United States 

Environmental Protection 

Agency 

Development, Community, 

and Environment Division (1808) 

Washington, DC 20460 

Our Built and Natural 

Environments 

EPA 231-R-01-002 

January 2001 

A Technical Review of 

the Interactions 

between Land Use, 

Transportation, and 

Environmental Quality 

Cover_Built&Nat'l Environ 12/22/00 11:19 AM Page 1 
Our Built and Natural Environments 

Acknowledgements 

This document has been peer reviewed. EPA acknowledges the helpful comments of Chester Arnold and 

James Gibbon, University of Connecticut; Robert Cervero, University of California at Berkeley; Susan 

Handy, University of Texas; Robert Johnston, University of California at Davis; and Peter Meyer, 

University of Louisville. 

Final document produced under EPA contract 68-W-99-054 for the Development, Community, and 

Environment Division, US EPA Office of Policy, Economics, and Innovation, by ICF Consulting. EPA 

staff members Geoffrey Anderson and Mary Kay Santore managed this project. 

To request additional copies of this report, call EPA's National Center for Environmental Publications at 

(513) 891-6561 and ask for publication number EPA 231-R-01-002. 

To access this report online, see <http://www.smartgrowth.org/>. 
Our Built and Natural Environments 

Contents 

Executive Summary i

Chapter 1 Introduction 1

1.1 Purpose 1

1.2 The Effects of Urban Form on Human Health and the Natural Environment 2

1.3 Topics of Discussion 3

Chapter 2 Land Use: Recent Trends and Environmental Impacts 4

2.1 Trends in Land Use 4

2.2 Causes of Land Use Change 8

2.3 Environmental Consequences of Land Use Trends 11

2.4 Summary 17

Chapter 3 Vehicle Travel: Recent Trends and Environmental Impacts 19

3.1 Trends in Vehicle Travel 19

3.2 Sources of VMT Growth 21

3.3 Environmental Consequences of Vehicle Travel 25

3.4 Summary 33

Chapter 4 Effects of Different Types of Development on the Environment 35

4.1 Compact Development 37

4.2 Design for Reduced Impervious Surfaces and Improved Water Detention 49

4.3 Design for Safeguarding Sensitive Areas 56

4.4 Mixed Land Uses 59

4.5 Transit Access 66

4.6 Support for Pedestrian and Bicycling Activity/Microscale Urban Design

Factors 71

4.7 Synergies: Combining Techniques 75

4.8 Summary 79

Chapter 5 Conclusion 80

Chapter 6 Bibliography 83
Our Built and Natural Environments 

Executive Summary 

In recent years interest has grown in Smart Growth as a mechanism for improving environmental 

quality. In Our Built and Natural Environments, the U.S. Environmental Protection Agency 

(EPA) summarizes technical research on the relationship between the built and natural 

environments, as well as current understanding of the role of development patterns, urban design, 

and transportation in improving environmental quality. Our Built and Natural Environments is 

designed as a technical reference for analysts in state and local governments, academics, and 

people studying the implications of development on the natural environment. 

The built environment has direct and indirect effects on the natural environment. Urban form 

directly affects habitat, ecosystems, endangered species, and water quality through land 

consumption, habitat fragmentation, and replacement of natural cover with impervious surfaces. 

Development patterns and practices also indirectly affect environmental quality since urban form 

influences the travel decisions that people make. Certain patterns of development encourage 

increased use of motor vehicles, which is associated with growth in emissions of air pollutants 

and the greenhouse gases that contribute to global climate change. Air pollution and climate 

change, in turn, can adversely affect water quality and habitat. 

Our Built and Natural Environments first examines trends in land use and their impacts. It then 

explores how various development patterns and practices can minimize environmental damage. 

LAND USE: TRENDS AND ENVIRONMENTAL IMPACTS 

Trends 

Development patterns have changed dramatically over the past century. In the early 1900s, urban 

areas tended to be compact, with a strong central business district and industrial facilities serving 

as large employment centers. Communities tended to be walkable and contained a mix of houses 

and convenience services such as shops. Today’s metropolitan areas extend over large areas and 

employment is frequently widely scattered. People must rely on automobiles for access to jobs 

and services, as residential and commercial areas are separated, and the pedestrian environment is 

increasingly inhospitable. 

In many regions, urbanized areas have expanded dramatically. Urbanized land area in the United 

States has quadrupled since 1954. From 1992 to 1997, the national rate of development more than 

doubled to 3 million acres per year. In most large metropolitan areas, urban land area rose more 

than twice as fast as population did between 1950 and 1990. The reasons for these dramatic 

changes in urban form are numerous, including income increases, living style preferences, and 

public policy on transportation investment, housing, and taxes that have facilitated these trends. 

Environmental Impacts 

Direct environmental impacts of current development patterns include habitat loss and 

fragmentation, and degradation of water resources and water quality. Building on undeveloped 

land destroys and fragments habitat and thus displaces or eliminates wildlife communities. The 

construction of impervious surfaces such as roads and rooftops leads to the degradation of water 

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Our Built and Natural Environments 

quality by increasing runoff volume, altering regular stream flow and watershed hydrology, 

reducing groundwater recharge, and increasing stream sedimentation and water acidity. A 1-acre 

parking lot produces a runoff volume almost 16 times as large as the runoff volume produced by 

an undeveloped meadow. Development claimed more than half of the wetlands in the lower 48 

states between the late 1700s and the mid-1980s. 

