Effectiveness: Effectiveness can be determined by calculating the estimated cost of the abatement measure per benefited receptor. NH DOT estimates the cost of noise barrier construction to be $30 per square foot. If the anticipated cost per benefiting receptor is expected to exceed $45,000 the abatement measures would not be considered cost effective. (For example: A barrier proposed to be 1,200 ft. long and 15 ft. high would total 18,000 sq. ft. At a cost of $30 per sq. ft., this barrier would be estimated to cost $540,000. If this barrier were anticipated to provide at least a 5 decibel reduction to 13 receptors, it would cost $41,538 per benefitted receptor. As this anticipated cost per receptor is less than $45,000, the barrier would be considered cost effective.)
- Dimensional Effectiveness: Effectiveness can also be determined by calculating the protective surface area per benefited receptor of a barrier or berm. Under this method, if the anticipated square footage per benefitted receptor is expected to exceed 1,500 sq. ft. the abatement measure would not be considered reasonable. (For example: A barrier proposed to be 1,200 ft. long and 15 ft. high would total 18,000 sq. ft. If this barrier were anticipated to provide at least a 5 decibel reduction to 13 receptors, it would require 1,384 sq. ft. of barrier per benefited receptor. As the anticipated square footage per benefitted receptor of this barrier is less than 1,500 sq. ft the barrier would be considered dimensionally effective.)
- Noise Reduction Design Goal - In order for an abatement measure to be considered reasonable, it must be able to provide at least a 7 dBA noise reduction for at least 1 benefited receptor.
- Views of the benefited receptors - Viewpoints from the entire project community, including benefited receptors, will be solicited for noise impacts and any noise abatement alternatives. If no objections to the proposed noise abatement are found at this level of public involvement, then the noise barrier will be deemed reasonable. If objections are identified, at least 51% of the total possible receptors (property owners and tenants) recieving at least a 5 dBA or greater noise reduction must be in support of an abatement alternative for it to be considered reasonable.
- Effectiveness: In order for an abatement measure to be considered reasonable, it must meet one or both of the following effectiveness criteria.
- Development History - In order for an area to be evaluated for noise impacts it must contain development which is already in place or is permited for development (as indicated by the issuance of a building permit or similar final approval from the local municipality) by the date of public knowledge (the date upon which design approvals are granted by the FHWA or NHDOT).
- 1996 NHDOT Noise Policy - For projects approved prior to July 13, 2011
- 2011 NHDOT Noise Policy - For Projects approved on or after July 13, 2011
- 2016 NHDOT Noise Policy - For Projects approved on or after November 30, 2016
- FHWA Traffic Noise Information
- AASHTO Center for Environmental Excellence - Noise
- Jon Evans - Air & Noise Program Manager, Jonathan.Evans@dot.nh.gov