ENERGY-EFFICIENT Urban Form

Reducing urban sprawl could play

an important role in addressing climate change.

JULIAN D. MARSHALL

UNIVERSITY OF MINNESOTA

The impact of cities—and urban design—on the global climate is becoming increasingly important. In 2008, urbanites will outnumber rural dwellers globally for the first time in human history (2). Chi-na’s population doubled between 1952 and 2003, but its urban population increased 7-fold; today, 170 Chinese cities have at least 1 million residents (3). The U.S. has 39 such cities (4). In coming decades, urban populations are expected to double while rural populations level off or decline.

Improving city layouts and transportation net-works could reduce carbon emissions more than replacing all gasoline with corn ethanol (1). Al-though much attention on mitigating climate change has focused on alternative fuels, vehi-

cles, and electricity generation, better urban design represents an important yet undervalued oppor-tunity. Fortunately, such decisions are well within the reach of local governments and leaders and can reduce long-term carbon emissions.

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Vehicle use is rising rapidly. From 1970 to 2005, U.S. total vehicle-kilometers increased 3× faster than the population (annual increases: 3.0% vs 1.0%) (5). Similar trends occurred in China (8.3% vs 1.7%, a 5-fold difference) and the world (4.3% vs 1.8%) dur-ing 1970–1990 (6). If current trends in total vehicle- kilometers continue, vehicle CO2 emissions may increase even if emissions per mile decline (7).

In an influential paper in Science, Socolow and Pacala (8) argue that climate stabilization during the next half century means reducing CO2 emissions by 175 GtC (33%) relative to a business-as-usual (BAU) scenario. They propose seven strategies, with each “stabilization wedge” representing emission reduc-tions of 25 GtC during 2005–2054 (each wedge grows from no reduction in 2005 to 1 GtC per year [yr] re-duction in 2054).

The race is now on to figure out ways to design and implement these wedges. Often neglected in the debate is the role of urban form (e.g., land-use pat-terns and the layout of transportation infrastructure) in meeting climate objectives. My estimates suggest that reducing urban sprawl in the U.S. alone could represent half or more of a stabilization wedge.

Impacts of urban form on transportation CO2Compact urban form can cut on-road gasoline emis-sions, the largest segment (62%) of transportation CO2 in the U.S. The transportation sector is the larg-est emitter (33%) of CO2, outpacing the residential, industrial, and commercial sectors. (Electricity gen-eration, when totaled for all sectors, accounts for 41% of CO2 emissions.) Records of automobile usage (Fig-ure 1) show an inverse relationship between popula-tion density and per capita daily vehicle-kilometers traveled (VKT) (4, 9). Evidence suggests that VKT is causally related to population density and other ur-ban form attributes, and therefore, sprawl reduction

policies may curtail VKT (10–14). In denser urban ar-eas, trip origins and destinations (e.g., home, work, shopping) are closer; driving disincentives (e.g., con-gestion, parking costs) are greater; and alternative modes of travel (e.g., walking, bicycling, mass tran-sit) are more common (15).

Sprawl encompasses many aspects of land use, including leapfrog development, segregated land use, and automobile dependence (16, 17). Calcula-tions presented next focus on one aspect of sprawl—declining urban average population density.

To estimate the potential carbon benefits of re-ducing urban sprawl, I considered five urban growth scenarios for the U.S., 2005–2054: high sprawl, BAU, reduced sprawl, no sprawl, and infill. Changes per de-cade in average urban population density are –47, –39, –13, 0, and +11%, respectively, for the five scenarios. In comparison, changes per decade in average ur-ban population density were –13% during 1960–1990 and –34% during 1990–2000 (18). For the no-sprawl scenario, average density is constant, so urban area increases at the population growth rate (1% annu-ally). The infill scenario approximates a strict urban growth boundary, with total urban area roughly con-stant (unrealistic, but a useful bounding estimate).

