A couple months ago we told you how ADOT works to minimize the noise impact a new freeway has on surrounding neighborhoods.

Well, similar efforts also are made when it comes to air quality …

ADOT is committed to improve air quality by making sure all projects comply with federal, state and local air quality laws and regulations.

That means ADOT’s Environmental Planning Group evaluates every ADOT transportation project to ensure air quality standards will be maintained.

ADOT Noise and Air Specialist Fred Garcia says smaller projects go through a qualitative review by his team to determine if the road work will have any impact on air quality. Larger projects (ones that increase traffic capacity or that have changes in the vertical and/or horizontal alignment) will go through a much more extensive quantitative analysis.

That quantitative analysis includes a sophisticated computer modeling process that helps predict localized air pollution concentrations in neighborhoods near the project.

According to Garcia, ambient background measurements must be taken first. ADOT gets these figures from either Maricopa County or from the Arizona Department of Environmental Quality.

That baseline number helps the modeling program generate an emissions report. From there, other conditions are entered into the program, including vehicle counts, atmospheric conditions and emission dispersion rates.

The program produces a report that can tell ADOT what the concentration of carbon monoxide will measure at different distances away from the future freeway.  The program is so sophisticated that it even takes the future design of cars into account. In coming years, vehicle manufacturers are required to make cleaner and more efficient engines … the computer program can factor that into its predictions.

Once the quantitative analysis is complete, ADOT can tell if air quality predictions will meet standards set by the U.S. Environmental Protection Agency. If standards are met, the project can move forward. If not, the project is modified to help meet those standards.

Transportation isn’t the only factor contributing to air pollution. Construction, manufacturing, and even wind, weather and Arizona’s terrain play a role. However, there are things everyone can do to help minimize their impact, including carpooling, using mass transit and driving less during rush hour.

For more on air quality and ADOT’s Environmental Planning Group, visit their webpage.

Most people don’t find the sound of traffic especially soothing …

ADOT certainly recognizes this and is constantly working to minimize the impacts to surrounding neighborhoods from the noise of new freeways.

You would think figuring out the impact of noise could get tricky because everyone perceives it differently – a sound that’s irritating to you might be tolerable to someone else.

But, federal law requires that ADOT estimate the future noise levels from new freeway projects, and if traffic noise approaches or exceeds 67 decibels, noise abatement must be considered. ADOT has taken it a step further, though, and considers 64 decibels to be the acceptable threshold to consider noise abatement for new projects.

The amount by which noise levels will change in a residential area also is a factor. A road project that will cause noise levels to increase by 15 decibels or more is considered an impact and noise abatement measures must be considered.

Just to give you an idea of what a decibel measurement means … a whisper could register at about 20 decibels, normal conversation comes in around 60 decibels and if you were standing about 15 feet from a loud rock band, you’d be hearing sounds measuring roughly 130 decibels.

Now you might be asking – how do you figure how noisy a new road will be before it’s even built?

Well, experts from ADOT’s Environmental Planning Group use sound pressure level meters to take readings of current noise levels in several areas near a proposed project site.

Measurements aren’t taken during rush hour because traffic is typically a little slower and quieter. Measurements are instead taken at the peak traffic noise hour, when traffic is free-flowing and cars are able to drive the speed limit, but the road is at maximum capacity.

Those measurements and the area’s topographic and weather information are plugged in to a very sophisticated computer program. It creates a model of the area and can tell ADOT what noise levels to expect if a new freeway is built, or an existing freeway is widened.

If it is determined a project will cause a noise impact to the area, ADOT has a few options, including:

• Install noise walls or berms
• Implement traffic control measures (speed limits, etc.)
• Modify the proposed freeway alignment

On top of everything, the abatement method chosen has to meet multiple criteria showing that it is reasonable and feasible … a noise wall isn’t reasonable if the residents of the neighborhood don’t want it or it is cost prohibitive and it isn’t feasible if it won’t effectively reduce noise levels.

A little more on noise walls …

Barriers, such as concrete noise walls, reduce traffic noise by blocking the path that sound waves travel from the traffic to the residential area.

When sound waves hit the noise wall, they must bend over and around it. This is known as diffraction, which absorbs the sound energy, thereby reducing noise. For every decibel of reduction, it is necessary to increase the height of the wall by two feet. Noise walls are most effective when the distance between the source and the receiver is no more than 400-500 feet.

ADOT and the Federal Highway Administration (FHWA) are currently studying the use of rubberized asphalt or “quiet pavement,” which can noticeably reduce traffic noise levels. When ADOT/FHWA completes the study, the results will determine whether quiet pavement can be used for noise abatement.

While no analysis, wall or special pavement can completely eliminate traffic noise, ADOT does have one of the most progressive noise abatement policies in the nation and that is something the citizens of Arizona may find soothing.