Density per net acre

After this discussion about 'The need for concentration' I found myself wondering about Jacobs method of measuring density in districts. She uses a metric that almost nobody else uses: dwelling or housing units per net residential acre, which counts only land designated for housing. Typical densities are expressed in terms of persons per square mile, counting all types of land. But this can include land that's really devoted to highways, parking lots, institutions, parks, or other places where people do not live. This has a tendency to skew the numbers because most boundaries are drawn to include arbitrary amounts of these non-residential uses. For example, Boston is said to have 12,752 people/sq mile, but residential neighborhoods such as Allston have a density of 18,505 people/sq mile and the North End is listed as nearly 30,000 people/sq mile, or more depending on how you draw the borders.

Thanks to a footnote in her book, we know that in 1960 the North End had a dwelling unit density of 275 per net acre of residential land, and Roxbury ranged between 21-40, according to planning commission measurements. But how does that compare to today, and to other neighborhoods? And what does it mean, anyway?

What are the proper densities for city dwellings? The answer to this is something like the answer Lincoln gave to the question, "How long should a man's legs be?" Long enough to reach the ground, Lincoln said. Just so, proper city dwelling densities are a matter of performance. They cannot be based on abstractions about the quantities of land that ideally should be allotted for so-and-so many people (living in some docile, imaginary society).

Still though, and perhaps somewhat uncharacteristically, she does attempt to give some hard numbers for what she feels is a reasonable categorization of different densities. Below ten dwelling units per net residential acre is typical for a "suburban" neighborhood. Densities from 10-20 indicate a kind of "semi-suburb" or a really dense suburb, but it cannot function well as part of a city. Then there is a range of "in-between" densities which are too dense to be a suburb, but not quite dense enough to generate necessary city vitality for safety and interest. According to her, this range seems to cover dwelling unit densities from above 20 to somewhere in the vicinity of 100 units per net acre. The exact numbers have to be determined on a case-by-case basis, asking yourself, "does this level of density produce true city life?" for each neighborhood.

With that in mind, I obtained both the Census 2010 data and the MassGIS Land Use 2005 survey. Using a PostgreSQL+PostGIS database I put together a series of queries to produce a map of housing unit densities per net residential acre for Boston. The query includes all land within a 10km radius of Park Street station. Then I uploaded it into a Google Fusion table and colored the areas using a scheme based upon her categories. I've adjusted the numbers somewhat though, for present-day Boston. In fifty years, I think that the way people live and their personal consumption patterns have changed considerably. In the interest of not spending a whole lot of time on this, I've settled on some choices which probably won't make everyone happy, but hopefully will be somewhat useful.

	1-10	Suburban
	10-20	Semi-suburban
	20-60	"In-between"
	60-80	Borderline urban
	80-100	Low density urban
	100-300	Urban
	300+	Extremely high density

The biggest caveat with this data is that one set is from 2005 and the other from 2010. I've already spotted one development on Morton Street that was grassland five years ago. Another problem is that the land use data was semi-automatically obtained using imagery from a surveying company, so it is not precise. It appears there are some spaces marked residential which are not, and vice versa. There are some missing zones, too. These problems seem to be relatively rare, but it is always a good idea to take a closer look at any numbers that seem off.

By this scale, the following neighborhoods include a piece of land that is settled at "urban densities:"

• Allston
• Back Bay
• Bay Village
• Beacon Hill
• Brookline Village
• Bunker Hill, Charlestown
• Central Square, Cambridge
• Chinatown/Leather District
• Cleveland Circle
• East Cambridge
• Fenway
• Kenmore Square
• Longwood
• Neighborhood Nine, Cambridge
• North End
• South End
• South Boston near W. Broadway

The North End seems to have dwelling unit densities ranging from 90-180 on this map, which is down from the estimate of 275 in 1960. On the other hand, Roxbury seems to fall pretty neatly into the category of 21-40 units per net acre even after all these years. Looking further, nearly all of the formerly separate towns that were annexed onto Boston still retain their suburban or semi-suburban density levels, the main exception being Brighton.

A lot of this list comes as no surprise. Everyone knows Beacon Hill has lots of homes. Actually, I expected to see more neighborhoods on it, based on "feel" which apparently turned out to be misleading. I thought there would be more density in East Boston, South Boston, Mission Hill, Somerville, parts of Roxbury, and more of Brighton. Especially since Somerville is the most densely populated city in New England (persons/sq mile). But on this map, it is almost entirely semi-suburban or "in-between."

So does this mean anything? Not necessarily. It is just one factor to be considered, among many. But it is potentially useful to get a feel for which parts of Boston have true density, and which parts might lack city diversity and vitality.