If a project will raise noise above a certain threshold once it is complete, it will be necessary to consider whether noise abatement measures are feasible. Once it has been decided whether abatement is reasonable and an acceptable abatement method has been chosen, the cost of abatement can be estimated and included in the project's overall costs.
Caltrans uses the following thresholds to determine whether abatement must be considered.
Example Thresholds for Noise Abatement
In addition, Caltrans requires abatement to be considered when a project increases existing noise levels by 12 dBA or more.
However, as the Federal Highway Administration (FHWA) explains, noise abatement criteria (NAC) "are not magic numbers. …The NAC should not be viewed as Federal standards or desirable noise levels" (FHWA 1995). In some cases, noise impacts must be abated even when the project's impact does not violate any agency's published standards. For example, a city government might object to the additional noise generated by a project and insist that it be abated.
If a project will create enough noise to require noise abatement, or if a government insists on abatement measures as a matter of policy, the next step is to choose a suitable abatement measure. Some of the most common abatement measures are:
Choose only abatement measures that will reduce noise impacts by at least 5 dBA. The following criteria can help determine whether an abatement measure is reasonable (Caltrans 2004):
Federal Highway Administration. Highway Traffic Noise Analysis and Abatement Policy and Guidance. Federal Highway Administration Office of Environment and Planning, Noise and Air Quality Branch, June 1995. Available at: http://www.fhwa.dot.gov/environment/polguid.pdf
California Department of Transportation (Caltrans). "Some Frequently Asked Questions About Highway Traffic Noise Analysis & Abatement." No Date. Available at: http://www.dot.ca.gov/hq/env/noise/pub/FAQ.pdf. Accessed May 2004.