> ## Content Index > Fetch the complete content index at: https://airport.mmalc.com/llms.txt > Use this file to discover other available public pages before exploring further. # The dispersal fallacy: mistaking more flight paths for a fair distribution of the noise - URL: https://airport.mmalc.com/dispersal-fallacy/ - Published: 2026-09-27T22:11:27.000Z - Updated: 2026-09-27T22:11:27.000Z - Description: Comparing flight paths means considering both how many people would hear the aircraft and how much they would be disturbed. The same jet can have a very different impact in a quiet valley than near a freeway. - Author: Malcolm Crawford - Tags: noise, airport, sonoma Spreading aircraft noise across more neighborhoods sounds fair, and it's become a recurring theme in the debate over jet noise in western Sonoma County. [A recent newspaper column](https://www.pressdemocrat.com/2026/08/25/close-to-home-complaints-about-airplane-noise-have-taken-off-lately-in-sonoma-county/?ref=airport.mmalc.com) is one example, proposing that the noise be dispersed across a wide landscape. Across the Mayacamas, Napa County is weighing a version of the same idea for its own airport. Spreading flights can be part of a fair arrangement. But judging any plan means looking at both how many people would hear the aircraft and how much the noise would disturb them. Neither more routes nor fewer people beneath them guarantees fairness: a plan can still make a small number of people bear a disproportionate share of the harm. Treating a spread of routes as proof of a fair share is the *dispersal fallacy*. It matters because the same jet is a very different burden in different places. A jet climbing out of the airport is one noise among several near a freeway; over a quiet valley, it can be the loudest thing in earshot. ## In a quiet valley, the same jet stands out more and can stay noticeable longer Take a departure that measures about 60 [decibels](https://airport.mmalc.com/glossary/) on the ground beneath it, roughly the level of a conversation at close range. Suppose traffic keeps the daytime background in a neighborhood near Highway 101 at around 50 to 55 decibels. The jet is still clearly heard there, standing out above the traffic by about 5 to 10 decibels. Now suppose a quiet valley in West County sits at around 30 to 35 decibels on a still day. There, the same jet stands out by 25 to 30 decibels. The jet intrudes in both places. It stands out far more in the valley, because the valley was so quiet to begin with. California's environmental law already recognizes that the same noise can matter more in a quiet setting than in a noisy one. How long the jet stays noticeable matters too. A jet's noise rises as it approaches and fades as it leaves. Against a louder background, more of that approach and departure can blend into the surrounding sound. In a quiet valley, the jet can remain noticeable for longer. That may help explain why people across the county describe jet noise so differently. Someone near Highway 101 in Santa Rosa may experience a brief interruption before the sound fades back into the traffic. Someone in a quiet valley may hear the aircraft approaching and departing for much longer, with the jet dominating everything else as it passes overhead. [The same aircraft, a different impact](https://airport.mmalc.com/same-aircraft-different-impact/) explains both effects in more detail, along with what Sonoma County measured at this airport in 2011. ## Spreading flights doesn't necessarily mean less disruption overall Suppose 20 departures a day follow one route, and they're split across five routes of four flights each. The community under the original route loses 16 flights a day. For the people there, that's a real improvement. The 16 flights haven't disappeared, though. Four other communities now hear four jets a day each, where they may have heard none before. If those places are quieter than the original route, and the jets pass over at similar heights, each new flight could cause more disturbance than a flight over the original route. On a route map, the new arrangement looks more even. No single route stands out. But the map shows only where the aircraft fly. It can't show how much each flight disturbs the people below, so it can't show whether the total disruption went up or down, or whether it's fairly shared. ## Napa's search for a new route raises the same issues In late 2024, a new approach into Napa's airport began directing aircraft over the Soscol Avenue area, with an expectation that their noise would blend into the traffic below. A year later, [Supervisor Belia Ramos said](https://www.pressdemocrat.com/2025/11/20/hail-mary-flight-path-proposal-emerges-as-napa-residents-demand-relief-from-jet-noise/?ref=airport.mmalc.com) that expectation hadn't worked out. That doesn't establish that quieter, less populated areas would be a fairer choice. It reinforces the point that the same aircraft noise can have very different effects depending on the surroundings. Soscol Avenue carries roughly a fifth of the traffic on Highway 101 through Santa Rosa; calling both places "busy" doesn't make them equivalent noise environments. A jet can stand out above street traffic and intrude still more sharply on a quiet rural setting. In November 2025, in response to persistent complaints about the noise, the county added $185,000 to a consultant's contract to develop an alternative approach for use in good weather. At a public meeting that month, a resident asked whether a second route would solve the problem or simply shift it elsewhere. That question remains relevant to the [three concepts the county presented in September 2026](https://www.pressdemocrat.com/2026/09/26/napa-county-holds-first-town-hall-meeting-on-alternative-flight-paths-meant-to-curb-noise-from-airport/?ref=airport.mmalc.com). All would send aircraft over less populated areas than the current path. Two would largely cross farmland, industrial land, freeways, and rural residential areas on the city's outskirts. Those settings could mean very different experiences for the people below. A comparable flyover could stand out more at a quiet rural home than at one near a freeway. ## A proposal to spread flights has to show its fairness Spreading flights can produce a fair arrangement, but each proposal needs to show what burden it would place on the people beneath its routes. A comparison should examine each affected community under each proposed arrangement: how many people would hear the aircraft, how often flights would pass, when they would come, how far their noise would rise above the background, and how long each interruption would last. It should also examine whether any community would bear a disproportionate share of the harm. More routes are a description of a map. A fair share is a claim about people, and it needs evidence about people. --- ## Notes - **"About twice as loud."** As a rule of thumb, every 10 decibels sounds about twice as loud. By that rule, the jet near the highway sounds up to about twice as loud as the traffic, while in the valley it sounds roughly six to eight times as loud as the background. The rule is an approximation that works best for the same sound at moderate levels. At levels as low as a quiet valley's, it probably understates the difference. - **Background levels.** The 30–35 and 50–55 decibel figures are illustrative of a quiet rural valley and a residential neighborhood near a highway, respectively. Right beside a busy highway the background is higher, but traffic noise falls off with distance and behind buildings, while a jet overhead reaches a wide area at much the same level. - **Busy roads differ.** Napa's Soscol Avenue carries roughly a fifth of the traffic on Highway 101 through Santa Rosa, so "a busy road" is too broad a description to settle a noise question. Caltrans's 2024 annual average daily counts, covering both directions, put the busiest state-highway sections of Soscol Avenue at about 26,500 vehicles a day, and Highway 101 between Hearn Avenue and Steele Lane at about 128,000 to 143,000 ([Caltrans traffic census](https://dot.ca.gov/programs/traffic-operations/census?ref=airport.mmalc.com)). At least five times the traffic makes roughly 7 decibels or more of extra road noise at the same distance, and the freeway's higher speeds usually add more. - **The number of flights is held fixed.** Everything in this piece concerns where flights go, not how many there are. *Research and drafting assistance: Claude (Anthropic).*