I have created another map of Seattle’s population density, using units of gross households/acre. This a measure of the built environment rather than of population, since household sizes can vary with time and between neighborhoods (maybe I’ll make a map of that also). Net housing units/acre is a commonly used measure in land use planning and zoning, which excludes roadways and non-developable land from the area calculation. This map is of gross density, incorporating all land, resulting in at least 40% lower density values.
Only a few hotspots, such as Capitol Hill and Belltown, exhibit greater than 50 units/acre gross (red on the map). At this level of household density most trips can be taken on foot or via mass transit. The controversial state bill promoted by Futurewise would have mandated zoning average >50 units/acre net around high capacity transit stations. A broader section of Seattle’s core has a continuous household density greater than 10 units/acre. This level of households is associated with robust local bus transit combined with lots of trips via foot or bicycle. In fact, the orange area on this map corresponds well with my personal experience of the zone in which you can catch a bus on a moment’s notice to go to the next neighborhood, without extensive pre-planning or OneBusAway use. You are always within two blocks of a bus stop, and a bus is usually coming within a few minutes.
There is a wider swathe of Seattle with 5 to 10 units/acre gross, which generally represent the portions of the city with sidewalks. While predominately single family homes, these neighborhoods were developed in an era of streetcars and small lots. Unlike central and north Seattle, the south half of the city is rather inconsistently developed, with a patchwork of low, medium and highly developed areas.