“Does
anybody know what the ‘right’ density is? I do. It is 12,000 to 60,000 persons
per square mile of residential area (20 to 100 persons per acre). In other
words, acceptable conditions can be created within a wide range of densities …
but there are upper and lower limits beyond which serious disadvantages
appear.” [H. Blumenfeld, “The Modern Metropolis: Its Origins, Growth,
Characteristics, and Planning”, p.172]
“If
sustainable development is so dependent on higher densities, then the question
is higher than what …?” [M. Jenks and N. Dempsey, ‘The Language and Meaning of
Density’, in Jenks & Dempsey, “Future Forms and Design for Sustainable
Cities”, p.287]
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I’d like to
spend a couple of posts driving around the veritable “Spaghetti Junction”
(http://en.wikipedia.org/wiki/Spaghetti_Junction) of issues related to the
subjects of urban density, the compact city, and sustainability. These complex
and interrelated topics have provided planners, scholars and urban historians
with material for robust debate since at least the early days of the commuter
suburb in the late 19th century. In recent decades, as technological
options have multiplied, the range of densities at which people live has widened
considerably, and as the question of the relationship (if any) between urban
density and sustainability has become globalised with rapid worldwide
urbanisation, the scope for complexity and confusion seems to have expanded
with them.
What I would
like to do as a contribution to unscrambling some of the complexity is to make
some of my definitions and assumptions explicit, and by simplifying a couple of
categories at least to pick out a couple of useful signposts along the route.
The first
point of simplification is that I will focus on the impact of urban
transport technologies on how the built environment has been (and can
be) shaped, and how that can potentially affect the issue of sustainability. Although
one can view the subject of urban sustainability through many lenses (such as
energy generation and consumption, food supply, water and sanitation, even no
doubt cultural and psychic) I believe that understanding the evolution and
dynamics of transport technology is the starting point for any meaningful
discussion. In this, I will draw heavily on Hans Blumenfeld’s wide-ranging
series of essays, written between 1940 and 1965 and published as “The Modern
Metropolis”.
The second
point of simplification will be to distinguish a number of abstract, “ideal type”
city structures in order to illustrate the symbiotic and dynamic interplay
between a given transport technology and population density. My aim in starting
with a very abstract categorisation of city types is twofold: first, to give a
mental image of the kind of city we are talking about; and second, to try to
focus on the essentials before reintroducing some levels of real world
complexity.
Let’s start
with a long quote from Blumenfeld which helps illustrate, using the example of 19th
and early 20th century technical progress, why the evolution of
transport technology is so critical to understanding the urban landscape:
“The 19th
century gridiron plan of our cities was designed without any mental image of
the body of the city, and the buildings were constructed without relation to
any design of the city as a whole. No one could guess from a map of Manhattan
from which field on this checkerboard rises the fantastic silhouette of the
city’s skyscrapers.
Yet it is
possible to discern a definite pattern of the modern city that has gradually
superimposed itself on the ubiquitous gridiron. This pattern is essentially a
product of the growth of transportation, which at different stages developed
centralizing and decentralizing tendencies.
In the first
stage interurban traffic was revolutionized by steamships and railroads. Where
they met, and only there, could modern industry assemble the masses of coal –
its driving power – and of raw materials and food that it needed. And only from
these points could it easily ship its products to distant markets. Factories
attracted workers, and the presence of many workers of various skills created
favourable conditions for more factories.
But while
steamships and railroads carried huge masses of goods and passengers to and
from the far corners of the earth, traffic within the city moved, as of old, on
foot or by horse and buggy; and while the telegraph carried news around the
world within a few seconds, communications within the city were still carried
by messengers. So factories and offices and dwellings all tried to be close to
the centre of the city, crowding each other.
Only after
several decades did the technical revolution reach the interior communications
of the city. Suburban railroads, streetcars, elevated trains, subways, buses,
automobiles, and the telephone overcame the distances within the urban area –
as steamships, railroads, and telegraph had already succeeded in overcoming
distances between cities. While interurban traffic continues to act as a
centralizing force, concentrating business and population in metropolitan
areas, intraurban traffic acts as a decentralizing force within the limits of
these areas. The densely crowded agglomeration of the 19th century
with its concomitant, the fantastic skyrocketing of urban land values, turns
out to have been a short-lived passing phenomenon necessitated by the time lag
between the transformation of interurban and intraurban traffic, respectively;
it was bound to disperse once this lag was overcome.