VEHICLE TRAVEL: TRENDS AND ENVIRONMENTAL IMPACTS 

Trends 

Vehicle travel has increased substantially in recent decades. Between 1980 and 1997, vehicle 

miles traveled (VMT) in the United States increased 63 percent. This growth rate was almost 

three times more rapid than population growth during the same period. 

Development patterns have contributed to increased vehicle use. Investment in highway capacity 

encourages more vehicle travel by temporarily reducing travel time and costs. Dispersed, lowdensity development with significant distances between housing, jobs, schools, and shopping 

make walking, bicycling, or use of transit difficult for most trips. Urban design that emphasizes 

the automobile, such as large surface parking lots, wide streets, and a lack of sidewalks, make 

vehicle use more comfortable and safer than walking or bicycling, even for short trips. 

Impacts 

The environmental consequences of vehicle travel and dependency include degradation of air 

quality, greenhouse gas emissions and increased threat of global climate change, and noise. 

Emissions from vehicle travel pose serious threats to ecological and human health. In 1991, air 

pollution from highways is estimated to have caused between 20,000 and 46,000 cases of chronic 

respiratory illness. Atmospheric deposition of vehicle pollutants into bodies of water also 

adversely affects water quality. The economic costs of air pollution in terms of health impact, 

crop damage, and building and materials damage are significant. 

Transportation is also a significant source of greenhouse gas emissions. The accumulation of 

greenhouse gases in the atmosphere is widely associated with changes in global climate that could 

raise sea level and increase the frequency and severity of extreme weather events worldwide. 

Although motor vehicle emissions of most air pollutants have declined since 1970 due to 

improved technologies and cleaner fuels, increasing VMT growth threatens to reverse this trend. 

Greenhouse gas emissions from motor vehicles have been increasing rapidly, fueled by increased 

vehicle travel. 

DEVELOPMENT PRACTICES TO REDUCE ENVIRONMENTAL IMPACTS 

Although the built environment inevitably affects the natural environment, communities can avail 

themselves of techniques for minimizing the “environmental footprint” of development. Patterns 

of development and its design greatly affect the level of direct environmental impacts associated 

with urban form. Environmentally sensitive development patterns minimize habitat and water 

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Our Built and Natural Environments 

quality impacts. They also decrease dependence on motor vehicles, reducing air pollution and 

greenhouse gas emissions. Environmentally sensitive development patterns and practices are 

summarized below. 

Compact Development 

Compact development can accommodate growth while minimizing use of undeveloped land. 

Techniques for encouraging compact development include the following: 

� Infill development. By accommodating new growth in an already urbanized area rather 

than using up new land on the periphery of a region, infill development minimizes 

growth in impervious surface area and thus runoff. Because infill takes place in a 

developed area, it is often accessible via transit or walking, and requires shorter trip 

distances than development on the periphery. 

� Brownfield redevelopment. EPA defines brownfields as “abandoned, idled, or underused 

industrial and commercial facilities where expansion or redevelopment is complicated by 

real or perceived environmental consequences.” A special type of infill, brownfields take 

advantage of the benefits of infill development. Developing brownfields reduces risk to 

communities by cleaning up contaminated sites. 

� Cluster development. By reducing lot sizes and employing narrow and shorter road 

widths and lengths, cluster developments typically have less impervious surfaces than 

more dispersed development patterns do. 

Because compact development uses up less land area, it can reduce habitat disruption and adverse 

impacts on wildlife, vegetation, and water quality. Regional travel studies have found that most 

compact development patterns produce less vehicle travel and fewer emissions of air pollutants 

than dispersed development patterns do. New development in a regionally central infill site can 

generate significantly less motor vehicle air pollution than the same development on a 

“greenfield” site. 

Reduced Impervious Surfaces and Improved Water Detention 

Land use measures can improve water quality by reducing impervious surface area and regulating 

the flow of stormwater. These measures include: 

� Narrowing and shortening streets and minimizing the provision of parking areas 

� Using porous surfaces when feasible, such as lattice blocks and bricks set in sand, rather 

than concrete and asphalt 

� Detaining stormwater for short periods in swales and filter strips and for longer periods in 

ponds and wetlands 

� Using special landscaping practices, such as the application of mulch to retain soil 

moisture and conserve water usage 

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Our Built and Natural Environments 

Safeguarding Environmentally Sensitive Areas 

The impacts of development depend not only on how much land is developed but also on the 

location and type of land. Sensitive natural areas such as streams, wetlands, floodplains, steep 

slopes, mature forests, swamps, critical habitat areas, and shorelines can be safeguarded through 

the following measures: 

� Minimize impacts. The first approach should always be to avoid disturbing sensitive 

areas. If impacts are unavoidable, then development should be designed to minimize 

impacts and limit disturbance to points of least sensitivity. 

� Create buffers and greenbelts. A “green corridor” can be preserved along the banks of 

rivers, streams, or other sensitive environmental habitat to protect these areas from 

development. 

