This map shows modeled outdoor noise across Johnston at 100-meter resolution, combining road, aviation, and rail sources. Green areas measure below 45 dBA. Orange and red exceed the EPA's 55 dBA outdoor threshold linked to long-term health effects. Use the layer toggles to view each source on its own or all together.
What the numbers sound like
- 30 dBAWhisper
- 40 dBASoft rainfall
- 45 dBAQuiet suburban street at night
- 50 dBAQuiet office
- 55 dBAEPA outdoor threshold: light traffic 100 ft away
- 60 dBANormal conversation an arm's length away
- 65 dBABusy restaurant
- 70 dBAHighway traffic 50 ft away
- 80 dBACity bus interior
Population Above the EPA Outdoor Threshold
The EPA's 55 dBA outdoor reference level is a common benchmark for residential noise exposure, especially for activity interference, annoyance, and long-term community noise concerns. About 5,669 Johnston residents, or 25.0%, live above that level. By land area, 28.4% of Johnston is above 55 dBA.
See how noise in Johnston compares to similar-sized cities.
Noise by Part of Johnston
Average noise levels for Johnston residents, grouped by direction from the center of Johnston. Central Johnston carries the highest population-weighted average; Northern Johnston carries the lowest. Just 13% of residents in Northern Johnston live in blocks above the EPA's 55 dBA threshold, roughly the same as the share in Central Johnston.
Central Johnston
15% of people above 55 dBA
Eastern Johnston
27% of people above 55 dBA
Northern Johnston
13% of people above 55 dBA
Southern Johnston
43% of people above 55 dBA
Western Johnston
20% of people above 55 dBA
Central Johnston sounds about 35% louder than Northern Johnston to the human ear, a 4.3 dBA gap. Every 10 dBA roughly doubles perceived loudness. Within any of these directions, two homes a quarter mile apart can still differ by 10 or more dBA depending on how close they sit to a major highway.
Loudest Road Corridors
The model evaluates every road in Johnston using federal traffic counts, posted speeds, heavy-truck ratios, and pavement type. The source level shown is the modeled noise at the road centerline, where it is loudest. Noise drops with distance, faster in vegetated areas and slower over open pavement.
How far back from I-35 do you need to be?
I-35 produces an estimated 76 dBA at its loudest centerline points. Noise drops logarithmically with distance, with the exact rate depending on what's between you and the road. Tree cover, walls, terrain, and pavement type all matter. At roughly a quarter mile back, traffic fades into the noise level of a soft rainfall.
Calculated from the model's calibrated attenuation formula. About 13% of Johnston sits under tree canopy (lighter than most cities) and roughly 27% is impervious surface like pavement and rooftops. Both are folded into the per-place decay rate above. Heavier canopy pulls noise down faster with distance; impervious surfaces slow the drop.
How Noise Is Distributed Across Johnston
The bar chart below shows the share of Johnston residents in each noise band. About 62% of residents live below the EPA's 55 dBA threshold, and roughly 3% live in blocks above 60 dBA. Long-term exposure in that range is linked to elevated stress hormones and cardiovascular risk.
How Johnston Compares
Johnston sits at the quieter end of the spectrum. Below: how Johnston's average outdoor noise and share of residents above the EPA threshold compare with Waukee, Clive, Grimes, and Altoona.
Average noise level (dBA)
Johnston's 52.9 dBA pop-weighted average is at the quieter end of the spectrum. Iowa as a whole averages 52.2 dBA and the U.S. averages 52.0 dBA. Both are lower than Johnston because most of either area is rural land away from major roads.
Share of residents above 55 dBA
About 25.0% of Johnston residents live in blocks where outdoor levels exceed the EPA's 55 dBA threshold. That's in the middle of its peer group. Measured by land area instead, 28.4% of Johnston's footprint sits above 55 dBA, against a Iowa average of 23.6% and a national average of 28.1%.
What This Means if You're Moving to Johnston
- Distance from highways matters more than the neighborhood name. Two homes in the same zip code can differ by 20 dBA if one sits 100 meters from I-35 and the other 500 meters away. The model captures this at 100-meter resolution, so noise exposure changes block by block.
- Tree canopy can help reduce modeled noise exposure. Roughly 13% of Johnston is under tree cover (lighter than most cities), and the dominant land cover is low-intensity developed land. Both are measured from federal USDA Forest Service and USGS satellite imagery at 30-meter resolution. Streets with 60% or higher canopy show 3 to 5 dBA lower noise than comparable streets with bare ground or pavement, which is why the per-place decay rate above already accounts for it.
Sources & Methodology
The BestNeighborhood noise model is calibrated against nearly one million federal ground-truth measurements across four states. Road noise is computed from segment-level federal traffic data and propagated outward using physics-based acoustic decay, with attenuation rates that depend on the surrounding land cover.
Federal datasets used:
FHWA Highway Performance Monitoring System: road geometry, traffic counts, lane configuration
U.S. DoT Bureau of Transportation Statistics National Transportation Noise Map: aviation and rail noise, road calibration ground truth
USGS / MRLC National Land Cover Database: land cover and impervious surface coverage
USDA Forest Service Tree Canopy Cover: vegetation density for sound propagation
U.S. Census Bureau TIGER/Line: block-level geography and population
U.S. EPA Levels Document: 55 dBA outdoor reference level
All inputs are published federal datasets. Block-level noise is computed by combining road, rail, and aviation sound sources in the energy domain, the same physics used in professional environmental noise assessments. Read the full methodology.