Though it
would disperse, it would not dissolve. Sources of power, raw materials, and
markets may be equally accessible outside the metropolitan area, but it is only
here that employees and employers have a wide range of mutual choice, as skills
become ever more varied and specialized. The modern metropolitan area is
primarily a labour market; it extends only as far as people can commute daily
to and from work.” [Blumenfeld, pp.31-2].
I think it
will be useful to look at three “ideal type” city formats, and the transport
technologies that underlie each of them and which are necessary to make them
work. I hope that, although the characterisation of the cities will be highly
simplified and abstract, it will nevertheless be useful in order to draw out
some of the basic constraints and interrelationships involved. The first city
type has a density of 75,000 people per square mile and is served by a public
transport network of subway lines – let’s for sake of argument call it “Paris”. The second type is a city
composed entirely of low density “automobile suburb” development (7,500 people
per sq m) and has no public transport network, being solely dependent on its
road network – let’s call it “Los
Angeles”. The third type is a hybrid with a density of 25,000 people per sq
m: characteristic of, for example, an outer suburb of a large city like London
or a smaller European city (let’s say Nottingham, Nantes or Freiburg). This
city is served by a mixed transport network of light rail (trams) and cars.
However,
before turning to look at each of the cities in turn, let’s first address a
couple of the definitional complexities and confusions that plague the debates
around urban density in order to clarify our terms a little. The first
confusion is “between high densities and overcrowding … The Garden City
planners and their disciples looked at slums which had both many dwelling units
on the land (high densities) and too many people within individual dwellings
(overcrowding), and failed to make any distinction between the fact of
overcrowded rooms and the entirely different fact of densely built up land.
They hated them both equally … and coupled them like ham and eggs, so that to
this day housers and planners pop out the phrase as if it were one word,
‘highdensityandovercrowding’” [J. Jacobs, p.268]. Clearly, one can find population
densities of 75k-100k people per sq m in either the central districts of Paris
or Hong Kong, or in a third world slum or in the slums of 19th
century London or Chicago. The differences are pretty obvious, and come down
essentially to the amount of space available to each person, so we don’t need
to be confused.
A second
area of confusion and complexity is of terminology and measurement. In their
survey of the British historical experience of recognising the significance of,
and attempting to legislate for, urban density criteria, Jenks and Dempsey
identify the “wide range of different measurements [which] have been used
including: persons per hectare; dwellings per hectare; habitable rooms per
hectare; bed spaces per hectare; and floorspace per hectare.” [p.293] Moreover,
there is the question of whether density is measured in gross (the whole area
including all uses) or net (solely concerned with residential square footage)
terms. “[D]ensity ‘can be life-threatening when in the wrong hands’. Using net residential
density alone fails to take into account wider issues of land capacity, mixed
uses, and gives no guide for assessing aspects such as ‘walkability’”. [p.293] Let’s
react by acknowledging these complexities, but assume that issues such as mixed
use are a given in the interests of thinking more abstractly: think “Paris” or
“L.A.” in terms of the general format of the city, or I will put in some
pictures to illustrate the sort of urban places I’m thinking of.
“PARIS”:
what, at least for the purposes of this analysis, is the point of a city? In
Blumenfeld’s view, the “basic raison d’etre of the modern metropolis is the
need for cooperation and communication resulting from the division of labour…
Primarily the metropolis is a labour market, a place for making a living. This
also sets its limits. It is a commuting area, extending as far as daily
commuting is possible and no farther.” [p.123]. Let’s go with this definition,
maybe broadened a bit to say that the point is ease of physical communication
of all kinds and therefore mobility – the ability of the individual to move
around the city reasonably fast and reliably, at a reasonable cost, for
whatever reason.
Starting
with a residential density of 75,000 let’s assume a mega-city of 15 million.
Purely abstractly this is a city of 200 square miles, or one that therefore
fits into a square with sides of just over 14 miles. Let’s further assume that
we are going to serve the city with a public transport network consisting
solely of a heavy rail underground system. (There is clearly always going to be
a requirement for the road network to deal with some private car usage,
transport for tradesmen and the delivery of goods, but let’s make the
reasonable assumption that with the great majority of intraurban trips made by
underground the road network is both adequate for high quality pedestrian usage
and occasional car/delivery van usage.)