Mixed-Use Developments 

In mixed-used developments, complementary functions are located close together. This kind of 

development has the potential to use parking and transportation infrastructure more efficiently, 

thus requiring less pavement and reducing runoff. By decreasing travel distances, mixed-use 

development can reduce average vehicle trip lengths and increase the potential for individuals to 

use nonautomobile travel modes. Examples of mixed-use development include the following: 

� Mixing retail and office uses with residential development 

� Mixing uses at employment and commercial centers 

� Developing a subregional balance of jobs and housing 

Transit Access 

Efficient transit networks can serve urban areas effectively, reducing fuel consumption, pollutant 

emission, and traffic congestion. Compact regions with a limited number of subregional centers 

linked by transit can support transit ridership and reduce VMT compared with other regional 

development patterns. Locating high-density commercial and residential development around 

transit stations improves accessibility to transit since more households are within walking 

distance of the facilities. 

Microscale Urban Design Features 

Enhancing the environment for nonmotorized travel such as walking and bicycling can lead to 

reduced vehicle travel. Microscale urban design features that improve the pedestrian environment 

include sidewalks, clearly marked crosswalks and walk signals, lighting, and other amenities like 

shade trees, benches, and streetscapes designed with the pedestrian in mind. Features that 

improve the bicycling environment include bicycle paths and lanes on streets, bicycle parking, 

and signage to identify recommended bicycle routes and raise awareness of drivers to bicycle 

traffic. 

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Our Built and Natural Environments 

SYNERGIES 

Many of the land use measures described in Our Built and Natural Environments can have 

positive environmental effects. Their efficacy in particular locations, however, depends on how 

well they are implemented and how they are combined with each other and with other programs. 

Communities that offer their residents lasting environmental, economic, and social benefits 

usually adopt a synergistic method of planning—one that incorporates multiple beneficial aspects 

of design, according to the particular needs and characteristics of a community. 

CONCLUSIONS 

Urban form directly and indirectly affects the quality of our nation’s air, water, and wildlife 

habitat. Our Built and Natural Environments provides a summary of the current understanding of 

these effects. The evidence suggests that the way we build our communities is an important 

concern as the United States attempts to meet its environmental goals. 

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Our Built and Natural Environments 

1. Introduction 

1.1 PURPOSE 

Environmental quality and human health are priorities for America. As a nation, the United States 

has committed to national standards for environmental performance to protect human health and 

welfare. Congress has set timetables for meeting health and environmental quality goals in every 

environmental medium, and the U.S. Environmental Protection Agency (EPA) is charged with 

implementing the federal laws designed to promote public health by protecting the nation’s air, 

water, soil, and climate. 

For most of EPA’s 30-year history, policy-makers have focused on technological approaches to 

reducing pollution. These efforts have met with significant success. Emissions from motor vehicles 

and point sources such as power plants and refineries have been reduced through the use of 

cleaner fuels and technology, and some of the most visible environmental problems (e.g., lead in 

gasoline, sulfur dioxide from industry) have been addressed. Despite these successes, 

technological solutions are unlikely to provide the solution to all of our environmental challenges. 

Increasingly, policy-makers are realizing that decisions regarding development patterns and 

transportation investment have significant impacts on the natural environment. In recent years, 

cities, states and regions around the nation have begun planning for Smart Growth. Concerns 

about community livability, loss of open space, traffic congestion, and air and water quality are 

spurring much of this interest. Recognition is increasing that land use and transportation decisions 

can either support community goals for livability and environmental protection, or interfere with 

those goals. Efforts to mitigate growth-related environmental impacts are being promoted by 

numerous organizations and implemented by communities nationwide. 

Our Built and Natural Environments is a technical reference work on the implications of land 

use and transportation for the environment, written as a resource for technical analysts in state 

and local governments, academics, and those studying the environmental impacts of urban form. 

The report: 

� Articulates current understandings of the relationship between the built environment and 

the quality of air, water, and habitat. 

� Discusses trends in development and transportation and their environmental implications. 

� Provides evidence for the view that communities can affect environmental quality 

positively through Smart Growth land use and transportation development decisions. 

The report concludes that together, the built environment and decisions made in response to it 

dramatically impact environmental health, and ultimately community quality of life. Thus, the form 

of built environment can help or hinder our nation’s ability to meet its environmental goals. As a 

result, the relationship between the built and natural environments deserves attention from EPA, 

the agency’s colleagues and clients, and communities across America. 

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Our Built and Natural Environments 

1.2 THE EFFECTS OF URBAN FORM ON HUMAN HEALTH AND 

THE NATURAL ENVIRONMENT 

The nation has recognized the environmental implications of industrial discharges for decades. The 

environmental effects of metropolitan development and transportation investment, however, are 

more difficult to define. As a result, only recently have they begun to attract widespread attention. 

As Figure 1-1 illustrates, development decisions have both direct and indirect impacts on the 

natural and human environments. The built environment directly affects habitat, ecosystems, 

endangered species, and water quality through consumption, fragmentation, and replacement of 

natural cover with impervious surfaces. Urban form also affects travel behavior which, in turn, 

affects air quality (with corresponding impacts on water quality), global climate, and noise. 

Figure 1-1: Direct and Indirect Effects of the Built Environment 

Built Environment 

Land use patterns 

Transportation infrastructure 

Building orientation and design 

Mobility and Travel Decisions 

Vehicle miles of travel (VMT) 

Road congestion 

Habitat, Ecosystems, and 

Endangered Species 

Water Quality 

Air Quality 

Global Climate 

Noise 

Direct 

Effects 

Indirect 

Effects 

Environmental Quality 

Direct Effects 

Land development affects the environment in two primary ways. First, development uses land and 

modifies habitats and ecosystems. The extent of land development, the type of development, and 

the location of infrastructure have direct and long-lasting implications for ecosystems. By 

interrupting feeding, dispersal, and breeding patterns, even a single roadway that cuts through 

wilderness can affect the population and diversity of species across a wide area. 

