(by Robert Butera, PhD, PE)
Background: The proposed “airport compatibility overlay district” (ACOD) in 2001 for neighborhoods around PDK airport was based on extrapolations from a 1997 environmental impact study of what the noise environment could be in the future. I have asserted in the past that the map was based on some flawed assumptions, and that the flaws in this map are revealed by the county's own noise monitoring data released in their monthly noise reports – see this page for more info.
Below is the ACOD map as proposed in 2001. The 3 colored regions represent the 65, 70, and 75 DNL lines, respectively. People in these regions would have lost some or all of their property rights under the proposed ACOD. The much larger black line was the 60 DNL line – the county also proposed some restrictions on residences in this much larger area, though there is (to my knowledge) no municipality in the country that has adopted a districting policy based on a 60 DNL measurement. The flags (added by me) show the location of the noise monitors for the Noise Monitoring System.
Below is a Noise Exposure Map presented by PDK Airport Director
Lee Remmel to the Dekalb County Commissioners on 16 November 2004.
This map was based on a fleet mix for an “average day” in
2004. I did not attend the presentation, but this data was based upon
assumptions apparently derived from a “Base Year.”
Below you will see the two maps overlaid. I only included the ACOD map for the neighborhood south of the airport for technical computer reasons (the ACOD map and 2004 map actually match pretty well north of the airport). I adjusted the
translucence so you could see the contour lines of both maps: use the
ACOD map above to get an idea of where the neighborhoods lie. While
it is hard to make out details of specific streets, pay attention to
the DNL lines for each map. The ACOD map shows 60 (black line) and
65/70/75 (increasing shades of red). The 2004 data shows DNL lines
for 55/60/65/70/75. The 75 and 70 DNL 2004 lines are similar to the
ACOD map. The 60 and 65 DNL lines are similar to the ACOD map north
of the airport, but there are notable differences south of the
airport. Look how far westward the 55 DNL line extends – past the Dekalb county line to what looks like Roxborough road (that
big development just west of that point is Phipp's Plaza).
For my neighbors in Drew Valley and others living south of the airport, the I've included a close-up
below. Compare the ACOD 65 DNL line (area shaded in red) with the actual 65
DNL line (light blue). Compare the ACOD 60 DNL line (black) with the
actual 60 DNL line (purple). The 55 DNL on the 2004 map is orange.
Here is what the 2004 noise exposure map shows. The 65 DNL zone barely
(if at all) hits Drew Valley. The 60 DNL zone covers about a fraction
of the neighborhood, roughly those on Drew Valley closest to Buford
Highway, Burch Circle, Skyland Trail, and parts of Cove Circle and
Winona Forest closest to the airport. The 55 DNL zone covers all of
Drew Valley.
How will this affect any future proposed policy in Dekalb county? One can assume that if such a policy like the ACOD is proposed again, it will be based on recent data like the above, and one could expect these lines to “grow” as air traffic increases in the future. The county included a “slide” showing such extrapolated data to the year 2009 in their presentation on November 16, unfortunately the contours shown for 2009 were identical to 2004 (an oversight, I hope). Hopefully we can obtain an updated graphic from the county.