Urban density is the number of people living in a particular urban area and is an important aspect of how cities function.  Many modern urban planners advocate higher densities because of the widely held theory that cities operate more efficiently when residents live in denser urban surroundings.  However, there are mitigating factors such as higher traffic congestion when traffic thinning and parking capacity reductions are not in place.  When cities have high densities, they tend to be more walkable and have greater transportation options.  However, when cities are allowed to expand from the center without benefit of smart growth planning, they can become relatively unsustainable.  Sustainability has several components germane to urban planners but the single most important of these is transportation – how people get around.  When cities rely on automobiles as their primary means of transit, they lack sustainability and quality of life choices that can only come about when urban fabrics are built for their human users rather than their cars.

Automobile Dependency and Sustainability

Automobile dependency developed in the mid-twentieth century as a result of policies favoring the construction of freeways and low-density tract housing far from city centers.  Moreover, the removal of streetcar lines from cities hastened the demise of walkability in our own downtowns, which – over time- were given over to large parking lots and empty sidewalks.  Automobile dependency is the condition of a place without other transit options whereby the only viable way to get around is by car,  where the built-environment consists of wide roads, parking lots, and space devoted to the expeditious flow of cars.  Automobile dependency deepens when roads are widened to accommodate larger numbers of drivers who then create more traffic and more 'demand' for parking.  When parking lots replace buildings and roads are expanded, cities are forced to grow horizontally.

Urban Sprawl

Urban sprawl occurs when cities grow haphazardly away from their centers over previously undeveloped land in low density patterns.  Typically, urban sprawl is characterized by new development that is dependent on automobile infrastructure and an expansion of municipal services into rural land.  Many towns and cities are constrained financially because their service areas cover large, resource-inefficient, low density development.  The tax-base of a 'big-box' store is a small fraction of a smart-growth development, as the per-unit revenue of dense, mutlti-functional construction is much greater.  Not only is sprawl characterized by low tax yields and a stretching of municpcle services, but it also is evident in the increase in air pollution from the use of cars for many lone-occupant, single-mission trips – those journeys's made with one motive, such as picking up lunch or the dry-cleaning.  Socially, sprawl can be described as a decentralization of cities, where the built environment is single-purpose, often residential with no connection with a town center.

Smart Growth

Smart growth is normally planned and centers around the concept of compact, town-centers built around high-quality transit stations such as tram or subway stops.  Smart growth, more specifically, is contained in an established urban corridor, and connected to the urban center or downtown through proximity if not mass transit.  Smart growth is such because it is planned to be self-supportive over a long time through its multi-functional uses and ease of access via walkability and bike trails, and public transit.  Such growth is smart because it is meant to last and not be solely dependent on automobile use.  Although many cities in Europe are built in 'smart growth' patterns, they were built that way long before the advent of the automobile and thus were layed out in more human-scale dimensions.  Modern planning terms such as 'smart growth' and 'transit oriented development' are used in low-density countries such as the U.S.A., Australia, and New Zealand to denote how future development should accommodate new residents.

Petroleum and Development Patterns

A clear distinction can be drawn between continents in their respective petroleum use and urban development patterns (see graph).  American cities have the greatest sprawl and average unit of land per resident, while European cities are the most efficient.  Cities in Europe are more concentrated and public transit tends to be superior to those of American cities, making it easy for people to leave their cars at home, especially during the week.  Australian cities are similar to American cities in their dependence on cars but are somewhat better planned.  As awareness and appreciation of a clean environment in our cities increases, the choice of more and more American and Australian cities is sustainability and smart growth.