Second, development can have significant implications for water quality as buildings, parking lots, 

roads, and other impervious surfaces alter the natural flow of water within a watershed. The 

amount of impervious surface as a percentage of land area in a watershed and the location of 

infrastructure in relation to specific natural resources can be correlated to the health of an area’s 

streams, river, lakes, and estuaries. 

These direct effects are relatively well understood and documented, and efforts to preserve 

wetlands and habitat of endangered species are common. Still, the implications for biodiversity, 

ecosystems, and water systems are often site-specific and are not universally recognized in 

community design and planning. 

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Indirect Effects 

Indirect effects of residential and commercial development include the distribution of employment 

opportunities. In addition, the transportation options available to link residential and commercial 

locations influence household travel behavior, including trip frequency, trip lengths, and mode of 

choice. Vehicle travel, in turn, generates air pollutant emissions, greenhouse gas emissions, and 

noise. 

The effects of urban form on travel behavior are less well understood than the direct effects of 

development, and the magnitude of those impacts is widely debated. Travel behavior is complex, 

with various factors simultaneously affecting decisions about how much, where, when, and how to 

travel. Still, significant evidence exists that urban design does affect travel behavior. As a result, 

there is increasing interest in using land use planning to address transportation, air quality, and 

greenhouse gas problems. 

Our Built and Natural Environments presents findings from researchers, academicians, policy 

analysts, and others as to how, where, and to what degree development practices and patterns 

directly and indirectly affect environmental quality. 

1.3 TOPICS OF DISCUSSION 

Chapter 2 outlines some of the more significant trends in land use and transportation infrastructure 

development, and their direct environmental consequences. Chapter 3 addresses trends in vehicle 

travel and environmental effects associated with vehicle use. Both chapters discuss the causes of 

change over time and suggest that development decisions, public policy, and changing 

demographics are all important factors in these trends. Chapter 4 provides evidence that different 

patterns of growth and development can have different impacts on the environment. Patterns and 

practices that minimize environmental harm include the following: 

� Compact development

� Reducing impervious surfaces and improving water retention

� Safeguarding environmentally sensitive areas

� Mixing land uses

� Providing transit accessibility

� Supporting pedestrian and bicycling activity (micro-scale urban design elements)

Chapter 4 also discusses “synergy,” the idea that a combination of strategies together can have 

greater impact than the sum of individual measures. The chapter illustrates how various planning 

strategies can work together to improve environmental quality. 

The final chapter provides a brief summary and conclusion about the effects of urban form on 

national environmental goals. 

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Our Built and Natural Environments 

2. Land Use: Recent Trends and Environmental 

Impacts 

2.1 TRENDS IN LAND USE 

Urban form has changed dramatically over the past century. In the early 1900s, most urban areas 

were compact and monocentric—that is, most had a strong central business district—and 

industrial facilities such as ports served as major employment centers. Small neighborhood shops 

were frequently fixtures in residential areas. Suburbs grew in tandem with extensions of streetcar 

and railroad lines, and residential developments were clustered along grid street networks. 

Residential development typically extended only as far from streetcar lines as a person might 

comfortably walk. 

The geographic size and the nature of urban development have undergone tremendous change. 

Metropolitan areas are polycentric, with multiple clusters of development dispersed over a large 

area. Increasing numbers of Americans live in suburban communities quite far from the old urban 

centers.2 The automobile has become central to mobility, as new subdivisions are constructed in 

locations separated from commercial services and business parks are built in areas that require 

vehicle travel not only for access but also for internal circulation. Interstate highways connect 

geographically dispersed locations, and shopping malls and business parks have become important 

places for commerce. 

One of the most noticeable trends in urban form is the dramatic expansion in the geographic size 

of metropolitan areas. Virtually every urban area in the United States has expanded substantially 

in land area in recent decades. Between 1954 and 1997 (the most recent year analyzed in the 

U.S. Department of Agriculture inventory), urban land area has almost quadrupled, from 18.6 

million acres to about 74.0 million acres in the contiguous 48 states.3 In 1997, developed land 

totaled about 7 percent of the nation’s nonfederal land area—a seemingly small amount. However, 

from 1992–1997, the national rate of development more than doubled. During this five-year period, 

more land was developed (nearly 16 million acres) than during 1982–1992 (about 13 million acres). 

The newly developed land has come mostly from forestland, pasture and range, and cropland.4 A 

1994 study by the American Farmland Trust showed that urban development already has 

consumed nearly a third of the country’s most highly productive farming regions.5 Developed land 

2 From 1950 to 1990, the percentage of the U.S. urban population in the central counties of today’s 50 largest 

metropolitan areas fell from 46.5% to 37.6%. (U.S. Census Bureau). 

3

 U.S. Department of Agriculture, Economic Research Service, Natural Resources and Environment Division. 1997 

National Resources Inventory. 1997. 

4

 U.S. Department of Agriculture, Economic Research Service, Natural Resources and Environment Division. 

Agricultural Resources and Environmental Indicators (AREI) Updates, No. 3. “Major Land Use Changes in the 

Contiguous 48 States.” June 1997. 

