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I-5 - Ship Canal Bridge Noise Study - Common Questions

Sept. 2012

Are the test panels performing as you expected?
No. The panels are reducing some noise (1-4 decibels), but not by as much as the models predicted (4-5 decibels). 3 decibels is considered the minimum change noticeable to the human ear. While the panels are working, it is difficult for local residents to actually hear the change.

Around the Ship Canal, four types of traffic noise can be heard at nearby homes:

  • Reflected noise from the underside of the main lines,
  • Diffracted noise curving down from the edge of the express lanes and main lines,
  • Direct path noise from vehicles to homes with an unobstructed view of traffic
  • Traffic noise from local streets, especially Harvard and Eastlake.

To make the most of the limited funding, acoustics experts recommended panels that focused on the loudest noise: reflected noise. With the panels, overall noise levels have been reduced by 1-4 decibels and don’t achieve the maximum predicted reductions of 4-5 decibels for some areas.

Why are they not performing as expected?

While noise is noise to anyone who lives near the express lanes, isolating and addressing certain kinds of noise is the scientific and best practice to approach noise reduction. Panels targeted reflective noise and measurements demonstrate that reflected noise has been reduced. The panels weren’t designed to reduce diffracted or direct path noise from the express lanes and these levels are unchanged. WSDOT believes that the lower than predicted noise reductions are because of the following:

  • The panels are not reducing low frequency noise by as much as was expected,
  • Traffic noise from local streets is contributing to neighborhood sound level measurements and was not modeled,
  • The space between the vertical noise panels and the uncovered area over express lane traffic allows reflected noise to “bounce” down into the neighborhoods in some areas, and
  • Traffic noise from the express lanes is diffracting around the hard edges of the panels and the edge of the express lanes.

What are neighbors reporting?

Residents who have a line of sight to the ceiling of the express lanes are reporting very little if any change in noise levels. This is consistent with the theory that reflected noise – the noise that bounces right off the ceilings and into nearby homes - is the loudest. Residents who live a block away and have no line of sight to the express lane ceiling report a noticeable reduction in noise.

What has WSDOT learned?
The current pattern and amount of noise absorbing material is not performing as well as expected.

WSDOT believes that additional coverage and a different panel alignment may improve performance appreciably. For example, adding panels directly above traffic and hanging the panels horizontally, instead of vertically.

The quilted texture panels are absorbing less of the low frequency reflected noise than expected. 

Direct path and diffracted noise are also contributing to area sound levels and need to be better understood.

Additional noise modeling of noise from local streets, particularly Eastlake and Harvard Avenue, could describe the contribution of local traffic to area noise and efforts to reduce direct and diffracted noise from the express lanes may also benefit the adjacent neighborhood. For example, adding absorptive material to the existing safety barrier.

What’s next?
WSDOT acoustics experts have completed four quarters of testing and generated the first annual report that summarizes what has been achieved and learned. At this time, it is difficult to chart a path forward. Experts have done extensive investigation, research and built the pilot project, but it's difficult to say what else can be done with any amount of money.

What is WSDOT testing?
Reducing noise from the Ship Canal Bridge poses a number of challenges because the bridge is a double-decked structure, travels over water, and runs through a dense urban neighborhood. Standard noise walls cannot be built on the structure without significant structural changes.

WSDOT worked with noise experts and a national expert review panel to develop a pilot study and panel configuration in an attempt to reduce noise in the area, within the project’s financial constraints.

The expert review panel recommended the test section on site today. The 500-foot test section consists of approximately 700 4-foot by 8-foot panels attached to the south end of the express lane ceiling. There are seven rows of panels on each side.

WSDOT is testing a noise-absorbing technology made of lightweight quilted vinyl panels. This material is typically installed on walls and ceilings to absorb noise in a variety of indoor environments, including classrooms and theaters. Since a ceiling treatment like this is not normally used outside on transportation structures, we installed a test section on the ceiling above the express lanes on the I-5 Ship Canal Bridge. We will monitor the acoustic performance and durability through 2014.

How much do the panels cost?
The construction cost to install the panels is $2.3 million. The overall budget for construction, and noise studies and design was $5.8 million, funded through the 2005 gas tax.

Where is the test section on the bridge?
The test section is on the south end of the Ship Canal Bridge. The test section is approximately 500 feet long and is located between E. Gwinn Place and E. Allison Street.

