How it's made
Quiet LA is a computer model, not a set of microphones. For every street in the mapped area, it takes how many vehicles pass, how fast they go and how many are trucks. It then works out how that sound travels: out from the road, over the ground, around every building, over hills and past freeway sound walls. Trains are added on the Valley main lines. For the 24-hour view, aircraft are estimated from the official airport noise contours.
We checked the model against 564 public measurements around the county. On average it is within about half a decibel. At any single spot it is typically off by about 6 dB, more or less, in either direction. Treat each number as a careful estimate for planning, not a reading from an instrument. Blank areas are not modeled yet, and blank never means quiet.
More detail
The physics
We use NoiseModelling 6, open-source software that implements CNOSSOS-EU, the European Union’s common method for mapping road, rail and aircraft noise. It calculates how sound spreads, how the ground absorbs it and how it bends over roofs, hills and walls. Every building wall is an obstacle and every road a source. Reflections between buildings are not included yet.
Roads and traffic
Every Census road is included. Traffic counts come from Caltrans for freeways and state highways, and from the Federal Highway Administration’s 2024 HPMS and the City of Los Angeles for other roads where they exist; elsewhere we use typical values for that kind of road. Evening traffic is set at 0.6 times, and night at 0.2 times, the daytime flow.
Ground and walls
Terrain comes from the 2023 USGS lidar scan at 10 m. Freeway sound walls found in the lidar and roads on bridges at deck height are included. Buildings come from LA County footprints and heights.
Open land
Parks, mountains and desert get a coarse estimate from freeways, ramps and major roads only, on a 100 m grid with no buildings and no local streets. Where the detailed model has a value, it always wins. Land with no freeway or major road within 3 km has no computed value and shows the lowest color; that is an assumption, not a measurement.
Distant traffic hum
The detailed model looks 1.5 km out from each road. Traffic farther away adds a low hum, so a coarser calculation of freeways and major roads out to 8 km, reduced for the buildings in between, is added to the map colors. It is not in the values for a single address, and it is the least certain part of the map.
Trains
Metrolink, Amtrak and Union Pacific freight on the Ventura and Valley main lines, from published schedules and federal crossing counts. Horns at public crossings are included; other lines are not included yet.
Aircraft
For the 24 h view, aircraft are estimated from the official airport contours, extended down to 55 CNEL where official maps stop at 65. Compared with 57 permanent airport monitors, the estimate is within about 3 dB on average. It reads low far out along some approach paths.
How well it matches
We compared the model with 564 public measurements. About 160 had a 24-hour value and a usable map position. The model is within about 0.5 dB of them on average (24 h level). A typical single site is off by about 6 dB; part of that comes from not knowing exactly where each microphone stood. Near freeways the model runs about 4 dB loud on average, with a wide spread, and for arterials the pattern differs by measurement year. We have not tuned the model to fix this: the evidence does not yet support one simple correction.
Not in the map yet
- Construction, events, nightlife, neighbors, helicopters and people.
- Weather: results use typical conditions.