5

 American Farmland Trust. Farming on the Edge: A New Look at the Importance and Vulnerability of Agriculture Near 

American Cities. 1994. 

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area has increased rapidly in many states, particularly in the South and Southwest, as shown in 

Table 2-1.6 

Table 2-1: Increase in Developed Land Area for Selected States, 1982-1992 

State Percent increase in 

developed land, 1982-1992 

Arizona 

California 

Colorado 

Florida 

Georgia 

Illinois 

Massachusetts 

Michigan 

Nevada 

New Jersey 

New York 

North Carolina 

Ohio 

Pennsylvania 

Texas 

Tennessee 

Virginia 

35.1% 

19.1% 

22.4% 

34.6% 

32.8% 

8.4% 

21.7% 

14.3% 

26.3% 

23.1% 

8.0% 

36.2% 

15.3% 

14.6% 

20.5% 

25.2% 

25.7% 

Source: Noss, R. and R. Peters. Endangered Ecosystems: A Status Report on America’s Vanishing Habitat and 

Wildlife. Washington, DC: Defenders of Wildlife, December 1995. Derived from data from the U.S. Bureau of 

the Census. Statistical Abstract of the United States, 1994. 

Many urban areas have increased in size by 50% during the past 30 years, with the increase in 

land development far outstripping population increases. U.S. Census data for the 34 metropolitan 

areas with populations of more than one million show that urbanized land area has grown on 

average 2.65 times as fast as population has.7 In urban areas with relatively slow population 

growth, the contrast between population growth and urbanized land area growth is especially 

dramatic. For example, the Census reports that from 1950 to 1990, the Pittsburgh urbanized area 

grew more than 21 times as fast as its population (from 1.533 million people and 254 square miles 

6

 Developed areas include urban and built-up areas in units of 10 acres or greater, and rural transportation. Most recent 

data available are for 1992. An inventory of U.S. land resources by type of use/cover was conducted by the Soil 

Conservation Service in 1982, 1987, and 1992, and were most recently published in the 1992 National Inventory of 

Land Resources (See Statistical Abstract of the United States: 1998. September 1998.) 

7

 The Census Bureau delineates urbanized areas to provide a better separation of urban and rural territory in the vicinity 

of large places. Urbanized area is defined as one or more places ("central place") and the adjacent densely settled 

surrounding territory ("urban fringe") that together have a minimum of 50,000 persons and a density of least 1,000 

persons per square mile. The urban fringe also includes outlying territory of such density if it was connected to the core 

of the contiguous area by road and is within 1 1/2 road miles of that core, or within 5 road miles of the core but 

separated by water or other undevelopable territory. Other territory with a population density of fewer than 1,000 

people per square mile is included in the urban fringe if it eliminates an enclave or closes an indentation in the boundary 

of the urbanized area. Metropolitan areas with population over 1 million in any decennial census from 1950 to 1990 

were analyzed. 

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Our Built and Natural Environments 

in 1950 to 1.678 million people and 778 square miles in 1990).8 As Table 2-2 shows, the trend is 

widespread. 

Table 2-2: Growth in Land Consumption Exceeds Population Growth in Metro Areas, 

1950-1990 

Urbanized Area Population 

Growth, 

1950-90 

Urbanized 

Area Growth, 

1950-90 

Ratio of 

Area Growth to 

Pop. Growth 

Pittsburgh 

Buffalo 

Milwaukee 

Boston 

Philadelphia 

St. Louis 

Cleveland 

Cincinnati 

Kansas City 

Detroit 

Baltimore 

New York 

Norfolk 

Chicago 

Minneapolis-St. Paul 

Atlanta 

Washington 

34 Metro Areas with Pop. > 1 million 

9.50% 

6.60% 

47.90% 

24.30% 

44.50% 

39.00% 

21.20% 

49.10% 

82.70% 

34.30% 

62.70% 

30.50% 

243.60% 

38.00% 

110.70% 

325.40% 

161.30% 

92.40% 

206.30% 

132.50% 

402.00% 

158.30% 

273.10% 

219.30% 

112.00% 

250.70% 

411.40% 

164.50% 

290.10% 

136.80% 

971.00% 

123.90% 

360.20% 

972.60% 

430.90% 

245.20% 

21.72 

20.08 

8.39 

6.51 

6.14 

5.62 

5.28 

5.11 

4.97 

4.80 

4.63 

4.49 

3.99 

3.26 

3.25 

2.99 

2.67 

2.65 

Source: The Public Purpose. “Demographic Briefs and Urban Policy.” Calculated based on 

data from the U.S. Census Bureau. 

A 1996 study of traffic congestion trends by the Texas Transportation Institute provides further 

evidence of the dramatic growth in urbanized area compared to population. Based on data 

collected from metropolitan planning organizations, the study shows urbanized area continuing to 

increase faster than population growth in most metropolitan areas, as shown in Table 2-3. Among 

the areas studied, urbanized area increased on average 43 percent faster than population growth 

between 1982 and 1996. 

Not only have people moved farther from the old city centers, but the nature of development has 

changed dramatically. Commercial and office developments are surrounded by large parking lots, 

with few sidewalks or connections to other developments. Hierarchical street patterns channel 

traffic to a number of large arterials, and wide streets and driveways are common in residential 

areas. As a result, people are using more land per capita than in the past. According to a report on 

development in the Chesapeake Bay Watershed,9 residential and commercial development used 

0.65 acre of land per person in 1988 compared with about 0.18 acre in the 1950s. According to the 

8

 U.S. Census Bureau. 