For each urban growth scenario, I considered no technology innovation and innovation (1% annual reduction in fleet-average gasoline CO2 emissions per kilometer). As a sensitivity analysis, I evaluated the impact of rapid innovation (3% annually). In-novation could include more-efficient vehicles or reduced-carbon fuels. Historically, annual improve-ments >1% have been achieved, but not maintained, for 50 yr. Annual changes in U.S. passenger vehicle fleet-average fuel economy were –0.4% during 1936–1973, +2.0% during 1973–1995 (23 yr), and near-zero (+0.03%) during 1995–2005 (19–21). Year 2005 pas-senger-vehicle average fuel economy was 20 miles per gallon (mpg; 21).

The Energy Independence and Security Act, signed by President Bush in December 2007, will in-crease fuel economy by ~1.5% annually for the next 23 years (assumptions: 10-yr vehicle turnover rate; con-tinuation of the ~20% gap between actual passenger-vehicle fuel consumption [20 mpg; 21] and Corporate Average Fuel Economy [CAFE]-rated consumption [25 mpg; 22, 23]). The act will increase CAFE standards (currently 27.5 mpg for automobiles and 22.2 mpg for light-duty trucks) to 35 mpg for both vehicle classes in 2020; this is below today’s CAFE-type standards in China (37 mpg) and the EU (44 mpg) (23).

For each urban growth and technology scenario, I calculated U.S. total VKT and the resulting on-road gasoline CO2 emissions. See the Supporting Infor-mation (SI) for details.

The results indicate that sprawl reduction could play an important role in addressing climate change. Reduced sprawl, without technology innovation, de-creases emissions by 10 GtC during 2005–2054 (by 0.5 GtC/yr in 2055) compared with BAU. These sav-ings represent 41% of a wedge. The no-sprawl and infill scenarios offer 53% and 60% of a wedge, respec-tively, compared with BAU, with no innovation.

Improvements in emissions per kilometer would

F I G U R E 1

Where do Americans drive the most?Vehicle usage records indicate an inverse relationship between population density and vehicle-kilometers traveled. Data are for passenger vehicles in the 47 U.s. urban areas with populations >750,000, for year 2000. (Adapted from Ref. 4.)

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reduce emissions for all urban growth scenarios and also would reduce the differences between them. For BAU growth, innovation reduces emissions by 38% of a wedge compared with no innovation—almost the same reduction as converting, without innovation, from BAU to reduced sprawl. Converting from BAU without innovation to reduced- or no-sprawl growth with innovation would offer 66% or 74% of a wedge, respectively. These improvements are impressive, es-pecially given that they involve action by the U.S. only (5% of the global population). Disruptive technolo-gies such as low-carbon fuels or rechargeable hybrid-electric vehicles that use low-carbon electricity could deliver environmental innovation rates significant-ly higher than historic levels. Increasing innovation from 0% to 3% annually would reduce U.S. emissions by 83% of a wedge under BAU growth.

Analyses above use the extant (Figure 1) relation-ship between population density and VKT (specifi-cally, a density-VKT elasticity of –30%; 9, 24). Greater attention and commitment to energy-efficient urban form (see below for example policies) could increase the carbon benefits from sprawl reduction. As an illustration, I repeated the analyses above, but as-sumed aggressive support for energy-efficient ur-ban form (represented by doubling the density-VKT elasticity to –60%). I found that, without innovation, reduced- and no-sprawl scenarios offer 125% and 149% of a wedge, respectively, compared with BAU. Sprawl reduction would have a greater CO2 impact if aimed at high-elasticity situations (9, 12) rather than low-elasticity ones.

Overall, these findings suggest that long-term cli-mate impacts from shifts in urban form could be comparable to those from technological innova tion, and that climate-mitigation strategies would have greater impact by addressing urban form and techno-logical innovation, rather than only one of those two.

Reducing sprawl could help minimize U.S. de-pendence on foreign oil and could ease economic impacts of price shocks while providing financial savings to help offset the costs of low-carbon en-ergy technologies. Conversely, if sprawl accelerates, it becomes another source of emissions to reduce or offset. In that case, additional mitigation wedges will be required. For example, compared with BAU, the high-sprawl scenario increases emissions by 14% of a wedge with innovation or by 21% without.

The values presented here are rough estimates only, involving several simplifications (see SI; even a highly detailed analysis would involve significant uncertainty). Still, the results highlight the potential importance of urban design for reducing CO2 emis-sions and suggest that further investigation is war-ranted. One source of uncertainty is the influence of urban form on vehicle choice. The perceived con-venience of a small vehicle is greater at high than at low density. The evidence suggests that the relation-ship is causal: lower densities and longer commutes increase preferences for light-duty trucks instead of automobiles (11). Including those interactions would increase the estimated benefit of sprawl reduction on CO2 levels. Perhaps the largest source of uncer-tainty is the density-VKT elasticity. If elasticity were

between –4% and –15% (9), the impact of reduced sprawl compared with BAU would shrink from 41% of a wedge to 4–17%. (If elasticity were zero, the CO2 impact of urban form shifts would be zero.) Using the elasticity of –30% to –50% reported by modeling (12) and empirical (24) studies would increase the estimated CO2 benefits of sprawl reduction.

Urban form is important outside of the U.S., too. Globally, the number of megacities (population >10 million) increased from 3 in 1975 to 20 in 2005 (25). However, most urbanites live in cities with <1 mil-lion people (2). Cities’ ecological footprint—account-ing for inputs and outputs such as food, consumer goods, energy consumption, and waste products—is many times larger than that of urbanized land areas; the difference is a factor of 60 in the U.S. (95 ecological footprint vs 1.5 urban area; units: 1000 m2/person) and a factor of 80 in India (~8 vs ~0.1) (3, 18, 26). Most (~90%) global population growth dur-ing 2000–2025 is expected to occur in urban areas in less-developed countries (2). The size and shape of these cities will have long-term impacts on trans-portation CO2 emissions. Although quantification is difficult because of a lack of relevant global da-tabases, if the above results were to hold globally, it is feasible that the difference in global average VKT between the reduced- and non-reduced-sprawl sce-narios would be a factor of 2 (roughly one stabiliza-tion wedge; 8).

The comments above suggest that urban form could offer a useful approach to climate mitigation, but important historic, economic, and social issues have not been explored here. The political challeng-es are daunting. Sprawl reduction may or may not be feasible and desirable in all cities.

Many disciplines, including planning and policy, sociology, economics, and engineering, explore the causes and consequences of transportation demand. Hundreds of empirical studies connect urban form and daily travel (10). Estimates presented here are broadly consistent with limited prior research re-garding CO2 emissions and urban form. Estimates have suggested that in 45–50 yr, compact develop-ment would reduce annual gasoline CO2 emissions by between 5% (12) and 9–16% (10). Analogous esti-mates for comprehensive smart-growth policies in-clude 3–14% in 15–20 yr and 10–25% in 45 yr (11). A life-cycle analysis found that CO2-equivalent emis-sions are 60% less for high-density than for low-den-sity development (27). Annual emission reductions estimated here vary by scenarios and by elasticity; for low sprawl versus BAU, with 30% elasticity, reduc-tions are 15–20% for 15–20 yr and 39% for 45 yr. As discussed next, achieving those deep emission cuts via energy-efficient urban form would involve poli-cies (e.g., mixed-use development) that supplement and support the increased density (10, 11).

Though not quantified here, urban form can in-fluence nontransportation CO2 emissions, for exam-ple, via district-scale building thermal management (heating and cooling); combined heat and power generation; availability of local food and products; and embodied CO2 in roads, buildings, and other infrastructure.

MAy 1, 2008 / EnvironMEntAl SCiEnCE & tEChnology ■ 3135

Strategies for improving urban energy efficiencyPatterns of urban growth (e.g., sprawl vs infill) re-sult from thousands of independent decisions about where to live, work, and shop on the basis of attri-butes like crime rates, housing affordability, and school quality. Factors supporting automobile- dependent sprawl in the U.S. include comparative-ly inexpensive gasoline, extensive highways (built mostly with tax dollars), building-height limits, and parking requirements (e.g., requiring new stores to build parking spaces before opening; 28).

Many important zoning and other land-use de-cisions occur at the local level, which offers both a challenge (e.g., lack of local political will regard-ing global issues) and an opportunity (e.g., for cities wishing to reduce emissions yet frustrated by lack of direct control over vehicle fuel economy and other factors). Aligning sprawl reduction with other goals (e.g., public health and economic development) will increase public acceptance.

