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SOUNDWALLS

In 1973-74, State and Federal agencies adopted formal policy and criteria for construction of noise barriers. California leads the nation in both completed and planned soundwalls. About 400 miles of walls have been completed.

There are three basic programs under which CALTRANS may under take soundwall construction:

  1. As part of a new freeway project
  2. As a part of a freeway widening project
  3. Under the Community Noise Abatement Program

The most frequent requests fall under the Community Program. Typically, the request is to build a soundwall on an existing freeway to shield adjacent residences from freeway noise.

The immediate key questions that need to be answered are:

  1. Does the area qualify? If yes...

In order for the area to qualify, it must meet all of the following criteria:

  1. When will it be built? If not soon...
    1. Normally, engineering and construction scheduling are not a problem. However, the availability of funds is usually the major stumbling block, which generally means waiting.

  2. Where does it stand on the waiting list?

Because the demand for soundwalls has far exceeded the funding to build them, a priority waiting list has been developed. This waiting list is based on a formula, which combines noise levels, number of living units and cost effectiveness to produce a ranking.

COSTS AND DESIGN FEATURES

Total Cost: Averages about $350 per lineal foot or $1.8 million per mile.
Type of Wall: Usually reinforced concrete, reinforced concrete block or combination earth mound/wall,
Footings: Trench, spread or pile footings are used as appropriate.
Typical Height: 8 to 16 feet, depending on specific design needs.
Aesthetic Treatment: Decorative concrete block, e.g. color split face, slumpstone, fluted is used.
Engineering of Plans: Typically 12 months,
Construction Project: Typically 12 months.

FUNDING METHODS

Traditional Financing

The California Transportation Commission is the approving body for program and project level funding. Recent legislation (SB 45 - STIP Reform) may have an impact upon the programming of soundwalls. During the implementation of SB 45, Caltrans works closely with the Los Angeles County Metropolitan Transportation Authority (LACMTA) and the California Transportation Commission (CTC) to program soundwall projects along with various other transportation needs.

Soundwalls, which come under new or major reconstruction projects, are automatically included as a part of the project design. Soundwalls, which are retrofitted to existing freeways, fall under the Community Noise Abatement Program. Under Commission policy, this program is subject to available funding. Since funding is limited, a priority list has been developed to rank future projects.

Payback Option

State law allows cities or counties to construct eligible soundwalls ahead of the time that they would be built under traditional funding. Then, when the funding priority is eventually reached, CALTRANS would reimburse the local agency for the actual cost. Its important to note that reimbursement does not include interest.

Benefit Assessment District

Some local agencies are considering a benefit assessment district whereby residents in effect tax themselves under some formula to generate funding. Under this method bonding could be used for early construction at the expense of a longer payback.

Special Legislation

Soundwalls have occasionally been funded and constructed by Special State legislation. These have occurred outside of CALTRANS' and the California Transportation Commission's process.

TYPICAL QUESTIONS

Q. How does my area qualify for a soundwall?

A. The two key factors under the Community Noise Abatement Program are:

    a. The residential area existed prior to the freeway opening date or major reconstruction completion.
    b. Outside noise levels exceed 67 decibels (Leg)

Q. Why is the 67 decibels (Leg) level so important?

A. This is the noise level established by Federal and State agencies, which must be exceeded before impacted neighborhoods are eligible for mitigation.

Q. What does Leg mean?

A. It is the steady noise level equivalent to fluctuating traffic noise over a given period of time.

Q. When are noise levels usually measured?

A. In the greater Los Angeles Area our studies have shown that the highest noise measurements usually occur between 9:00 a.m. and 3:00 p.m. and not at peak congestion times.

Q. Why aren't noise measurements taken during the peak congestion time?

A. Traffic noise is speed related. i.e., as vehicles move faster; they produce more noise. Likewise, when traffic is stop-n-go or at low speeds, noise levels are also lower.

Q . Why does it seem noisier late at night and early morning?

A. Due to the fact that the surrounding area is quieter at these times, the masking effect of other noise does not screen the freeway noise. This usually makes the freeway noise more prominent but lower than the midday level.

Q. Why are noise measurements taken for only ten minutes?

A. Our measurements on heavily traveled roads have shown that a ten-minute period is sufficient to reliably reflect an hourly noise level.

Q. Why is there a soundwall on the other side of the freeway or just down the road and not in my area?

A. There are many factors which affect noise levels even when traffic volumes are the same. These differences usually happen when the terrain changes the freeway curves in a different direction the freeway elevation changes from above to below ground level. Also developers of the adjacent property have sometimes constructed soundwalls.

Q. Why can't you place a soundwall to protect our area from cars running off the freeway?

A. Soundwalls are not intended to act as safety barriers. There are other reliable methods used such as installing guardrails to protect against vehicles running off the road. All improvements, whether to reduce noise or enhance safety, have to meet specific criteria and be justifiable on their own merits. Cost is always a factor.

Q. Why are soundwalls built to protect commercial property in some locations?

A. Commercial property in itself is not eligible for soundwall protection. However, when designing a wall in a particular location safety, aesthetics or continuity will sometimes dictate gap closures, which can end up protecting non-eligible property. Also in some instances, the walls were privately funded.

Q. Are soundwalls required by Federal, Caltrans, and/or City offices?

A. As part of the general environmental review process associated with all projects, Caltrans is required to evaluate traffic noise impacts. Impacted areas are considered for noise mitigation, by implementing noise abatement when reasonable and feasible.

Q. What is "decibel energy" and how is it accurately measured?

A. Decibel Energy is the relative intensity of sound defined on a logarithmic scale. The unit of measurement for sound intensity is the decibel (dBA) as measured on the "A" scale of a standard Sound Level Meter. The "A" scale most nearly approximates the response of the human ear to sound. While an increase of 2 or 3 dBA may be hardly noticeable, and increase of 10 dBA doubles the apparent noise level.