9

 2020 Panel. “Population Growth and Development in the Chesapeake Bay Watershed to the Year 2020.” Annapolis, 

MD: 1988. 

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Our Built and Natural Environments 

report, Dorchester County, on Maryland’s rural Eastern Shore, actually lost population between 

1980 and 1990, yet gained hundreds of new homes. During that time period, less than one-tenth of 

new population consumed half the newly developed land—due to development on large suburban 

and rural lots. 

Table 2-3: Growth in Land Consumption Exceeds Population Growth, 1982-1996 

Urbanized Area 

Population 

Growth, 

1982-96 

Urbanized Area 

Growth, 

1982-96 

Ratio of 

Area Growth to 

Pop. Growth 

Detroit, MI 

Rochester, NY 

Buffalo-Niagara Falls, NY 

Pittsburgh, PA 

Harrisburg, PA 

Boston, MA 

Chicago-Northwestern IN 

Cleveland, OH 

New York-Northeastern NJ 

St. Louis, MO-IL 

Baltimore, MD 

Nashville, TN 

Tucson, AZ 

Las Vegas, NV 

Los Angeles, CA 

Houston, TX 

Avg. Of 70 U.S. Metropolitan Regions 

-1.1% 

-3.1% 

0.0% 

6.6% 

14.5% 

5.6% 

10.9% 

6.3% 

2.9% 

9.2% 

26.2% 

25.0% 

42.2% 

138.9% 

23.4% 

27.5% 

20.2% 

19.6% 

15.5% 

52.0% 

39.0% 

72.0% 

26.9% 

44.2% 

23.8% 

10.1% 

30.8% 

64.4% 

53.9% 

86.7% 

243.8% 

22.7% 

9.8% 

28.8% 

-– 

-– 

-– 

5.9 

5.0 

4.8 

4.1 

3.8 

3.4 

3.3 

2.5 

2.2 

2.1 

1.8 

1.0 

0.4 

1.43 

Source: Calculated based on data from Texas Transportation Institute. Mobility Study 

(Urban Roadway Congestion: Annual Report 1998). 

Urban decentralization and growth in transportation infrastructure, mostly in the form of highway 

construction, have dramatically increased total developed land (and impervious) surface area. 

Figure 2-1 provides a visual image of growth in urban development for the Baltimore-Washington 

region, based on a study sponsored by the U.S. Geological Survey. 

Figure 2-1: Baltimore-Washington Growth 

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Temporal databases and mapping document the dramatic increase in urban development following 

World War II and continuing to the present day in a number of metropolitan areas. Each urban 

database focuses attention on the forces influencing current spatial patterns and corridors. As 

seen in the Baltimore-Washington maps, the amount of land area developed has expanded 

tremendously in the 20th century. In 1900, urban development was located almost exclusively in 

the cities of Baltimore and Washington, with a vast amount of undeveloped and agricultural land 

between the two. By 1992, development was so widespread that the two formerly distinct areas 

shared common suburbs. The U.S. Census Bureau now recognizes the region as one large 

consolidated metropolitan area. 

An analysis of development and transportation clearly demonstrates the influence that 

transportation infrastructure (roads, railroads, and seaports) exerted on development patterns. 

Roads alone take up considerable space. Nationwide, roads take up approximately 11.1 million 

acres of land, or 17,375 square miles, not including road shoulders and medians.10 This area equals 

the geographic size of the states of Maryland and Delaware combined. Buildings and parking lots 

also take up considerable area. According to one estimate, land devoted to parking ranges from 

1,910 to 3,035 square miles (or approximately 1.2 to 1.9 million square acres) in the United 

States.11 

The exact percentage of land area in U.S. urban areas consumed by streets and parking is 

unknown.12 Nonetheless, road and parking areas are known to be increasing. Although the rate of 

new highway construction has slowed from the growth rates characteristic of the 1950s and 

1960s, transportation infrastructure continues to claim new land. A net increase of 91,357 road 

miles occurred between 1990 and 1997.13 Each year from 1993 to 1997, an average of 16,389 

miles of roadway projects financed with federal aid were underway. 

10 Figure is for 1997. U.S. Environmental Protection Agency, Office of Policy, Planning, and Evaluation, Indicators of 

the Environmental Impacts of Transportation: Highway, Rail, Aviation, and Maritime Transport. October 1999, EPA 

230-R-99-001. 

11 Delucchi, M.A. “Bundled Private Sector Costs of Motor Vehicle Use.” Report No. 6 in Series: The Annualized Social 

Costs of Motor-Vehicle Use in the U.S., Based on 1990-91 Data. Davis, CA: University of California, Davis, Institute 

of Transportation Studies. 1995. 

12 There are many problems inherent in calculating this sort of statistic. In particular, some road or parking areas are 

rarely used by vehicles and may actually function as open space, such as a driveway alongside a house. Many pre-auto 

cities contained wide streets and boulevards, suggesting that it is inappropriate to consider all roads and parking as 

associated with automobiles. Still, it is certain that impervious surface area has increased with the development of new 

roads and parking. There is no data on the share of urban land devoted to parking, and some estimates of land area 

consumed by roads have been found to be of questionable basis. See Shoup, Donald. “The Pedigree of a Statistic.” 

