The U.S. Department of Transportation’s Bureau of Transportation Statistics (BTS ) released its first National Transportation Noise Map last week. Data represents weighted 24-hour equivalent sound levels and will be used to track trends in transportation-related noise from aviation and highways. Rail and maritime noise sources are planned for future versions of the map.

According to the study, more than 97 percent of the U.S. population may be exposed to highway and aviation noise at levels below 50 decibels (equivalent to a refrigerator humming), while 10 percent experience noise levels greater than 80 decibels or more (equivalent to a garbage disposal or lawn mower).

The areas of central North Carolina with the greatest amount of transportation noise are, unsurprisingly, near the Raleigh–Durham International Airport (RDU).

The southwest-northeast orientation of RDU's parallel runways is prominent in the noise map with a dog bone shape developing as departing aircraft turn north and south over Morrisville and the Wake-Durham County line near Falls Lake.

According to the map, combined aviation and highway noise levels drop to below that of conversational speech (~60 dBA) north of Interstate 540. To the southwest, noise levels drop off to that of a quiet office (~50 dBA) by N.C. Highway 54 in Morrisville.

Airport noise is concentrated near the ends of the runways, where departing aircraft throttle up and landing aircraft deploy thrust reversers to reduce speed, at average levels exceeding 80 dBA. At the midpoint of the runway, where terminals and other services are concentrated, noise levels average out to that of a vacuum cleaner (~75 dBA). Doctors warn against habitual exposure to noise levels above 85 dBA.

But RDU isn't the only noise source in the area. BTS data also shows noise levels along Interstate 40 approach that of RDU’s runways.

I-40 between exit 285, Harrison Avenue, and exit 284, Airport Boulevard, is among the noisiest in the area according to the map. This stretch of road also coincides with an interesting part of North Carolina geology and topology.

If you’ve ever tried to drive this stretch of road in the winter, you know of the rise in elevation between Lake Crabtree and the Harrison Avenue exit. At the base of that rise is the old, inactive Jonesboro fault line. I-40 drops 130 feet and doesn’t regain that elevation until the road begins to rise in elevation again near exit 270, 15-501 in Chapel Hill.

The bottom of that hill at Lake Crabtree also marks a change in the general makeup of the bedrock from Wake County’s largely metamorphic rock, which has been changed by heat and pressure over time, to Durham and Orange Country’s more sedimentary rocks, made up of pieces of other rocks.