Metabolism of Urban Location: Travel time and the morphology of cities

William W. Braham, CEBD, Weitzman School of Design, University of Pennsylvania, Philadelphia, PA, USA
Jae Min Lee, School of Architecture, University of Ulsan, Korea 

Throughout this evolution [of cities] there is only one factor which defines the extent of human settlements: the distance man wants to go or can go in the course of his daily life. Doxiadis, 1970

Abstract

This project examines the metabolism of urban location using the method of emergy synthesis. Location is an emergent property of urban settlements, a spatial and temporal property that underlies the morphology of cities. But how does the real estate logic of location enter the stock and flow calculations of urban metabolism, and how can it help us better understand the physiology of a more sustainable city? A new, locational quantity, EC, is defined as the additional emergy value obtained by a tract of urban land due to interconnection with the other land parcels in the city. It is based on their level of development and proximity measured in terms of travel time. The article uses the greater Philadelphia region as a case study to examine the metabolic value of location, and its role in the transition to a renewable economy.

Premise

The formative logic of cities is proximity. Despite the many powerful modes of long distance communication, people continue to gather in larger and larger cities to get closer together. According to the first law of real estate, the fundamental principle is location, location, location, which ultimately means, how long does it take to get from that place to other places of interest? An empty lot in the center of a city is valuable because of its proximity to nearby jobs, customers, transportation, and infrastructure. The same lot in a rural setting is only as valuable as the adjacent farmland, unless it is close to a transit system connecting it to the city, then it becomes a potential site of development. But how does the real estate logic of location enter the stock and flow calculations of urban metabolism, and how can it help us better understand the physiology of a more sustainable city? This article uses the greater Philadelphia region as a case study to examine the metabolic value of location.

The most evident value of land is the assets built on it or buried under it, which in turn attract the people and resources that drive economic activity. But location is neither a stock nor a flow of resources; it is a property of the urban system that emerges from the spatial arrangement of human activities on the landscape, which largely dictates the density of urban settlements. The value of location cannot be measured directly and is typically established by a real estate market (that also considers many other factors like regulation, pollution, etc.). This paper proposes a method for assigning a metabolic value to urban location, using the average travel time between locations as a factor to determine how much value a lot obtains from or shares with the lots that surround it. Applied across a city or region, this not only provides us with a method to evaluate specific locations, but to understand the spatial organization of cities as they grow into the polycentric mega-cities that now characterize urbanized areas.

In a 1970 article explaining his theories of urban planning, the Greek urbanist Constantinos Doxiadis presented a sequence of plan diagrams illustrating the effect of available energy on the growth of cities. Figure 1. The examples range from a compact, pre-modern village with a “daily per capita consumption of 8,000 calories” to a sprawling “central settlement of a system of villages during the era of the automobile and industry” (33,000 to 100,000 calories per person per day). In his account, each increase in available energy results in a corresponding increase in size, which certainly describes much of the transition from pre-modern to contemporary cities. According to the Energy Information Agency, the average energy consumption in North America is now about 173,000 calories per person per day, while the global average is 53,000 and the lowest is 11,000 in Africa (EIA 2016).

Energy is only a first requirement for growth, however, and other limiting factors quickly come into play, especially travel time across the city, which is determined by much more than the amounts of power directly available for movement (a fast car is little use without a road or faced with a traffic jam). Travel time is determined by the whole system of the city, by all the resources invested in vehicles and transportation systems, as well as in the buildings and resources needed to support the higher densities of population. The greater density will eventually crowd the more popular routes, and need larger roads and faster networks, in a classic example of self-organizing and self-limiting growth.

A generation of urban scholars have charted, explained, and criticized the sprawling urban areas that have appeared through the twentieth century (Garreau 1991, Geddes 1915, Batty 2018, Doxiadis 1968). Many denounce urban sprawl as inefficient and wasteful, but it is a surprisingly predictable form that cities assume when energy resources are plentiful and populations grow. H.T. Odum described sprawl as a “weedy” form of growth, useful for quickly covering large portions of land and preparing for more durable forms, but that initial coverage has grown “lumpy” over time, producing multiple points of intensity that often rival the original city center (Odum 2007, 46, Krugman 1996). According to Marchetti, the fundamental factor shaping city growth is the hour that most people were willing to commute each day and empirical data from many different periods and cities still supports that limit (Marchetti 1994, Zehavi 1974, Ahmed and Stopher 2014). The constraint of the half-hour one-way commute means that at a certain distance from the center, smaller regional centers become attractive destinations for commuters located further out, allowing those centers to expand and intensify.

Locational value is an emergent and constantly shifting property that shapes cities, so the transition to a renewable urban economy cannot be achieved simply with reduced consumption or greater operational efficiencies. People are remarkably mobile and relocate quickly to avoid inconvenience or to seek new opportunities or. In the classic view, more effective transit enhances sprawl, while slower transit increases density, but existing buildings and infrastructures cannot move as quickly as people, so any transition will begin as an adaptation of contemporary sprawl.