Access. University of California Transportation Center. Berkeley, CA: Fall 1997, 11: 41. 

13 U.S. Department of Transportation, Federal Highway Administration. Highway Statistics (1990 and 1997). Table 

HM-12. 

8 
Our Built and Natural Environments 

2.2 CAUSES OF LAND USE CHANGE 

Observed increases in rates of urbanization and population dispersion are enabled and influenced 

by a variety of factors. New communications and transportation technologies, along with changes 

in public policy have facilitated increases in the size of metropolitan areas. Reductions in the 

monetary and time costs of travel play a particularly important role in these trends. The 

widespread adoption of the automobile as the predominant travel mode and major investments in 

transportation infrastructure have been important factors in encouraging the dispersal of residential 

and commercial development within metropolitan areas. However, social and economic dynamics 

have also played a large role in this shift. 

Highway Development and Reduction in Travel Costs 

New highways open up land for development by increasing the accessibility of locations that once 

were remote or difficult to reach.14 Just as the opening of streetcar lines allowed the development 

of streetcar suburbs in the early 1900s, the development of interstate highways and other 

freeways allowed the expansion of residential development into formerly rural areas. 

According to economic theory, accessibility is one factor that affects the desirability and therefore 

the price of land. All else being equal, reduced travel costs increase the relative value of land at 

the periphery of a metropolitan area to urban residents and workers. The increased price leads 

potential residents to outbid farmers for land near the urban region’s edge. At the same time, 

transportation infrastructure enables more land to be available for development. The availability of 

more land allows people to use more transportation and land resources without an increase in 

transport cost. However, as a World Bank report emphasizes, “where transport prices do not 

reflect full social and environmental costs, the land market can generate inefficient land-use 

patterns.”15 

One of the basic tenets of modern location theory, well articulated since the 1920s, is the influence 

of transport costs on a company’s choice of location. As the relative importance of transportation 

costs decreased (both due to lower transportation costs and economic shifts from manufacturing 

to services), transportation’s role diminished in location choices, and more locations become viable 

site alternatives. Reduced transportation costs thus greatly facilitated the dispersal of employment. 

For manufacturers, improvements in highway connections simplified the use of trucks for intercity 

traffic. As manufacturers switched from trains and ships to trucks, they were freed from 

dependence on railheads and ports, generally located near the city centers, and were able to move 

manufacturing facilities to sites accessible to interstate highways. The shift from rail and ship to 

trucks also facilitated the evolution from concentric development around major transport facilities 

(the port and railyard), to axial and polycentric development patterns.16 

Finally, changes in commodity flows and the development of modern logistics concepts such as 

14 World Bank. Sustainable Transport: Priorities for Policy Reform. Washington, D.C.: 1996, p. 59. 

15 Ibid. p. 61, citing Egal, Y. 1994. “Des effets positifs de la densité urbaine.” Transports Urbains. 83 (avril-juin) 27-31. 

16 Button, K.J. Transport Economics. Hampshire, England: Gower Publishing Co. Ltd., 1982, p. 261-263. 

9 
Our Built and Natural Environments 

just-in-time delivery reduced dependence on central locations. The freight-generating 

characteristics of land uses were surpassed in importance by the truck trip-generating 

characteristics, reducing the influence of manufacturing and industrial facilities on land use and 

development patterns of cities.17 

Firms with highly skilled work forces have also changed their locational patterns in recent years. 

Because these firms draw employees from the entire metropolitan area, they need to locate at a 

point accessible from all points in the area. In the streetcar city, the city center (hub) was the most 

accessible location. In the modern automobile city, firms may choose to locate in the urban core, 

but often they are drawn to locations along an urban highway or suburban beltway, since such 

sites are accessible to a large population via automobile.18 This change is further reinforced by the 

tendency for the peripheral suburbs to contain a relatively high proportion of wealthy, welleducated, well-trained workers.19 In sum, increased highway development loosens ties to the 

central business district (CBD), allowing the suburbanization of firms. Highway development 

facilitated similar suburbanization of retailers. If shoppers have access to private vehicles, 

suburban stores may be more accessible than center city locations. In fact, as residential 

suburbanization occurred, many retailers followed consumers out to the suburbs to be closer to 

their markets. 

Public Policy 

Public policy plays an important role in shaping the built environment. After World War II, federal 

funding for highway construction opened vast areas of land for development. A survey of experts 

by the Federal National Mortgage Association (“Fannie Mae”) declared “More than any other 

single measure, the 1956 act created the decentralized, automobile-dependent metropolis we know 

today.”20 

Efforts to promote homeownership—through the G.I. Bill of Rights and mortgage subsidies, for 

example—encouraged movement to new subdivisions. In addition, the federal tax deduction of 

interest paid on home loans provides a subsidy for homeownership. Federal mortgage programs 

promote housing consumption and increase the relative attractiveness of the suburbs. 

17 Ogden, K.W. Urban Goods Movement: A Guide to Policy and Planning. Hampshire, England: Gower House, 1992. 

18 This has been associated with both the growth of new outer rings of land-use development, and subsequently of edge 

cities. Burchell, R. “Understanding Sprawl.” On the Ground. Vol. 2, No. 2, 1996. 

19 Largely in consequence to standard rent-bid curves that have been influenced by auto ownership patterns, mortgage 

subsidies, and rent control and public housing policies, among other factors such as school quality, perceptions of 

crime, and racial bias. 

