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What is the optimal density for a 'green' city?

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What do you think is the optimal density for a city to be as environmentally sustainable as possible? Here are my random thoughts on the subject.

1. High Rise

At first glance, it would seem that piling as many people on top of each other as possible would conserve the most land, reduce transport and hence be the best. But wait - it's not so simple. There are many problems with ultra-high-density built environments.

A) Transport of goods that cannot be produced in the high-density area (especially food).

B) Transport of garbage out of the area.

C) Elevators use energy and use up some of the floorplate. So the taller the building, the more elevators, the more floorplate used up, the wider the building, the more energy used. Also if you have a very tall (and hence very wide) building and you want any light to still hit the street, you have to set it back more and more, reducing its overall density.

D) If the high rises are generally for jobs, you have to consider transportation to those jobs. That is why Manhattan, with the largest and densest business district in the USA (3 million jobs), also generates the longest average commutes (in both time and distance) and is hence probably the least environmentally sustainable business district.

E) If the ultra-high-density city is where people live AND work, you have problems of severe overcrowding. 100 years ago, when Manhattan had twice the population it does now but fewer jobs, it was extremely dense and had a very close job/housing match. And it was one of the most atrocious living environments humanity has ever produced, with squalor that could barely be imagined today.

These problems lead me to look beyond ultra-high-density for a sustainable solution...

2. Spread Out

Many urbanists including Frank Lloyd Wright and recently, Robert Breugman (who wrote the famous/infamous pro-sprawl book Sprawl: A Compact History), have proposed that spreading out may be more sustainable. "A single family living on a two acre plot can potentially produce all the energy they need themselves" (from a recent lecture in Manhattan, paraphrased). Also, many "off the grid" hippy-types who set up "sustainable communities" tend to remove themselves from cities, and set up small land plots for each homesteader.

Although this may be viable, I'm not sure if the two-acre sustainable lifestyle is amenable to most people in the advanced world these days. So if you give them an inch, they'll take a foot, that is to say, give them two acres and they'll buy and SUV and shop at Wal Mart. Also I'm not sure if there is enough land in the world for 6 billion sustainable 2-acre plots.

3. Walkup

Lastly, there is the density proposed by Joel Crawford for Car Free Cities: four stories maximum (I would say four to six stories). The idea is that buildings this height do not require elevators, and create a very pleasant street environment where people want to live. Especially if you remove cars from the city as Joel suggests, you can achieve a good enough density to preserve land and support high-quality public transportation.

Another point is that if we want to think about sustainability, the best model to look at is most likely the pre-modern city, which existed in a very tightly defined bio-region (due to lack of transport) and was therefore by definition sustainable. Super dense environments like Hong Kong and Manhattan are entirely the product of modern, unsustainable transport.

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I'm not sure that any of these schemes would ever become the norm. People are just to different in all aspects. Some people are well adapted to dense living and prefer that, others can't stand that style, and yet others will be happy to settle for the happy medium you described at the bottom.

What is better for sustainability itself, I'm not sure. But I'm fairly certain that a mix of all of the above is will be the outcome for the foreseeable future.

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Great post. This is very provocative. Though I wonder if trash disposal isn't more efficient in very dense places. Consider high versus low density residential buildings. In the suburb garbage trucks have to slowly drive by every home to collect its trash, idling often. In a residential high rise the trash from many households can be removed all at once. The same could go for other services such as mail and package delivery.

A consistent landscape of 4-6 story residential buildings makes for very high average density. In Somerville, Massachusetts a consistent spread of just 2-3 story double and triple decker detached houses averages about 20k/sq m.

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