What does
moving around the city look like? In order to go door-to-door anyone making a
trip within the city will obviously need to walk from the starting point to the
closest subway station, take the subway to the nearest subway stop to his/her
destination, and walk to the final destination. Pretty straightforward. Making
the standard assumption of an average 3 miles per hour walking speed, a
10-minute walk is half a mile. Therefore if we are going to say that every
inhabitant is a maximum approximately 10-minute walk away from the nearest
subway station, a subway station needs to be placed at the centre of every
square mile block of the city – i.e. 200 stations which are connected by 14
13-mile west-to-east subway lines and one (higher capacity) 13-mile
north-to-south line. So the longest trip
anyone will have to make in the 15 million inhabitant city is: 10 minutes’ walk
to the nearest subway station; a subway trip of 28 stops with 2 changes of
train line; and 10 minutes’ walk to the final destination.
What are the
advantages of a modern subway system for moving people around a city? Obviously,
it is independent of the surface shape of the city, in particular the design of
its street network. This means that there is no trade-off between the design
choices that can be made to enhance pedestrian accessibility and liveability on
the one hand, and the design choices necessary to ensure car mobility on the
other. The second point is really a subset of the first: that its capacity (the
number of people that can be moved per hour) is for practical purposes
unlimited. This is significant because intraurban trips are not evenly spread
throughout each 24 hour period but rather are heavily concentrated in the
2-hour morning and evening commuter rush hour periods; therefore in terms of
capacity any transport system has to deal not just with the total number of
trips made during the day but with the 2 daily “spikes” of demand on working
days – therefore 20 hours of the average working week in comparison to the
total 168 hours of a week.
If we want
to get a little bit nerdy, there are a number of ways the carrying capacity of
a subway system can be increased through the design of trains and stations:
trains can be enlarged – widened, lengthened or made double-decker; headways
(the time between each train) can to some extent be decreased (although there
are technical constraints that mean that minimum headways are not lower than around
90 seconds or 2 minutes); moving passengers on and off the trains can be made
faster by having platforms on both sides of the train, one for passengers
entering and the other for exiting. If necessary, more tunnels can be dug. There
are some technical limits to train speed stemming from the fact that a train
has to accelerate and then decelerate within (in our example) one mile;
however, even this can be radically increased by having two sets of trains and
tunnels on the highest-capacity routes, one set for a smaller “local” service
stopping at each station and one set for an “express” service stopping every 4-5
stations. Anyway, the net result is that there are no meaningful limits on the
design capacity of underground rail to move very large numbers of people, with
high concentrations at rush hours, efficiently and reliably.
The main
problem with underground rail systems is capital expense – primarily digging
the tunnels and the stations, and equipping them with signalling, control
equipment and trains. Once built, the operating costs are relatively low and
stable – electricity, maintenance and salaries for the relatively small number
of people necessary to run the system. A subway system is also by definition
inflexible – it can’t cheaply be adjusted if part of the city suddenly becomes
relatively depopulated and/or the city expands rapidly in terms of area. However,
the debt incurred through the high capital costs of a subway system is
relatively straightforward to amortise over a long period of time when you have
a large and densely populated city which is relatively stable in land area and
population over extended periods of time, and which will tend to have
relatively a high and stable tax base (in terms of individual incomes, business
productivity and property values.) Such cities go together with subway systems,
as Jane Jacobs would put it, like “ham and eggs”.
Having
looked at an idealised high-density city, let’s begin to look at the dynamics
involved in the transportation options available to a low-density city, “Los
Angeles”. Blumenfeld points out that “[i]n every big metropolis a battle royal
is raging between the advocates of a rapid-transit system and the partisans of
a freeway system … But neither rapid-transit lines nor freeways constitute a
system. Neither can carry persons or goods from door to door. Given the money,
anyone can build a rapid-transit line or a freeway … The problem is: how do you
get to it from where you are, and from it to where you want to go? A transit
system is no better than the feeder system that brings people to and from the
stations, and a road system is no better than the interchanges, streets,
parking and loading spaces to which the cars and trucks must get from the
freeway.
If one
thinks in terms of a transit system and a road system, it is not difficult to
define the role that each can play best. If you go back to your home in one
suburb from a party in another suburb at two in the morning, you cannot expect
a bus or train but have to drive your car. But when a thousand people want to
go from point A to point B in the same five minutes, it is obviously more
sensible to carry them in one train than to have a thousand cars compete for
street and parking space. High density requires transit, and transit makes high
density possible. Low density requires individual car driving, and universal
use of the car requires low density.” [p.141]
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