Analysis of noise impacts associated with highway projects are performed in accordance with the procedures and provisions of Title 23, Code of Federal Regulations (CFR), Part 772, U.S. Department of Transportation, Federal Highway Administration (FHWA), and Procedures for the Abatement of Highway Traffic Noise and Construction Noise. These regulations establish a requirement for a noise assessment for any proposed federal or federal-aid project. It is the policy of the Arkansas Department of Transportation (ARDOT) that highway traffic noise prediction requirements, analysis, and abatement criteria comply with the noise standards established by 23 U.S.C 109(i). Procedures for noise impact and abatement analysis can be found in ARDOT’s Policy on Highway Traffic Noise Abatement.
A noise analysis generally includes:
- Identification of existing activities, developed land and land currently under development.
- Measurement and/or estimation of existing noise levels.
- Prediction of Design Year No-Action noise levels.
- Prediction of Design Year Build noise levels for all alignment alternatives considered by the project.
Noise levels for existing or ambient conditions are measured in the field with a sound level meter. This information is then used as a baseline to determine if the predicted noise levels will result in a substantial increase.
Predicted noise levels are modeled by utilizing FHWA’s Traffic Noise Model (TNM), version 2.5. This model utilizes traffic counts, vehicle speeds, truck percentages and roadway cross-section information as the input components. Predicted noise levels are then compared to the FHWA noise abatement criteria (NAC) to determine if an impact will occur.
When the predicted noise levels do not approach or exceed the FHWA NAC, then substantial increase criteria (SAC) is also considered. If the predicted noise levels indicate a 10dBA or greater increase over the existing noise levels, then a noise impact exists.
Noise abatement must be considered for proposed projects when the predicted noise levels at any receptor location approach or exceed the FHWA NAC and or exceed existing noise levels by 10 dBA or more. Studies generally include efforts to avoid or minimize noise impacts to sensitive receptors through alignment shifts and overall avoidance of residential areas. If this is not possible, there are several types of noise reduction measures that can be considered for mitigation of highway noise impacts.
These measures include:
- Alteration of vertical and horizontal alignments.
- Traffic controls.
- Construction of noise barriers.
These noise abatement measures are evaluated for use in a location on the basis of being reasonable and feasible. Reasonable usually relies on factors such as cost effectiveness, number of receptors protected, residential support for the abatement measures and concerns for the physical and visual access of adjacent properties. The feasibility of an abatement measure usually relates to topography, access points, drainage, safety and maintenance requirements. Where noise abatement considerations are warranted, every reasonable effort will be made to achieve adequate noise level reductions for locations that approach or exceed the NAC and or where the projected noise levels exceed the SAC.