In The Death and Life of Great American Cities (1961), Jane Jacobs argued that lively streets require four conditions, all at once. This index computes a proxy for each condition for every tax block in New York City, from the Department of City Planning's PLUTO file (the Primary Land Use Tax Lot Output, version 26v1) and the Department of Finance digital tax map.
The entropy of each block's floor area across five categories — residential, office, retail, industrial (factory, garage, storage) and other commercial — normalized to 0–1. A block that is all one thing scores 0; an even mixture scores 1.
Mean block-face length, estimated as the block polygon's perimeter divided by four. Faces of 200 feet or shorter score 1; faces of 900 feet or longer score 0, linearly in between.
The share of dated building area built 40 or more years ago (1986 or earlier). The score rises with the old-building share, peaks at an even mingling of old and new, and declines gently to 0.7 for uniformly old blocks. All-new blocks score 0 — Jacobs's central warning.
Residential units per acre of lot area, capped at 100 dwelling units per acre — roughly where Jacobs put the end of "in-between" densities too thin for city life. Note she measured per net residential acre; this index divides by all lot area, which makes the score conservative.
The geometric mean of the four scores, times 100. Jacobs insisted the conditions are jointly necessary, so failing any one condition drags the composite sharply down — a block that scores zero on one condition cannot score above about 18 overall, however well it does on the rest. Blocks with under 10,000 square feet of building area, with no buildings that carry a construction year or without a matching tax-map polygon are left unscored.
Full methodology, code and data: METHODOLOGY.md. Sources: NYC Department of City Planning PLUTO; NYC Department of Finance digital tax map, via NYC Open Data. Computed July 2026.