If done well, reducing sprawl can improve qual-ity of life while reducing emissions. Successful approaches likely differ among cities, especially between developing versus developed countries. In some cases, improving urban schools or reducing crime rates would decrease migration to suburbs and exurbs. Other cities may need to increase the supply of affordable, attractive medium- and high-density housing. Pedestrian- and bicycle-friendly neighbor-hoods, convenient mass transit, and land-use mix-ing (e.g., allowing retail near residences) can allow people to drive less each day if they wish (potentially increasing the density-VKT elasticity magnitude).

As one example, the “Living First” campaign in Vancouver required high-density neighborhoods to be aesthetically pleasant and full of amenities (e.g., easy access to parks, child-care facilities, and gro-cery stores; streetscapes with shops and row housing rather than blank high-rise walls; and safe, conve-nient mass-transit and pedestrian facilities; 29). The resulting neighborhoods benefit not only CO2 emis-sions (two-thirds of trips are by mass transit, bicycle, or walking) but also public health: by reducing au-tomobile usage, compact development also reduces traffic fatalities and obesity (29).

Similarly, Singapore’s convenient mass transit, mixed-use development (shops near residences), and disincentives for driving (e.g., vehicle taxes, con-gestion pricing) result in a commuting mode split of 53% of the population using mass transit (subway or bus), 31% using a private vehicle or taxi, and 15% not commuting or using “other” modes (e.g., bicy-cle) (30). Mass-transit use is 47% and 56% for pro-fessionals and associate professionals, respectively (30). Globally, mode share for transit/walking/bik-ing varies from >55% (Berlin, Madrid, Vienna) to <6% (Houston, Atlanta) (31).

London’s congestion toll reduced vehicle CO2 emissions by 20% in the charging zone; emissions on the surrounding ring road did not increase because lower per-kilometer emissions from less congestion offset minor increases in vehicle usage of ring roads (32). Lower VKT and reduced congestion contributed equally to the inside-zone emission reduction (32).

These examples further emphasize that urban de-sign and transportation policy can influence trans-portation CO2 emissions.

Calls for transportation- or energy-efficient urban form are not new—the 1973 oil crisis motivated mul-tiple studies (33)—but recent concern about climate change has reinvigorated discussion and provided political motivation for near-term actions yielding short-, medium-, and long-term benefits. For exam-ple, Washington State’s greenhouse gas legislation (Bill 2815), enacted in March 2008, seeks to reduce annual per capita vehicle-miles traveled by 18% by 2020, 30% by 2035, and 50% by 2050.

Of interest to policy makers, sprawl reduction might require less government intervention, not more. A common myth in the U.S. is that urban de-velopment patterns (e.g., sprawl) are a “natural” re-flection of free markets. Land and transportation are not free markets; subsidies, fees, regulations, and externalities abound. The largest barrier to infill development cited by U.S. developers is regulation (78%), not insufficient market interest (34).

Climate change mitigation will require a combi-nation of approaches. Fuel efficiency deserves atten-tion, and current research could generate a viable low-carbon fuel. But not all climate solutions require new technologies. Energy-efficient urban design of-fers an important tool that local governments can use to begin tackling this global problem.

Julian D. Marshall is an assistant professor of environ-mental engineering in the department of civil engineering at the University of Minnesota. Address correspondence about this article to Marshall at julian@umn.edu.

AcknowledgmentsThoughtful comments by Michael Brauer, Hadi Dowlatabadi, Kevin Krizek, David Levinson, Eric Mazzi, Conor Reynolds, and three anonymous reviewers are gratefully acknowledged.

Supporting InformationAn equation, input data, results, and discussion are present-ed free of charge in SI at http://pubs.acs.org.

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Correction: the Viewpoint article “Why Large-scale Affor-estation Efforts in china have Failed to solve the Desertifi-cation problem” (Environ. Sci. Technol. 2008, 42, 1826–1831) contained an error in the legend of Figure 1. the corrected figure appears below.

F I G U R E 1

Total area of afforestation in China from 1952 to 2005Data from Ref. 8.

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