Improvements to major roadways can benefit thousands of drivers every day. However, they also have the potential to increase noise for the people who live next to the road. The Maryland Transportation Authority (Authority) takes this concern seriously. From the beginning of project planning through detailed engineering design, the Authority looks for ways to minimize increases in noise levels

Common Sounds in Decibels (dBA)

Sound is measured in decibel units. The higher the decibel level, the louder the sound. Most humans cannot "hear" any difference in loudness unless the difference is greater than three dBA. A 10 dBA increase, however, is perceived as a doubling of noise level

Whenever transportation improvements are being made, the Authority considers noise barriers for existing properties along the road if future noise levels approach or exceed 67 dBA or will increase more than 10 dBA.

The Process for Determining Noise Level Impacts

The Authority uses the Federal Highway Administration's (FHWA) noise policies and criteria as the basis for its noise guidelines. FHWA defines noise levels approaching 67 decibels (dBA) near homes, parks, schools, hospitals, and the like as having the potential to disturb sleep, disrupt conversation, and interfere with other tasks. Following is the general process the Authority uses to determine how noise levels from transportation projects will impact surrounding neighborhoods.

1. Identify areas within the project limits that could be impacted by increased noise levels Identify all homes, parks, churches, hospitals, schools, and businesses near the existing road where noise-sensitive activities occur.

2. Determine when current noise levels are the highest Noise measurements are taken at different locations within the project area for a 24-hour period to determine when the noise level is the highest (peak noise hour). Once the peak noise hour is identified, the number of cars, buses and trucks traveling during that time period are used in the traffic noise model. In addition, the speed that the vehicles are traveling is taken into consideration.

3. Estimate future noise levels and areas of impact for each transportation alternative that is being considered Using existing maps of the road and the surrounding area and information collected from noise measurements, a computer model is calibrated to estimate the existing noise levels and how much noise levels are likely to change for each alternative under consideration.

4. Decide the best approach to mitigate noise Should an impact be identified, there are many ways to mitigate roadway noise. Most people are aware of the most visible one-noise barriers. However, noise barriers are just one of the tools that are used. During the design process, engineers also consider ways they can reduce noise through engineering.

Whenever transportation improvements are being made, the Authority considers noise barriers for existing properties along the road if future noise levels approach or exceed 67 dBA or increase more than 10 dBA.

Noise barriers

• can reduce noise levels for residences located next to highways
• are most effective within 200 feet of a highway
• can reduce the perceived loudness of highway traffic noise by as much as half
• do not completely block all traffic noise
• must be tall and long with no openings

What are Noise Barriers?
Noise barriers are solid obstructions constructed between the highway and homes along a highway. They can reduce perceived noise levels by as much as half but cannot completely eliminate highway traffic noise. Typical reduction in noise levels with the use of a noise barrier is about 5 to 10 decibels (dBA).

What materials are used for Noise Barriers?
Barriers can be formed from earth mounds landscaped with attractive plantings (known as "berms"); from high, vertical walls; or from a combination of earth berms and walls. Earth berms have a very natural appearance and can reduce noise by approximately 3 dBA more than vertical walls of equal height. However, an earth berm requires more space to construct than a wall of equal height. Walls are usually limited to 26 feet in height for structural and aesthetic reasons.

Noise walls may be constructed from concrete, masonry, wood, metal, and other materials. To effectively reduce sound transmission through the barrier, the material chosen must be rigid and sufficiently dense. All noise barrier material types are equally effective, acoustically, if they have sufficient density.

Are Noise Barriers Effective?
Traffic noise is loudest right at the roadway and decreases as you move away from the source. As the diagram below shows, two things will happen to sound waves traveling out from the roadway. Some of the sound waves will be absorbed by the noise barrier or reflected back to the roadway. Other sound waves will travel over the barrier, and can reach houses on the other side of the barriers.

Houses close to the barrier received the most protection from highway noise. Houses further from the barrier receive less protection from the barrier-although distance helps to reduce noise. Houses that are higher than the top of the noise barrier (within line of sight of the traffic) are not protected.

What is REASONABLE and FEASIBLE?

There are several factors that need to be taken into consideration before the Authority decides whether to install noise barriers. First it has to be determined that it is reasonable to install a noise barrier. If the answer is yes, then the Authority must decide if it is feasible to do so.

Is a noise barrier reasonable?

• Will noise levels increase by 10 dBA over existing noise levels?
• Will noise levels increase by at least 3 dBA over what future noise levels would be if the project weren't built?
• Do residents want a barrier? At least 75 percent of impacted homeowners must approve the noise barrier.
• Would the barrier impact recreation areas, parks, historic sites on or considered eligible for the National Register of Historic Places or wildlife areas?

• Will a barrier reduce noise by 7 to 10 dBA?
• Can the wall be built? Is it feasible from an engineering standpoint? Generally, noise barriers are no higher than 26' tall.
• What is the cost of the barrier per benefited residence? Currently, the Authority will construct a barrier if the cost is less than $50,000 per benefited residence.
• Will a barrier cause a safety problem such as blocking a driver's view around a curve?

For More Information
Maryland Transportation Authority
2310 Broening Highway
Baltimore, MD 21224
(410) 537-1029 (voice)
(410) 537-1027 (fax)
www.mdtransportationauthority.com