New Urbanism, sprawl, and density

Density is a crucial factor when measuring the environmental impact of development, because it affects land consumption, stormwater runoff, automobile usage, and transit usage. There is much misunderstanding on this subject, mostly because density is defined in various ways. The bottom line is this: the New Urbanism (NU) is significantly more compact on average than conventional suburban development (CSD), and comparable in density to historic urban neighborhoods. Density figures are given in three forms. Gross density for developed areas includes all kinds of residential units, commercial land, office/industrial uses, civic/institutional uses, and open space of various kinds. This calculation is easy (just divide total units into total land area), but may have little meaning, because it often includes many kinds of urban forms. Gross density for specific projects usually includes residential land, roads, and easements in the denominator. This calculation is easy for single-use CSD, but difficult for NU, which includes all kinds of uses in individual projects. Net density includes just the land used for residential units in the denominator, and this is most commonly understood meaning of the term. Net density is also the best way to compare density on the scale of individual projects, according to Eliot Allen of Criterion, an expert in environmental analysis of development. Comparing NU and CSD Of the several hundred projects that Criterion has measured, net densities in CSD commonly fall in the range of four to seven units/acre. Net NU densities commonly fall in the 15 to 30 units/acre range — that is to say, about four times higher than CSD. These are average figures — individual CSD, NU, or traditional urban places can be higher or lower. Another way to compare NU and CSD is to look at gross densities for developed areas. According to researcher Rutherford Platt of the University of Massachusetts, gross densities for urbanized areas in the US averaged three units/acre in 1920. Since 1960, all development has averaged less than one unit/acre, and this figure has been trending downward. According to a recent article in the Christian Science Monitor, development since 1982 has averaged less than 0.5 units/acre. When measured in this fashion, new urbanist projects yield approximately three units/acre — similar to 1920. Densities are not constant throughout the urban-rural Transect. In the Core, one would expect net densities ranging from 50 to more than 100 units/acre. A town center will likely yield net densities ranging from 20 to 75 units/acre. The Core and the Center require densities that will support retail, a lively street life, and mass transit. In the neighborhood General areas, net densities likely will range from six to 25 units/acre. Net densities in the neighborhood Edge may range from 3 to 8 units/acre. In an interconnected neighborhood, lower densities in the Edge are balanced by higher densities in the more urban tiers of the Transect. One final note: high density is no substitute for good urban design. A high-rise apartment building located on a suburban arterial road will not reduce automotive use if there are no walkable destinations. And, dense modern public housing projects failed because of their lack of diversity, disconnection from surrounding neighborhoods, and general poor design. As planner Andres Duany notes, good street life is the compensation for high density. Without a walkable, rewarding public realm, high density may mean low quality of life.