How does WSDOT decide which areas receive funding for noise reduction projects?
WSDOT identifies sites where noise is a concern, this neighborhood existed prior to the construction or major expansion of a roadway, and noise was not previously evaluated. These locations are added to a statewide Noise Retrofit List that is prioritized by a number of factors. This information is provided to the Legislature who decides which areas to fund and the amount of funding. The noise list is new locations are identified and construction projects are completed.

Why are we evaluating noise reduction methods on the bridge?
Highway traffic noise is a concern for many residents living along highways. The I-5 Ship Canal Bridge is one of several projects where we are working to learn more about how to reduce noise in challenging environments. Knowing that people live next to highways and will always live next to highways, we want to learn more about noise in these urban environments so we can develop reduction strategies that are realistic, cost-effective and feasible. This particular area is ideal to test some theories about noise.

This test site on the Ship Canal Bridge is located in a heavily traveled urban area and traffic noise from the bridge reaches the densely populated Seattle neighborhoods of Eastlake, Roanoke Park, University District and Wallingford. Due to the large number of affected residents whose homes were built prior to May 1976, high noise levels, and the close proximity of communities to the highway, we identified the area near the I-5 Ship Canal Bridge as one of the top ten priorities in the state for reducing traffic noise.

How much noise is coming from the bridge?
Existing noise levels in the neighborhoods surrounding the bridge range from 66 to 87 decibels. For comparison, a train passing by at 50 feet away is approximately 70 decibels and a jackhammer at 50 feet is approximately 90 decibels. Above 66 decibels, it is difficult for two people standing three feet apart to hold a conversation.

Other than this study, what else has WSDOT done to reduce noise?
In response to community input, we currently close the I-5 express lanes on the bridge from 11 p.m. to 5 a.m. on weekdays and 11 p.m. to 7 a.m. on weekends. This reduces the amount of highway traffic noise residents near the bridge experience during the night.
With the automation of the I-5 express lanes, residents may experience more quiet time as we reduce the amount of time it takes to open and close the lanes.

Why don’t you just build noise walls instead of testing a new technology?
We use concrete noise walls where it’s cost-effective and where we are likely to meet our noise reduction goals. Noise walls on the bridge, even it is even possible, would require major structural changes to the bridge.

How did you select the south end for the test section?
The results of our structural analysis showed that we would need to modify the north end and steel truss portions of the bridge in order to accommodate the panels, while the south concrete section would not require any modifications.

What sort of safety measures are in place to make sure the panels don't fall down?
There are redundant safety mechanisms which include tying the panels together laterally and transversely to prevent the panels from detaching from each other, the ceiling or moving during high winds.

How will WSDOT determine if the noise-absorptive ceiling treatment is effective?
WSDOT noise experts are methodical in their approach to noise measurement. They took “pre-construction” noise readings to develop a base line. They are taking “post-construction” noise readings to compare with the pre-construction measurements. To measure the long-term effectiveness of the noise panels, the noise experts took quarterly measurements for the first year and annual measurements for the next two years. All information will be published to our web site.

We will also monitor the wear and tear of the panels over time and evaluate its maintenance requirements and qualities. This monitoring will help us understand how the noise-absorptive material works and determine if it should be used in future projects.

What happens if the test section doesn’t work?
We are measuring the noise reduction and the product’s durability. The monitoring will last up to three years and will help us understand how the noise-absorptive material works and determine if it should be used in future projects. The results of the monitoring will allow us to develop recommendations, which we will document in a final report. Since it is a test section, the ceiling panel installation is designed to be temporary. The test section may be removed at some point in the future.

What will you do with the results of the test?
The results from the test section will help us learn more about noise and innovative reduction strategies. It will allow us to determine if these types of noise panels could be used on other transportation structures in other areas. A copy of the final report at the end of three years will go to the State Legislature who will evaluate whether further funding would be of value.

What other noise reduction technologies could be pursued if more funding becomes available in the future?
Other technologies that could be pursued if we had additional funding include:

  • Installing a ceiling treatment on other portions of the bridge to address the reflected and reverberant noise from the bridge—this would be contingent on the results of the test section.
  • Modifying the existing barriers on the bridge to add noise reduction capabilities to structures already in place.
  • Constructing new noise walls at ground level in the area near the bridge approaches.

These options were not pursued in the initial phase of the study because they would require additional funding and/or structural modifications to the bridge.
How can I provide comments regarding this study?
You can provide your comments or sign up for study updates by contacting us: E-mail: