(Draft version.)

Density is a hot topic.  Environmental groups tout it as good for the environment, developers like it because its profitable, yet neighbors and other citizen groups fight density almost universally.  Given population increases, our choices range from sprawl to high density.  We know the problems with sprawl:  the loss of farm land, forest land and recreation area; habitat loss, and impacts on water and air quality.  We also know that, given a choice many people don't want to live in urban areas--the suburbs wouldn't exist otherwise, so is there a reasonable balance?  Alternatively the question is: how do we stop sprawl yet deal with a growing population?1

The primary concern from the sensible house perspective is not density itself, but how density affects buildings contained in it.  There is no simple answer to this because there are many ways to do density and many definitions of what density is (ie anywhere from say 12 dwellings/acre to over 100 dwellings per acre).  For the most part, density has historically been done with large disregard for access to solar power, for quality daylight, for green space and for noise reduction--if anything its only the latter that is addressed.

The Issues with Density

There are two broad issues here: (1) land use: how much of the land is covered by buildings and (2) lot layout: how each of the dwellings relate to each other.  Each of those issues can then be further broken down into subcategories.  Land use gets divided into three broad categories: buildings, infrastructure (ie streets, sidewalks, power, water) and green space, while lot layout addresses the spacing between buildings, the size the buildings can be and how many dwellings can be in each building.

Density can be achieved by varying each of these subcategories: allowing more of the land to be covered by buildings, shrinking the footprint of the infrastructure (eg larger blocks, smaller streets, one way streets etc), and allowing the buildings to be larger and in particular allowing more dwellings per building.   When daylight and solar access are considered, there is a limit to how many units per acre you can build without creating adverse affects: lots, like buildings, should be long in the E-W direction, and the N-S spacing must be large enough to prevent the shadow cast by a house from limiting the sun available it's northern neighbor.  Admittedly this model will never produce a neighborhood as dense as current urban cores, but it is more dense than suburbia, and hence could serve as transition between the two.4

The only remaining questions then are what the E-W spacing between buildings is, whether more than the required green space is left, how much of the green space is private versus public, and whether other green space (here we mean not just nature, but ball fields, playgrounds, gathering places, community gardens etc) is also set aside.  In the suburban model, green space is largely private, and much, especially the space between buildings end up as unused dead space.  In the urban model, there is often no space between buildings, but inadequate public green space is allocated, leaving the area with a net deficit.

spacing North-South spacing determined by winter sun angle.
layout Lots oriented on long axis East-West. Separate pedestrians and cars.

What we have now

Urban core: buildings here range from high-rise to low-rise to row-houses.  Density ranges from around 30 units/acre to well over 100/acre.  Unit size is often less than 2000SF. Often there is little or no private green space, and sadly there is often very little public green space either.  The neighborhood is usually walkable (although not always safe), and well served by public transit, but bicycling is usually dangerous (although that is changing).  Driving and parking are challenging.  From an energy consumption point of view this is the most green, but looked at another way, many urban environments are just warehouses for people.  From a storm water perspective, the urban core is an environmental nightmare: there isn't enough pervious surface.

One limitation of the energy efficiency of the urban core appears as a sore thumb in poor neighborhoods: the lack of retail, particularly grocery stores forcing residents to travel some distance on a regular basis.

 Outer Urban: buildings here range from row-houses to small low-rise apartments to single family dwellings on lots up to 5000SF.  Density ranges from around 30 units/acre to 8/acre. Unit size is often less than 2000SF. These buildings usually have some private green space, and depending on the city may have good public green space also.   Walkability is usually (but not always) good, bicycling is often possible on quieter streets, but public transit is sometimes lacking.  Energy footprint can be good, but it depends on how far one typically has to drive and weather walking/bicycling is an option. Storm water infiltration is often effective, but not always and fertilizer and pesticide pollution is common.

Suburban: buildings here are predominately single family with some low-rise multi-family units mixed in.  Lot size ranges from 5000SF to 10000SF (1/4 acre), and house size also varies from under to 2000SF to well over.  Density is 4-8 units/acre.  While some of these neighborhoods have sidewalks, often retail such as grocery stores are too far away for most people to walk.  Bicycling is often quite good,  but its equally often terrible, particularly in neighborhoods where the only thru streets also have heavy traffic.  Public transit is often poor as these neighborhoods were developed around the car, but sometimes the two co-exist well.  When it does exist, its bus/trains often only run during commute hours.  By design, most trips are by car, although the distance driven is not necessarily always far. Storm water infiltration is usually good, but the water is often polluted with fertilizer and pesticides.

Exurban: these are the areas outside the suburban areas, usually quite a distance from any city. While lots here might be as small as 5000SF, they are typically bigger and often enforced by zoning.  Density is almost always no more than 4/acre. Sometimes these areas used to be small towns, and sometimes there are active farms.  House sizes vary widely here, from very small to very large.  Walkability is often non-existent, as are sidewalks.  Bicycling is mixed depending on how much thru traffic there is on local streets, although as the areas get denser, local traffic is also a problem.  Public transit is almost non-existent, other than occasional commuter buses.  As these are car-centric areas where every errand is often a distance, and people often commute long distances to jobs in the city, so energy efficiency is poor.  The problems with storm water here, if there are any, are stream overflows that typically originate somewhere else.

Rural: these are areas that are not near any city.  Traditionally almost everyone that lived in a rural area was involved in agriculture or resource extraction (ie mining, timber), or part of the support network for those.  Now those areas might be centered around tourism, or alternatively a jail or landfill.  Lot size and unit size vary, but aren't relevant since most of the land is not dedicated to buildings.  Such a small percentage of the US (and any of the developed world) lives in rural areas that their environmental footprint isn't that relevant, which is a good thing because its often quite a bit higher than urban areas.  It is often impossible to walk in rural areas, unless they are very rural; bicycling can be good, but it depends on the quantity of thru traffic on local roads. Public transit simply not possible, and wouldn't be efficient even if it was.

In many cases rural areas are being broken up into ranchettes, often an acre or two: big enough to make walking and public transit difficult, these developments are not as connected as traditional rural communities since the residents are often wealthy and don't rely on each other- in fact the lifestyle here is often quite different from that of traditional rural communities.  Many argue that these developments ruin the rural lifestyle--which is no surprise--essentially they are exurbs transported to a rural location.  Needless to say, the people who live there appear to be quite happy there.

Where we are heading

Sprawl is continuing, but is now more constrained by growth management acts, farmland preservation, public purchases of open space and other legal measures than ever, so development is mostly occurring in already built-up areas.  In places where there is a less dense urban core, its becoming more dense; outer-urban areas are seeing more large multi-family units, suburbs, particularly older have areas converted from single family to multi-family, and exurban areas become more suburban.

Yet local residents in each area are almost unanimously opposed to such development and consider it to be ruining their neighborhoods.  In fact, most suburban neighborhoods, and many city ones have strict zoning ordinances to prevent such development.  Then inevitably some neighborhood, usually one with little political clout, gets thrown under the bus, and all the development is concentrated there.

There are a few simple economic truths behind this situation:
  1. On a given piece of land,  the more units a developer builds, the more money they make
  2. hence, the price of land is largely dictated by the allowed density
  3. Larger homes sell for more profit than smaller homes, so the bigger each unit is, the more money the developer makes
  4. hence, zoning setback and height limitations therefore also affect land prices
  5. It's hard to profit from green public green space, so its rarely provided unless required.  Often the only green space dictated by zoning: its the part of the lot that can't be covered.

What are the variables?

Privacy/Quiet - Most people like a bit of personal space, and clearly some want more than others.  Likewise, most people desire quiet, but many like some kind of background sound, just not necessarily planes, train, automobiles and industrial equipment.  Privacy can lead to isolation, so maximizing privacy tends to increase isolation, hence there is a tradeoff.  Since people vary, density needs to vary as well.

Access to green space/nature - people like nature, but many prefer managed nature rather than the actual thing.  The majority does like to both look at, and be in, green space of some kind, yet the availability of public green space is often limited or non-existent. Currently the availability of green space is not tied to allowable density, and hence if any green space exists, its in your own yard.

When the green space is in your yard, it is often at the expense of public green space, hence the effect is of privatizing the space.  While it has it benefits, for many homeowners its represents just a unpleasant Saturday chore or the expense of a lawn care company.  As a place for children, its often confining and less appealing than the town park.  In addition the standard 5-7' side yards on suburban lots are often dead space, as well as are most front yards--people don't use them.  Yet urban locations often lack both, and for many are just horrible places to live.  The question then is: why can't we have some density and green space along with it, and the answer is that we probably can, but economics and lack of political will get in the way.

Community space - in Europe many communities are built around town squares, and often these are well loved community space.  In the US sometime city parks serve the purpose, but often gathering is down in the private realm of the backyard.  Like green space, as we increase the size of private space, the quantity of public space goes down, and the space that does exist becomes, on average, further away.

Local food - this is not yet a popular topic, but could become one.  Community gardens are having a resurgence all over America.  Again, in suburbia,  gardens tend to be private, while in cities they tend to be a mixture, and space tends to be very limited.

Transportation - there are two aspects of this, which are often mutually exclusive: convenience and energy efficiency.  Cars are often very convenient, at least until your stuck in traffic.  Public transit is convenient only when its easily accessible and runs frequently, which is not typically the case.  Statistically a huge percentage of trips are under 3 miles, so many could be done by walking or bicycle, which are both very energy efficient, but not always considered convenient.2

Environmentalists hate cars, because they use a huge amount of energy, but they generally get you from point-A to point-B with the least hassle.  Public transit (bus) is more energy efficient with lots of passengers, but local buses get only 2-4mpg, so empty they're horrible, and hence density makes buses efficient.3  Electrified train is probably better, but they're dramatically more expensive to build, and hence are never likely to go everywhere. Commute vans (ie vanpool) get 10-12mpg, so with 8-10 passengers are quite efficient. Cars probably could average over 50mpg, which might be good enough, especially if the average miles driven can be reduced. Another intriguing idea is car-share, where you essentially rent a car by the hour, as well as innovative ideas like bus-rapid-transit and personal-rapid-transit.

But the other way to look at transportation is to look for ways to make more things local, particularly local enough so you can walk.  This turns out to be drastic change from how zoning laws currently allocate development.  By changing this we can concern ourselves less with the energy efficiency of transport. The key is having many small retail areas that serve common needs, like groceries, bank, pharmacy, restaurants and coffee shops, and of course just enough density to support them.  This also means that the schools need to go back to being smaller and local rather than the current trend toward large schools.

Daylight/Solar - this is an issue not just for density but how the density is done.  Multi-family housing tends to have much worse daylighting than single family due to shared walls preventing windows.  Buildings all too often block the sunlight falling onto others, and communities are beginning to pass solar access laws that guarantee some kind of access.  Part of the issue here is about where the open space is: the sun is generally in the south, so empty space in that direction, even roads will provide solar access.

Storm water - land in its natural state tends to be quite porous and will absorb rainfall readily (except deserts where runoff is common), which tended to naturally limit flooding.  Urban areas tend to be a combination of impervious surfaces like buildings and streets, causing much more runoff than would otherwise be, and in some cases causing sanitary sewer overflows.


The Radburn, NJ model - this small 1930s development in suburban NJ features smaller lots with a strip of public green space every backyard: the green space is both pedestrian corridor and parkland, allowing a separation of people and cars. Radburn also features cul-de-sacs reminiscent of modern suburbs, so that most residents live on dead-end streets.  This part of the Radburn model was heavily copied, but most subsequent developments left out the green space.

Village Homes, Davis, CA - this 1980s development is a variation on the Radburn model where the individual lots are all laid out for solar access.  Rather than Radburn's cul-de-sacs, Village homes features narrow, winding streets, and the homes face the common space rather than the street.  Village homes also makes use of swales for storm water control, which avoids much of the traditional storm water infrastructure.

Cottage housing - these are essentially detached condos surrounded around green space.  In this model, much of the private green space is given over to public, although the public in this case is only the development's residents, not the general public.  Units are typically small, and there is often a common building for gatherings.  Parking is communal, and not next to the individual unit.

Accessory dwellings - these are apartments located in single family neighborhoods, typically distinguished from duplexes in that the auxiliary unit is usually limited in size.  Sometimes these are in attics or basements, and sometimes they are separate detached units with are either standalone or over a detached garage.  The net effect is a doubling of the number of units.  The main concern is parking.

New Urbanism/Transit oriented development -  these are a series of related ideas: to increase density and provide walkability (unlike Raburn, in new urbanism development, walking is in the front of the house and houses have front porches), and part of that density in done by having some units be smaller than others.  The most dense areas, and commercial core surround public transit.  A web search for "new urbanism" will yield much more info.


1: Many would argue that population reduction is the only viable solution, but that path is a political/cultural problem so will not be addressed here other to admit its another way to address these issues.

2: We have an obesity epidemic on our hands that could be partially cured by walking more, but American are somewhat culturally opposed to walking.

3: intercity buses however get more like 10-12mpg: its basically the same bus, but all the stops the local bus makes kills its efficiency.  Hybrid buses are a bit better (ie they're the 4mpg local versions).  It seems theoretically possible to make a 10mpg local bus, but the challenge is capturing all the regnerative braking.

4. Although this model is similar to the dominant development scheme of Europe, dense cities will probably always exist as well as less dense suburbs.  The argument here is that even if global population were to drop precipitously, this model is preferable to suburbia.


Geography of Nowhere - James Kunstler

A Pattern Language - Christopher Alexander, et al