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Executive Summary

The phrase “you can’t get there from here” has a new application.  A commonly accepted US target for climate stabilization is achieve a 60 to 80 percent reduction in CO₂ emissions by 2050 relative to 1990 levels in the United States.  But this paper demonstrates that the transport sector must contribute to this goal, and the transport sector cannot do its fair share through vehicle and fuel technology alone.   We have to sharply reduce the growth in vehicle miles driven across the U.S.’s sprawling urban areas, reversing trends that go back decades.

This report is based on an exhaustive review of existing research on the relationship between urban development, travel, and the carbon dioxide (CO₂) emitted by motor vehicles.   It provides evidence and insights on how much transportation-related CO₂ savings can be expected with compact development, how compact development is likely to be received by consumers, and what policy changes will make compact development possible.  Several related issues are not fully examined in this report. These include the energy savings from more-efficient building types, the potential to preserve forests that act as carbon sinks, and the effectiveness of pricing strategies—such as tolls, parking charges and mileage-based fees—when used in conjunction with compact development and expanded transportation alternatives.

The term compact development doesn’t imply high-rise or even uniformly high density, but rather higher average “blended” densities.   As important as density is, it is no more fundamental to compact development than mixing of land uses, development of strong population and employment centers, interconnection of streets, and the design of structures and spaces at a human-scale

Driving Up CO₂ Emissions

The U.S. is the largest emitter worldwide of the greenhouses gases that cause global warming. Transportation accounts for a full third of CO₂ emissions in the U.S., and that share is growing as others shrink in comparison, rising from 31% in 1990 to 33% today.  It is hard to envision a “solution” to the global warming crisis that does not involve slowing the growth of transport CO₂ emissions in the U.S.

The Three-Legged Stool Needed to Reduce CO₂ from Automobiles

Transport CO₂ reduction is a three-legged stool: one leg related to vehicle fuel efficiency; a second to the carbon content of the fuel itself; and a third to the amount of driving, or vehicle miles traveled (VMT).  Energy and climate policy initiatives at the federal and state levels have pinned their hopes almost exclusively on the first two legs of the stool: developing more efficient vehicles (such as hybrid cars) and lower carbon fuels (such as biodiesel fuel).  A stool cannot stand on two legs.

As the research compiled in this report makes clear, technological improvement in vehicles and fuels are likely to be offset by continuing, robust growth in VMT.  Since 1980, the number of miles Americans drive has grown three times faster than population, and almost twice as fast as vehicle registrations (see Figure 1.3).  Average automobile commute times in metropolitan areas have risen steadily over the decades, and now many Americans spend more time sitting in their cars commuting than they do vacationing. 

This raises some questions, which this report addresses: Why do we drive so much? Why is the total distance we’re driving growing so rapidly? And what can be done to alter this trend in a manner that is effective, fair and economically acceptable?

The growth in driving is due in large part to urban development, or what some refer to as the built environment.  Americans drive so much because we have given ourselves little alternative. For 60 years we have built homes ever farther from workplaces, created schools that are inaccessible except by motor vehicle, and isolated other destinations—such as shopping—from work and home.  From World War II until very recently, nearly all new development has been planned and built on the assumption that people will use cars virtually every time they travel. As a larger and larger share of our built environment has become automobile-oriented, car trips and distances have increased as walking and public transit use have declined.  Population growth has been responsible for only a quarter of the increase in the last couple decades. More of the increase has stemmed from factors related to the sprawling environment, such as longer trips and people driving alone.

As with driving, land is being consumed for development at a rate almost three times faster than population growth.  This expansive development has caused CO₂ emissions from cars to rise even as it has reduced the amount of forest land available to absorb CO₂.

How Growth in Driving Cancels Out Improved Vehicle Fuel Economy

CO₂ is more difficult to control through vehicle technology than are conventional air pollutants.   Conventional pollutants can be reduced in automobile exhaust with sophisticated emission control systems (catalytic converters, on-board computers, and oxygen sensors).  Carbon dioxide, meanwhile, is a direct outcome of burning fossil fuels; there is no practical way to remove or capture it when used in moving vehicles. At this point in time, the only way to reduce CO₂ emissions from vehicles is to burn less gasoline and diesel fuel.

An analysis by Steve Winkelman, a co-author of this report, finds that CO₂ will continue to rise despite technological advances, as the growth in driving overwhelms planned improvements in vehicle efficiency and fuel carbon content. The U.S. Energy Information Administration (EIA) forecasts that driving will increase 59 percent between 2005 and 2030 (red line, Figure 2.3), outpacing the 23-percent increase in population. The EIA also forecasts a fleet-wide fuel efficiency improvement of 12 percent over this time frame, primarily due to new federal fuel efficiency standards for light trucks (green line, Figure 2.3).  Despite this improvement in efficiency, however, CO₂ emissions would grow by 41 percent (dark blue line, Figure 2.3).

U.S. fuel economy has been flat for almost 15 years, as the upward spiral of car weight and power has offset the more efficient technology. Federal and state efforts are underway to considerably boost vehicle efficiency and reduce greenhouse gas emissions.  In June 2007, the U.S. Senate passed new Corporate Average Fuel Economy (CAFE) standards that would increase new passenger vehicle fuel economy from the current 25 mpg (cars and light trucks combined) to 35 mpg by 2020. California is implementing a Low Carbon Standard for Transportation Fuels that calls for a 10 percent reduction in fuel carbon content by 2020.

Even if these more stringent standards for vehicles and fuels were to go into effect nationwide, transportation-related emissions still would not come close to meeting goals for averting global climate change (see Figure 2.4).  The rapid increase in driving would overwhelm both the increase in vehicle fuel economy (green line) and the lower carbon fuel (purple line) so that CO₂ emissions would still be 12 percent above 2005 levels.  This would be 40 percent above the 1990 level (turquoise line), which is a benchmark for climate stabilization. Instead, the U.S. must bring CO₂ levels back to 15-30% below 1990 levels by 2020 to keep in play CO₂ reductions of 60-80% below 1990 levels by 2050.

As the projections show, the United States cannot achieve a 60-80% reduction in transportation-related CO₂ emissions relative to 1990 levels without sharply reducing the growth in miles driven.

Changing Development Patterns to Slow Global Warming

Recognizing the unsustainable growth in driving, the American Association of State Highway and Transportation Officials (representing state DOTs) is urging that the level of growth be cut in half between now and 2055. But how does a growing country—one with 300 million residents and another 100 million on the way by mid-century—slow the growth of vehicle miles driven? There are aggressive measures available, certainly: Imposing ever stiffer fees for driving and parking, or establishing no-drive zones or days. Some countries are experimenting with such measures. However, many in this country likely would view such steps as punitive, given the reality that most Americans don’t have a viable alternative to driving. The body of research surveyed here shows that much of the rise in vehicle emissions can be curbed simply by growing in a way that makes it easier for Americans to drive less.    In fact, the weight of the evidence shows that, with more compact development, people do drive less, at minimal or reduced cost, while reaping other fiscal and health benefits.

How compact development helps reduce the need to drive

Better community planning and more compact development help people live within walking or bicycling distance of some of the destinations they need to get to every day—work, the store, schools, and parks, as well as transit stops.  Rather than building single-use subdivisions or office parks, communities can plan mixed developments that put housing within reach of these other destinations.  The street network can be designed as a network of streets that connect, rather than ending in cul-de-sacs or funneling traffic onto overused arterial roads. Individual streets also must be “complete,” with safe and convenient places to walk, bicycle, and wait for the bus. Finally, by building more homes as condominiums, townhouses, or detached houses on smaller lots, and by building offices, stores and other destinations “up” rather than “out,” communities can shorten distances between destinations.  This makes neighborhood stores more economically viable, allows more frequent and convenient transit service, and helps shorten car trips. 

This type of development has seen a resurgence in recent years, and goes by many names. “Walkable neighborhoods” are becoming a popular selling point for real estate agents; “new urbanist” neighborhoods are reviving many features of pre-automobile communities; and “transit-oriented developments” (TOD) take advantage of the access provided by transit stations. “Infill” and “brownfield” developments put unused lots in urban areas to new uses, taking advantage of existing nearby destinations and infrastructure.  Some “lifestyle centers” are now replacing single-use shopping malls with open-air shopping on connected streets with housing and office space as part of the new development. And many communities have rediscovered and revitalized their traditional town centers and downtowns, often adding more housing to the mix.  These varied development types are referred to as “compact development” or “smart growth.”

How we know that compact development will make a difference: the evidence

As these forms of development have become more common, planning researchers and practitioners have documented that residents of compact, mixed use, transit-served communities do less driving. Studies have looked at the issue from varying angles, including:

• Research that compares overall travel patterns among regions and neighborhoods of varying compactness and auto-orientation;
• Studies that follow the travel behavior of individual households  in various settings;
• Models that simulate and compare the effects on travel of different future development scenarios at the regional and at the project level.

Regardless of the approach, researchers have found significant potential for compact development to reduce the miles that residents drive.

Studies that compare different types of regional or neighborhood development patterns find strong evidence that places with more compact development generate less overall driving.  A comprehensive sprawl index developed by co-author Dr. Reid Ewing of the National Center for Smart Growth, University of Maryland ranked 83 of the largest metropolitan areas in the United States by their degree of sprawl, measuring density, mix of land uses, strength of activity centers, and connectedness of the street network.  Even accounting for income and other socioeconomic differences, residents drove far less in the more compact regions.  In highly sprawling Atlanta, vehicles racked up 34 miles each day for every person living in the region.  On the other end of the scale, in Portland, Oregon, vehicles were driven fewer than 24 miles per person, per day. 

This relationship holds up in studies that focus on the travel habits of individual households, while measuring the environment surrounding their homes and/or workplaces.  The link between urban development patterns and individual or household travel has become the most heavily researched subject in urban planning, with over 100 rigorous empirical studies completed. These studies are able to control for factors such as socioeconomic status, and can account for the fact that higher-income households tend to make more and longer trips than lower-income families. 

One of the most comprehensive studies, conducted in King County Washington by Dr. Larry Frank, found that residents of the most walkable neighborhoods drive 26 percent fewer miles per day than those living in the most sprawling areas. The study showed that land-use characteristics are responsible for about 24 percent of the variation in household CO₂ production[x].  An analysis of many of these types of studies finds that households living in developments with twice the density, diversity of uses, accessible destinations and interconnected streets when compared to low-density sprawl drive about 33 percent less.

Many studies have been conducted by or in partnership with public health researchers interested in how the built environment can be better designed to encourage daily physical activity.  These studies show that residents of communities designed to be walkable both drive fewer miles and also take more trips by foot and bicycle, which improves individual health.  A recent literature review found that 17 of 20 studies, all dating from 2002 or later, had established statistically significant relationships between some aspect of the built environment and risk of obesity.

Two other types of studies also find relationships between development patterns and driving: simulations that project the effect of various growth options for entire regions; and project-level simulations that predict the impact for individual development projects sited and designed differently.  In regional growth simulations, planners compare the effect of a metropolitan-wide business-as-usual scenarios against more compact growth options.  Co-author Keith Bartholomew’s (University of Utah) analysis of 23 of these studies found that compact scenarios averaged 8 percent fewer total miles driven than business-as-usual, with a maximum reduction of 31.7 percent.  The better performing scenarios were those with higher degrees of land-use mixing, infill development, and population density, as well as a larger amount of expected growth.  The upshot is that the studies that projected accommodating a greater share of future of growth in walkable, compact areas, and less in conventional sprawl, saw greater reductions in VMT.  The travel models used in these studies would be expected to underestimate the impacts of site design, since most only crudely account for travel within neighborhoods and disregard walk and bike trips entirely.

Of the project-level studies, one of the best known evaluated the impact of building a very dense, mixed-use development at an abandoned steel mill site in the heart of Atlanta, versus spreading the equivalent amount of commercial space and number of housing units in the prevailing patterns at three suburban locations. Analysis using transportation models enhanced by co-author Jerry Walters of Fehr & Peers Associates, and supplemented by EPA’s Smart Growth Index (to capture the effects of site design) found that the infill location would generate about 35 percent less driving and emissions than the comparison sites.  The results were so compelling that the development was deemed a transportation control measure by the federal government for the purposes of helping improve the region’s air quality.   The Atlantic Station project has become a highly successful reuse of central-city industrial land.

Total estimated VMT reduction and total climate impact

When viewed in total, the studies on land use and driving show that compact development will reduce the need to drive between 20 and 40 percent, when compared with development on the edge that spreads out and isolates homes, workplaces, and other destinations.   

The research consensus is that compact, walkable development can reduce driving by an average of 30 percent.

Taking a midrange estimate, cutting back on VMT by 30% would be equivalent to substantial savings from an increase in fuel efficiency.  Returning to the projections made by Steve Winkelman, if between now and 2030, 60 percent of new development and redevelopment is compact, the resulting reduction in driving would save 53 million tons of CO₂ in that year, equivalent to a 16 percent increase in CAFE standards. It would be as if the fleet-wide efficiency for new vehicles had risen to 31 mpg by 2020.  Every resident of a compact neighborhood would provide the environmental benefit expected from, say, driving one of today’s most efficient hybrids. That effect would be compounded, of course, if that person also drove such an efficient car whenever he chose to make a vehicle trip. Smart growth would become an important ‘third leg’ of a comprehensive transport policy stool, along with more efficient vehicles and lower-carbon fuels.

Making reasonable assumptions about growth rate, market share of compact development, and VMT reduction with compact development, smart growth could by itself reduce total transportation-related CO₂ from expected trends by 7 to 12 percent by 2050. This estimate is based on land use change alone. It does not include additional reductions from other measures that have been proposed to reduce driving, such as higher fuel prices and carbon taxes, peak-period road tolling, pay-as-you drive insurance, the reduction or elimination of free parking, and other policies designed to make drivers pay more of the costs of driving. 

This estimate also does not include the energy savings gained in buildings with compact development. It is important to note that land-use changes provide a permanent climate change benefit that would compound over time.  The second 50 years of smart growth would build on the base reduction from the first 50 years, and so on into the future.  More immediate strategies, such as gas tax increases, do not have this degree of permanence.

The potential for smart growth

The potential for smart growth to curb the rise in greenhouse gas emissions will, of course, be limited by the amount of new development and redevelopment that takes place over the next few decades, and the share of it that is compact in nature.  There seems to be little question that a great deal of new building will take place as the population grows toward 400 million. According to the best available analysis, by Dr. Chris Nelson of Virginia Tech, 89 million new or replaced homes, and 190 billion square feet of new offices, institutions, stores, and other nonresidential buildings will be constructed through 2050.  If so, two-thirds of the development on the ground in 2050 will be built between now and then.Pursuing smart growth is a low-cost climate change strategy, because it involves shifting investments that have to be made anyway.  

Smart growth meets growing market demand for choice

There is no doubt that moving away from a fossil-fuel based economy will require many difficult changes.  Fortunately, smart growth is a change that many Americans will embrace.  Evidence abounds that Americans are demanding more choices in where and how they live—and that changing demographics will accelerate that demand.

While prevailing zoning and development practices typically make sprawling development easier to build, developers who make the effort to create compact communities are encountering a responsive public.

In 2003, for the first time in the country’s history, sales prices per square foot for attached housing, i.e., condominiums and townhouses, was more than detached housing units.  The real estate analysis firm Robert Charles Lesser & Co. has conducted a dozen consumer preference surveys in suburban and urban locations for a variety of builders to help them develop new projects. They have found that in every location, about one-third of respondents prefer smart growth housing products and communities. Other studies by the National Association of Homebuilders, National Association of Realtors, the Fannie Mae Foundation, high-production builders, and other researchers have corroborated these results—some estimating even greater demand for smart growth housing products. If smart growth also offers shorter commutes, it appeals to another quarter of the market, because many people are willing to trade lot or house size for shorter commutes.

Because the demand is greater than the current supply, price-per-square foot values of houses in mixed-use neighborhoods show price premiums ranging from 40 to 100 percent compared to houses in nearby single-use subdivisions, according to a study by the Brookings Institution.

This market demand is only expected to grow over the next several decades, as the share of households with children shrinks and those made up of older Americans grows with the retiring of baby boomers. Households without children will account for close to 90 percent of new housing demand, and single-person households will account for a 1/3^rd.  Professor Nelson projects that the demand for attached and small-lot housing will exceed the current supply by 35 million units (71%), while the demand for large-lot housing will actually be less than the current supply.

A climate-sparing strategy with multiple pay-offs 

Addressing climate change through smart growth is an attractive strategy because, in addition to being in line with market demand, compact development provides many other benefits and will cost the economy little or nothing. Research has documented that more compact development helps preserve farmland and open space, protect water quality, and improve health by providing more opportunities for physical activity.  

Studies also have confirmed that compact development saves taxpayers money, particularly by reducing the costs of infrastructure such as roads and water and sewer lines.  For example, the Envision Utah scenario planning process resulted in the selection of a compact growth plan that will save the region about $4.5 billion in infrastructure spending over a continuation of sprawling development.

Finally, unlike hydrogen-fueled vehicles and cellulosic ethanol, which get a lot of attention in the climate-change debate, the “technology” of compact, walkable communities exists today, as it has in one form or another for thousands of years.  Using this technology in the service of a cooler planet can start right now.

Policy Recommendations

Growing Cool: Removing Barriers and Key First Steps

In most metropolitan areas, compact development faces an uneven playing field. Local land zoning and development codes in most places remain geared toward producing auo-oriented development. Public spending often is skewed toward supporting development at the metropolitan fringe rather than supporting growth in already-developed areas. Transportation policies remain focused on accommodating rising rates of driving, rather than proving alternative options.

The policy options summarized below are based on three principles. First, government needs to remove barriers that prevent the private sector from meeting the market demand for more walkable, convenient neighborhoods. The second is that, rather than prescribe individual measures, the federal and state governments should set targets for the reduction of VMT-generated greenhouse gases (GHGs), and incentivize and reward innovative approaches by localities, metropolitan areas and the private sector. The last is that public spending should be pegged to performance in pursuit of a smaller carbon footprint, the realization of which would promote energy independence, economic efficiency and the environment. 

Federal Actions

Although land use regulation and growth management are primarily local and state responsibilities, the federal government plays a strong role in where that growth goes. Every year, national taxpayers spend more than $70 billion on infrastructure critical to urban development. Federal funds account for 40 percent of public money spent on transportation and water projects. Recommended changes in federal policy include:

(1) Enact Climate Legislation that Promotes VMT-Reduction and Smarter Growth.

Federal climate change legislation should require communities to create – and follow – plans for holding the line on VMT-generated emissions.  California’s Global Warming Solutions Act is a good model. Transportation plans should be required to conform to GHG reduction goals, just as they are for other pollutants. Revenues from any cap-and-trade system, as well as other resources, should be directed to help states and localities rewrite their planning and development rules and meet the demand for more travel-efficient communities.

(2) Enact a Transportation Law that Prioritizes Energy Security, Climate and Travel Options.

Fifty years ago the United States adopted the Interstate Highway Act, launching an unprecedented engineering project that in short order changed everything about the way we develop and get around. Today, with the interstates complete, we are entering a new era and the U.S. needs the equivalent -- an ambitious investment in completing the transportation system. The current national transportation funding law, known SAFETEA-LU, should be refashioned as “Green-TEA”. While preserving the highway system, it should invest more aggressively in public transit, intercity rail, cycling and walking. But spending alone is not the answer. Green-TEA also should:

• Reduce the need for travel by encouraging compact, mixed-use development;
• Meet performance goals and develop new funding formulas for reducing VMT growth and GHG emissions;
• Require state and regional entities to be accountable for meeting performance goals for GHG/VMT reductions and for planning that coordinates transportation and land use; and,
• Reduce the hurdles that now hamper federal approval for new transit projects.

(3) Put More Housing within Reach.

Today, close-in and transit-oriented housing is often scarce and expensive, in part because it is difficult to build.  Congress should create tax incentives for developers who build or rehabilitate developments in convenient locations, with efficient use of land close to travel choices. These incentives will decrease household transportation costs, and should also be coupled with incentives and programs for more-affordable housing in convenient locations.

(4) Fix It First.

In order to accommodate more efficient growth, the nation should make the most of what we’ve already built, and make a priority of repairing crumbling facilities over building new infrastructure on the fringes of our communities. Critical infrastructure such as roads, bridges, public transportation, intercity rail networks, water and sewer systems, levees, and schools should receive increased funding from new infrastructure spending and from this shift in priorities. This strategy should tie investment to performance and needs, not to funding formulas.

State Actions

Like the federal government, most states do not make local land use decisions. However, they do establish guidelines, standards and accountability for planning, state spending on infrastructure, environmental protection and many other areas. Some key policy changes that would promote smarter land use and reduce the growth rate of driving include:

(1) Set State Targets for Transportation Greenhouse Gas Emissions.

To meet national targets for greenhouse gas reductions (and sustainability goals such as those of AASHTO), states must play their part in slowing the growth of vehicle travel.  The first step would be for states to establish a “budget’” for transportation GHG emissions, as has been done for other pollutants. Localities and metropolitan areas would then develop suitable plans for achieving those goals. States should provide technical assistance and financial support to help localities fulfill those plans. In doing so, local communities will realize significant additional benefits, such as improved housing and transportation choices, fiscal savings and expanded services for underserved neighborhoods. Green-TEA would help by providing the right signals and incentives.

(2) Adopt State Transportation and Land Use Policies that Achieve GHG Targets.

Achieving a state transportation greenhouse gas target will require cooperation between localities and state transportation agencies. The state’s transportation policies should reinforce efforts to reduce VMT through the provision of greater transportation choices, smarter land use and development practices, and a vision for meeting the needs of existing travelers and an aging population. Policies also should emphasize a fix-it-first approach to infrastructure spending, safety for all users, and fiscally disciplined and performance-driven spending on transportation projects. States can also require developers to assess and mitigate climate impacts of large projects, following the example of Massachusetts’s revision of its state Environmental Policy Act.

(3) Align State Spending with Climate and Smart Growth Goals.

Funds for housing, economic development, infrastructure, water and sewer, schools, transportation, state facilities, and recreation should be allocated based on criteria that promote efficient growth. One model for this is the scorecard developed for the Commonwealth Capital Fund in Massachusetts, which awards points when local governments change their development rules and funding to promote more compact, mixed use, walkable neighborhoods.  Communities that score well receive greater access to some types of funding, linking state spending to results.

Local Actions

Many local governments are committing to action to reduce greenhouse gas emissions. More than 600 mayors have signed on to the U.S. Conference of Mayors’ Climate Protection Agreement.  In their efforts to reduce greenhouse emissions, these mayors and their counterparts in other cities and counties can have their greatest impact with policies that reduce vehicle miles traveled. Here are a few promising approaches:

(1) Change the Rules to Encourage Compact Development

Many communities have outmoded zoning and land-use codes that make it difficult for developers to build smart.  Local codes for building design, setback requirements, parking lots, street widths, crosswalks, and other design elements should be re-written to help create a pedestrian-friendly environment.  To end the time-consuming special approvals now required in many jurisdictions, localities may adopt regulations that make smart growth development permissible “by right,” as long as the projects comply with other environmental and community protections. Cities and towns also may offer incentives and streamlined approvals to developers who propose compact development projects with transit access, and affordable housing. Form-based codes and overlay zones also help to shape higher-density nodes to communities’ advantage. State and local governments should also find ways to expedite and reward exemplary projects that meet the Green Building Council’s LEED for Neighborhood Development certification standards, and consider adopting those standards as their own.

(2) Create Safe and Welcoming Environments for Pedestrians, Cyclists, and Transit Users.  

If communities make it easier for people to walk, bike, or ride transit, they create new options for people besides driving. Localities should adopt “complete streets” policies and design guidelines to create safe and welcoming environments for pedestrians, cyclists, and transit users. Accommodating new, walkable development on land that once held dead shopping centers or factories, or creating transit-oriented developments at rail stations, is likely to require investments in building or retrofitting a street network for pedestrians and cyclists. A great example of a place that has put all the elements together is Arlington County, Virginia, a suburb of Washington, D.C.  Arlington County’s master transportation plan includes elements for transit, bicycling, and pedestrians.

(3) Invest in Citizen Engagement to Help Residents Choose Their Community’s Future.

For plans to be as successful as possible, the people who will be living and working in the community must be involved in creating them. Metropolitan leaders can conduct regional visioning efforts to help local residents plan for future growth, such as those held in the Sacramento, CA, region and along the Katrina-affected Gulf Coast. The recent Louisiana Speaks process attracted over 27,000 participants, all of whom now have a stake in a plan for a more sustainable future. At the neighborhood level, localities can sponsor design workshops known as charrettes to allow residents to have hands-on involvement in shaping development that affects them.