By 1879, the date of the Taylor map on the historical slider, most of Manhattan was densely covered with masonry structures of three to six stories – the majority of which were residential, either row houses or tenements. Almost all of these buildings were built on private land as properties to rent, or sell at a profit.

As the slider shows, most neighborhood blocks changed little from the early 20th century through the 1950s, when the first urban renewal projects began to bulldoze great swaths of the city. The touch screens allow you to explore details of the individual projects and to compare their density, both of population and lot coverage.

Optimal Density?

For more than a century housing reformers have been debating and regulating “bad density.“ But what is the measure of good density? Can we put numbers on the optimal lot coverage and persons per acre that create a vital neighborhood?

In the 1960s, critics of contemporary urban renewal Jane Jacobs and Lawrence Halprin offered their answers. In Death and Life of Great American Cities in the chapter “The Need for Concentration,” Jacobs summarized her detailed analysis of the qualities of good neighborhoods to settle, reluctantly, on a number. She concluded that, with a mixture of uses and building types as a necessary part of the equation, 500 people per acre constituted optimal density.

Landscape architect Lawrence Halprin, who in 1968 authored a critique of recent urban renewal projects for the Lindsay Administration, titled New York, New York, proffered a redesign of the Penn South project that added retail and residential buildings to the site, raising the density to 500 to 600 people per acre.

What do you think is good density? The diagrams you can compare on the interface give the numbers for population and built density for the projects featured in the exhibition. But look at the models and the images around you to decide what buildings and districts seem the most appealing.

Think about how and where you’d like to live, but also consider that density is about many people. How should the city think about density so that the scarce resource of valuable urban land is best used for all its citizens?

Chelsea District Model

This district model of Chelsea illustrates strongly contrasting density strategies of private-market and publicly-assisted housing in this now fashionable area on Manhattan’s west side. The gray blocks of low- or medium-rise buildings that remain today are mostly tenements, row houses, commercial, or industrial structures like those that once covered the area.

Highlighted in white are the big redevelopment projects that reshaped Chelsea over the past century. The oldest is London Terrace, which when completed in 1930, was the densest building block in the city. Privately built by the real estate entrepreneur Henry Mandel, the fourteen contiguous buildings of 17 to 19 stories created a population density that equaled the most crowded tenements of the Lower East Side and today numbers 931 people per acre. The lot coverage was high at 68 percent, but a landscaped interior court afforded light, and cross ventilation. Two recent market-rate rentals, The Tate (2002) and 555 W 23 (2005), frame a block west of Tenth Avenue and pack apartments into 14-story structures, creating densities of 775 and 800 pp/acre, respectively, by taking advantage of special zoning incentives implemented for the High Line district.

The other major projects highlighted in the model are public, or publicly-assisted housing. The western blocks of cruciform and slab towers set in ample open space are NYCHA’s Elliott (1947) and Chelsea Houses (1954), which both have densities of less than 300 pp/acre and cover only about one-fifth of their land with buildings.

The expansive open space at the center of the model is the superblock of Penn South (1962), a limited-equity housing cooperative housing project that de-mapped cross streets between 23rd and 29th streets and Eighth and Ninth avenues. Even though Penn South's towers rise to 22 stories, the low lot coverage of 20 percent yields an average per person density across the site of just 302 pp/ acre – one third the population density of London Terrace.

The underlying transparent yellow acrylic indicates blocks that are under-built in relation to the zoning law implemented in 1961 and revised in subsequent decades. The transparent red acrylic shows blocks that are overbuilt according to contemporary zoning.