“Most blocks must be short; that is, streets and opportunities to turn corners must be frequent.”
-The Death and Life of Great American Cities; Chapter 9, “The need for small blocks”.
It’s a little bit tricky to define precisely what it means for a block to be short. It appears that Jacobs’s intended metric is not so much the actual size of blocks, as the frequency of street intersections, and the number of sensible routes between various points A and B.
Creating an Index
Measuring the number of sensible routes between various points is a time-consuming task. For now let us focus solely on the frequency of street intersections, and use that as our metric for satisfying Jane Jacobs’s Condition 2.
Here I compare 4 cityscapes at the same scale, and thus each map has the same area.
Let us give a weighted value to each intersection, depending on how many streets intersect there: value = number of streets – 2. Then a three-way or “T” intersection will have a value of 1, and a four-way intersection will have a value of 2. This way, ambiguity about whether streets meet at a single 4-way or adjacent 3-way intersections won’t affect the result.
Short Block Index: 5 (2 four-way, 1 three-way)
Central Kobe, near City Tower
Short Block Index: 13 (5 four-way, 3 three-way)
Short Block Index: 18 (1 five-way, 1 four-way, 13 three-way)
Short Block Index: 9 (4 four-way, 1 three-way)
Summary of Results
Area :: Short Block Index
Kobe, Kitano-Cho :: 18
Kobe, City Tower area :: 13
Minneapolis, downtown :: 9
Berkeley :: 5