On the Nature and Likelihood of Soundwalls Coming to Rockridge

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Saturday, May 5, 2012

The possibility of the construction of soundwalls along Highway 24 through parts of the Rockridge-Temescal area has raised many questions. This fourth article on soundwalls (following up on those in the May, June and July 2011 issues of The Rockridge News) will consider the nuts-and bolts of soundwalls, some "urban legends" surrounding them, and the processes by which they are or are not constructed. San Leandro lightweight transparent soundwall

Sources of Freeway Noise

Freeway noise comes from many sources: tires on pavement, including noise as they hit bumps, potholes, and cracks; engine noise, which varies with the type of engine— cars make some noise, diesel trucks and motorcycles make much more. While the noise originates in the engine, most of it comes out from the exhaust pipe, reduced somewhat by the muffler; and the noise of air displaced by cars and trucks as they travel. Trucks create a distinctive and particular sound when the driver applies engine braking or Jake's Brakes, a compression release mechanism that releases a mechanical gargling sound.

All of this noise increases with the number of cars and their speed. However, as the number of vehicles nears the freeway's capacity, speed diminishes, as does noise. Thus, a traffic jam produces far less noise than free-flowing traffic. Weather also affects noise levels, with perhaps the biggest effect being increased noise from tires moving on wet pavement.

What Can Soundwalls Do?

A soundwall's basic act is to block or deflect freeway traffic sound. Sound travels as waves, but often behaves as if it were a series of little balls. When the soundwave balls hit a soundwall barrier, they bounce off the wall in an organized manner (reflection); they bounce off the wall in random directions (diffusion); or they stick to the wall (absorption). A small amount of sound will pass through the wall.

Because sound generally travels in straight lines, placement of a soundwall blocking the straight-line path between the noise source and the listener serves to deaden the noise. Conversely, if the lineof- sight isn't blocked, neither is the sound. Soundwalls usually don't help people living on the upper stories of buildings.

People often worry whether the sound reflected off lightweight soundwall ground viewa soundwall will increase noise on the opposite side of the freeway. The answer is: generally, no.

First, sound dissipates over distance. A truck's diesel exhaust noise, for example, drops by five decibels (dB) with each doubling of the distance from the truck. [A decibel is a unit of "loudness." A 10 dB noise difference will make a noise sound half as loud.] The truck noise that travels across the freeway, hits a soundwall and bounces back, will be five dB softer when it gets back to where it started.

The second factor is diffusion. If the soundwall has a rough surface (as do most, by design), a lot of the sound will bounce back in random directions, spreading out and dissipating its energy and noise level. Even when sound bounces back the way it came in, it is reflected back, like light hitting a mirror, at the same angle that it came in. Most freeway noise starts near the roadway surface. When it hits the sound wall, it's going up. When it bounces off the soundwall, it keeps going up. By the time it gets to the other side of the freeway, it's many feet above the freeway. That reflected sound generally dissipates harmlessly.

A Legacy of the Past

If Highway 24 were built today, soundwalls would most likely be installed. Federal and California regulations mandate sound protection when new freeways are constructed. However, Highway 24 was built before these regulations were in effect. Therefore, soundwalls, if built, would be a retrofit. That complicates matters. Not only was the roadway not designed to accommodate soundwalls, but the surrounding area comes very close to the freeway, leaving little room to put them up.

The Good, the Bad, the Ugly

A plus for soundwalls is the protection they can offer to people living next to the freeway. This is important in Rockridge where the homes and the freeway are old. Most Rockridge homes are not equipped with double-paned windows and newer insulating materials that could reduce noise in the home. It is possible that soundwalls could reduce freeway noise by 5 to 10 dB for homes near the freeway.

However, the esthetics of Caltrans' concrete block soundwalls are stark, at best. Happily, that type of soundwall is not likely to be built in Rockridge since Highway 24's elevated structures weren't built for the extra weight of heavy concrete walls. Caltrans has recently installed transparent soundwalls on some nearby overpasses, as shown in the accompanying photos. While somewhat less effective in blocking noise, they preserve views and a sense of the roadway's openness.

Building Soundwalls, or Not

The Alameda County Congestion Management Agency (ACCMA*) Freeway Soundwall Policy adopted in 2002 and later revised, defined a review and ranking system for retrofit soundwall projects. The policy defines four criteria for determining if a soundwall is warranted: ■ The existing or future predicted exterior noise level is 65 dB. ■ A reduction of at least 5 dB resulting from the installation of a soundwall can be achieved. ■ The projected cost will not exceed $45,000 per dwelling unit (based on year 2002 costs) affected by the soundwall. ■ The residences were developed prior to opening the freeway to traffic.

The process for implementing sound walls is twofold: 1. the initial screening process; and, 2. the Noise Barrier Scope Summary Report (NBSSR) process, a detailed noise study analysizing the four criteria above.

The screening process starts with a request for a soundwall. The jurisdiction, in this case the city of Oakland, agrees to sponsor the request and take responsibility for coordinating public input. As one element of the city's settlement agreement with Caltrans, an initial screening, or pre-NBSSR study, was conducted by Caltrans' consultant along Highway 24 between Highway 13 and I-580 to determine whether soundwalls were warranted. The pre-NBSSR study identified four areas along SR 24 that meet the criteria of the ACCMA soundwall policy and are considered cost-effective.The study, accepted by the ACCMA, is available at http://tinyurl.com/6oejbee.

The next step is evaluating support in the neighborhood for soundwalls through property owners' petitions. A petition favoring construction of a soundwall must be signed by a property owner from 100 percent of the households with a property line immediately facing the proposed soundwall and 75 percent of the households with a property line not immediately facing the proposed soundwall, but experiencing a minimum 5 dB in noise reduction (typically 200-300 feet from the roadway). If the required quotas of petition signatures aren't submitted, the city can appeal to the ACTC, if it feels public support is nonetheless strong.

If enough qualifying signatures are gathered and the city of Oakland supports moving forward, a Noise Barrier Scope Summary Report (NBSSR) process is initiated. It must include: ■ A detailed cost estimate. ■ Life cycle of the soundwall. ■ Consideration of the environmental impacts such as blocking residents' views or scenic vistas. ■ Engineering feasibility.

Upon completion of the report, the city of Oakland must hold a public meeting and adopt a City Council resolution in support of the proposed soundwall. The final step is funding the actual soundwall construction.

The Oakland Settlement Agreement provides Caltrans funding for Highway 24 NBSSRstudies, should they progress to this stage, significantly shortening the development process. Two soundwall sections, eastbound from Vicente Way to Broadway and westbound from Ross Street to Telegraph Avenue, would receive $554,000 and $628,000 respectively for the studies.

If the NBSSR studies will not be pursued, the agreement's $8 million funding line will be moved down to projects lower on the final list (see http://tinyurl.com/7na7jhj).

An informational meeting on soundwalls and the process of having them studied, approved, funded and constructed will be held in the near future. Check the RCPC website at rockridge.org for more information or contact info@rockridge.org.

* The ACCMA has since been replaced by the Alameda County Transportation Commission (ACTC). Same job, different name