HIGHWAYS
67 dBA outside the home
For federal highway projects, homes count as impacted when predicted noise approaches or exceeds 67 dBA as an hourly average outdoors, or rises substantially over today's level.
Compare levelsWHAT THE NOISE RULES SAY
Quiet is
a health matter.
Traffic noise is more than a nuisance.
Check a noise source
65 dBFAA line for aircraft noise
- Night counts most
- Levels are outdoors
- Windows make a difference
- Know the numbers behind the noise
Federal agencies use noise levels to decide when highways and airports need to act, and the World Health Organization sets stricter health guidelines. A federal map estimates that almost one in four Americans live with transport noise above 45 dB.
Updated
THE SHORT ANSWER
Different sources,
different limits.
US rules decide when agencies act.
The WHO guidelines are stricter, and aimed at health.
AIRCRAFT
DNL 65, or WHO 45
The FAA treats DNL 65 dB as the threshold of significant noise exposure for homes. The WHO's health guideline for aircraft is far lower: 45 dB Lden, and 40 dB at night.
See the numbers
AT HOME
A quiet side helps
These levels are outdoor figures. Closed windows cut the level indoors, and studies the WHO reviewed link a quiet side to the home with less trouble falling asleep.
What you can doA QUICK LEVEL CHECK
Which rule,
which level?
Pick a rule or guideline to see the noise level it uses. The bar shows how strict that level is compared with the others.
Levels are averages outdoors, over an hour (Leq) or a whole day (DNL, Lden), so they are not directly comparable. US rules decide when agencies act; the WHO values are health guidelines. General information, not medical advice.
TWO NUMBERS WORTH KNOWING
Common outside,
softer inside.
One figure on how many Americans are exposed,
one on what a window does.
23%
Americans potentially exposed above 45 dB
The Bureau of Transportation Statistics estimates that 74.7 million people, 23.1% of the US population in 2018, were potentially exposed to aviation and highway noise of 45 dB or more as a 24-hour average.
EXPOSURE · BTS 2018 DATA
≈ 25 dB
Typical drop indoors with windows closed
The WHO says the difference between outdoor and indoor levels is usually estimated at around 10 dB with windows open, 15 dB tilted or half-open and about 25 dB closed.
WINDOWS · WHO
The BTS figure counts noise at about the level of a quiet office, and comes from modeling rather than measurement. BTS says the map is for tracking trends, not for regulatory decisions.
FOUR THINGS TO TRY
A quieter night,
one room at a time.
You cannot move a highway, but how you use your home changes how much noise reaches you, especially at night, when DNL and the WHO guidelines both weigh noise most.
Sleep on the quiet side
If one side of your home faces away from the road, railroad or flight path, consider using that room as the bedroom. Studies link a quiet side with less difficulty falling asleep.
Close the window at night
A closed window typically cuts the level indoors by about 25 dB, against about 10 dB when open, according to the WHO.
Keep the bedroom quiet, cool and dark
That is the National Heart, Lung, and Blood Institute's advice for healthy sleep; a dim night light is fine if you need one.
Look at the noise maps
The National Transportation Noise Map shows modeled aviation, highway and rail noise across the country. Near an airport, its noise exposure maps show DNL contours over the surrounding land.
HOW THE US MAPS NOISE
Modeled,
not measured.
The Bureau of Transportation Statistics builds the National Transportation Noise Map with simplified noise modeling from FHWA, FRA and FAA data. The 2018 update added passenger rail to aviation and highways. BTS says it is meant for tracking trends, not for assessing impact or making regulatory decisions.
Airports have their own maps. Under the FAA's Part 150 program, noise exposure maps show DNL contours, lines of equal noise, laid over the land uses around the airport, and DNL 65 dB marks where homes stop being considered compatible.
Questions
Questions people ask
What does DNL mean?
Day-night average sound level: a 24-hour average for the average day of the year, with a 10 dB penalty added to each aircraft operation between 10 p.m. and 7 a.m. It is the FAA's standard metric for aviation noise.
Are these levels measured inside or outside?
Outside. The FHWA criteria for homes are evaluated at the exterior, and the WHO levels refer to the most exposed wall of a home, outdoors. The WHO did not set indoor guideline values.
Why is the WHO aircraft level so much lower than the FAA's?
They answer different questions. The FAA uses DNL 65 dB to judge whether homes are compatible with airport noise; the WHO set 45 dB Lden from evidence that aircraft noise above it is linked to adverse health effects.
How many Americans are affected?
The Bureau of Transportation Statistics estimates 74.7 million people, 23.1% of the 2018 population, were potentially exposed to aviation and highway noise of 45 dB or more as a 24-hour average.
Does noise really affect sleep?
The WHO links night-time road, rail and aircraft noise above its levels with adverse effects on sleep. The National Heart, Lung, and Blood Institute advises keeping the bedroom quiet, cool and dark.
Where the facts come from
- FAA: Community Response to Noise
- FAA: Environmental Desk Reference, Chapter 17 (Noise)
- eCFR: 23 CFR Part 772, Procedures for Abatement of Highway Traffic Noise
- govinfo: 24 CFR 51.104, HUD noise standards
- Bureau of Transportation Statistics: Under a quarter of the population exposed to office-type transportation noise (2020)
- WHO Regional Office for Europe: Environmental Noise Guidelines for the European Region (2018)
- NHLBI: Healthy sleep habits