SR 14 - I-205 to SE 164th Avenue Auxiliary Lanes - Noise wall assessment

Noise wall overview

In order to provide a fair and equitable process for all, we follow a uniform procedure using consistent criteria following federal noise regulations outlined by the Federal Highway Administration (FHWA) and WSDOT, for all transportation projects across the State to identify the noise effects on neighborhoods and to determine where noise barriers will be built. The evaluation of noise for new roadway construction projects is a mandatory program under federal law.  Noise evaluations take into account many factors, including:

  • Future highway noise
  • Area topography
  • Residential density
  • Cost effectiveness
  • Levels of noise reduction a wall would provide

Assessing noise impacts

To evaluate noise associated with a new roadway project, we take field measurements outdoors along the existing roadway segments in areas frequently used by people. The study area for noise on a transportation project extends from the point on the highway where the project starts to the point where it ends. In addition, we look out from the pavement edge into the community on both sides of the roadway to the point where noise impacts stop. We limit the noise study area to where impacts occur for three reasons:

  • Noise barriers are typically effective in reducing noise within 200 to 300 feet of the barrier.
  • We want to ensure that all noise impacted areas are identified in the noise model.
  • In areas farther from the highway, other noise sources, such as local roads, commercial exhaust fans, leaf blowers and so on, tend to be the dominant sounds a person hears.

Computerized noise models are developed to predict future traffic-noise levels. To be conservative (worst case) in our estimates, we often model with rush hour traffic volumes, traveling at the posted speed limit, a condition that would seldom occur at the same time.

Any applicable area predicted to have a future traffic-noise level of 66 decibels (dBA) or greater qualifies as an impacted area. All impacted areas are considered for noise abatement. We make every attempt to qualify these impacted locations for noise barriers based on the reasonable and feasible criteria. A noise wall must provide a minimum noise reduction to the majority of homes, or other noise sensitive land uses, closest to the road.

Reasonable and feasible criteria

A barrier must meet both reasonable and feasible criteria to be constructed:

  • Feasible means that the noise barrier reduces sound sufficiently for the majority of people.
  • Reasonable means the noise barrier is cost effective for the amount of noise reduction and the number of people we aim to protect.

Barriers do have limitations. For a noise barrier to work, it must be high enough and long enough to block the view of the road. Noise barriers need to be very tall to provide any protection for homes on a hillside. Noise barriers are not designed for buildings that rise above the barrier. Additionally, as is the case with this project, when homes are not densely spaced, noise barriers cannot be built at a reasonable cost.

Noise wall costs

Current statewide construction costs average $51.61 per square foot. This translates into a fourteen-foot high wall (typical) costing about $3.9 million per mile. We use modeling software to determine appropriate wall heights and alignments.

Noise wall evaluation for this project

In accordance with FHWA and WSDOT guidance, we evaluated sources and patterns of noise in neighborhoods near the project limits, in compliance with FHWA regulations for noise walls, following a three-step sequential process to determine:

  1. Whether a location has noise levels of 66 decibels (dB) or higher. We also model noise projections up to the year 2035 to warrant further consideration of a noise wall;
  2. Whether it is feasible to construct a noise wall. Meaning, will the proposed noise wall provide a substantial reduction in noise and does it avoid extreme efforts to construct; and
  3. Whether it is reasonable to construct a proposed noise wall, based on a per residence construction cost, which is related to the density of homes in the area.

The noise study boundary for this project was set at 500 feet from the edge of the travelled lane to be certain that all noise impacts are included within the noise study area.

Result  

Since future noise levels at some of the homes located within the project area were at or above the 66 dBA threshold (step 1), noise reduction, in the form of a noise wall, was evaluated as part of a noise study recently completed. The results found that a noise wall was feasible (step 2) in most of the evaluated areas of the project area, but construction of several walls did not meet the reasonable criteria in all but one location (step 3) since they were not cost effective to construct, due to the low density of homes affected by noise in those areas. This means that per federal guidelines, one noise wall on the north side of the highway will be constructed as part of this project. 

A public open house for the SR 14 - I-205 to SE 164th Avenue Auxiliary Lanes project was held on Thursday, Jan. 31, 2019.

More information about the WSDOT noise wall process is available online.