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Saturday, November 13, 2010

Transit Density

While I've blogged elsewhere about the danger of using 'average density', averages are good for making back of the envelope calculations, which tell you which calculations are actually worth making. The danger lies in using the averages themselves to make the decisions.

Again, my hat is off to the people at We Alone on Earth

10^4 is 10,000 people per square kilometer. There are about 247 acres in a square kilometer, giving it an average density of about 40 people per acre. I'll call that 'Transit Density'. With an average household size of 2.5, that is about 16.2 units per acre. Historic 'Streetcar Suburbs' built out at an average density between 8-12 units an acre, about 5,000 square feet per lot. That is about as tightly as you can pack detached single-family units without turning them into townhouses or apartments. Ergo, ANY detached housing is unsuitable for Transit Oriented Development. That provides a very simple, and very useful metric for assessing developer submitted plans for a 'TOD'.

Also notable is the density 'cap', a level of density at which even NYC has very few people living. 10^5 is about 160 units per acre. What does that kind of density look like? How tall is it? What is considered 'maximum' human density varied by use:
  • Prison Design: 35 sq. ft/prisoner
  • Schoolchild: 50 sq. ft/child 
  • Home Design: 200 sq ft./inhabitant
  • Call Center: 100 sq ft/user
  • Class A Office: 600 sq. ft/person
  • Dorm Room: 180 sq. ft/person 
  • Average 1950's House: 290 sq. ft/person
  • Average 2000's House: 900 sq. ft/person 
 The average American 2 bedroom apartment runs about 1100 square feet, so an acre of ground could be sectioned into about 40 units. Shocking. That means that 'extreme' NYC density averages about four stories of 1100 sq. ft. 2 bedroom apartments.

Even assuming a car for every apartment, at 200 sq ft/car, (for a total of 1300 sq. ft per unit), and 10% of the land area for roads/paths/circulation, that yields about 30 units/acre. Suggesting that the 'most extreme' NYC density of 160 units/acre is only about 5-6 stories tall.

That puts a whole different spin on density--its not how tall the buildings are, but what percentage of the ground acreage they are using. Devoting half the lot to parking and landscaping halves the buildings 'floor area', and doubles the number of stories.

That also puts the employment intensity of NYC in perspective. A 40 story office building on a quarter-acre lot, at 430 sq. ft./employee represents 1,000 jobs.