20 Robert Fishman, “The American Metropolis at Century’s End: Past and Future Influences.” Washington, DC: Fannie 

Mae Foundation, Winter, 1999. Dr. Fishman’s survey included a list of 25 possible influences on the American city 

over the past century and a list of 19 potential future influences. Fishman surveyed 149 urban and regional historians, 

planners, and practitioners and asked them to rank the 10 most powerful influences--looking at the past 50 years and 

into the next century. 

10 
Our Built and Natural Environments 

Government also subsidized suburban development by funding the construction of new water and 

sewer infrastructure. The effect of subsidizing any kind of infrastructure is to subsidize the type of 

growth that depends on the infrastructure, which in this case was growth on the urban edge. 

Inequitable support for suburban development comes not only from public policies, but sometimes 

also from private companies. For example, anecdotal and statistical evidence indicates that many 

insurance companies either resist writing insurance in minority and low-income communities, or 

only provide over-priced, inferior insurance policies in such areas—a practice commonly referred 

to as “insurance redlining.” Although states are working to eliminate this practice through testing 

and litigation, redlining still remains a serious deterrent to investment in many areas. 

Zoning also has played a large part in the nature of land development. Minimum parking standards, 

put in place to reduce parking problems, encourage vehicle travel.21 Similarly, zoning that 

separates residences from commercial and office development often makes mixed-use 

communities impossible—the walkable neighborhoods with neighborhood stores that were once 

common would be illegal to build in most places today. In addition, competition among localities for 

economic development has led to intra-regional competition for jobs, which has hurt older city 

centers. 

Growing Affluence and Social Change 

Changes in the national economy and social attitudes contribute to the dispersal of urban 

development. Because land is a “normal good” in the parlance of economists, increases in income 

lead to increases in the demand for land. Historical increases in income led to greater demand for 

large homes and lot sizes.22 In turn, single family detached homes on large lots became 

increasingly associated with monetary success and social prestige. This combination of rising 

incomes and changing values has encouraged land use patterns marked by low density 

development, declining urban cores, and a strong reliance on the automobile. 

Central city problems, real and perceived, also contribute to movement from city centers. 

Empirical studies of the suburbanization process provide support for the theory that central city 

attributes have had some effect on the pace of suburbanization.23 The appeal of newer and larger 

21 Donald Shoup, “An Opportunity to Reduce Minimum Parking Requirements,” Journal of the American Planning 

Association, Winter 1995, pp. 14-28; Willson, Richard W. “Sububan Parking Requirements: A Tacit Policy for 

Automobile Use and Sprawl.” Journal of the American Planning Association. Vol. 61, No. 1 Winter 1995. pp. 29-42. 

22 On the other hand, as real income increases, so should the opportunity cost of commuting. As a result, an increase in 

income should also lead to greater demand for dwellings close to work, all else equal. However, increases in income 

occurred concurrently with reductions in travel costs associated with highway development. As a result, increased 

opportunity costs of commuting have been minimized. 

23 See the following: Wasylenko, Michael. “Disamenities, Local Taxation, and the Intrametropolitan Location of 

Households and Firms.” Research in Urban Economics. Vol. 4. Ed. Robert Ebel. Greenwich, CN: JAI Press, 1984; 

Bradbury, Kathernine, Anthony Downs, and Kenneth Small. Urban Decline and the Future of American Cities. 

Washington, DC: Brookings Institution, 1982; Frey, W.H. “Central City White Flight: Racial and Non-Racial Causes.” 

American Sociological Review. 44, 1979, pp. 425-88; Grubb, W.N. 1982. “The Flight to the Suburbs of Population and 

Employment, 1960-1970.” Journal of Urban Economics. 11: 348-67. 

11 
Our Built and Natural Environments 

homes and lots in suburban areas, coupled with issues of race, ethnicity, education, and safety, 

have contributed to considerable out-migration from the urban core. Fiscal problems in the central 

city, including high taxes and in some cases lower-quality schools and services, also have led many 

people to leave for the suburbs.24 A study based on 121 metropolitan areas for the period 1970-75 

found that cities experienced relatively rapid suburbanization if their central area had (1) a 

relatively old housing stock, (2) relatively high taxes, and (3) a relatively large African-American 

population. In addition, the larger the number of suburban governments to choose from, the more 

rapid the suburbanization.25 Another study found that metropolitan areas with high taxes, high 

crime rates, and low educational expenditures experienced the most rapid suburbanization.26 

2.3 ENVIRONMENTAL CONSEQUENCES OF LAND USE TRENDS 

Although changing development patterns provide many social benefits, they also come at a cost to 

the natural environment and our communities. This section addresses the two major direct 

environmental impacts of development: (1) habitat loss and fragmentation and (2) the degradation 

of water resources and water quality. 

The effects of land development on the environment are particularly important because 

development patterns have long-term effects that are not easily reversible. Degradation of water 

resources can have adverse effects over many generations of wildlife, and habitat and species 

loss may forever alter ecosystems. As a result, the cumulative effects of development decisions 

are important when considering the long-term health of the environment and communities. 

Habitat Loss and Fragmentation 

Habitat loss and fragmentation are two of the most direct impacts of development on previously 

undeveloped land.27 According to recent studies, habitat destruction is the main factor threatening 

24 The causality goes both ways: fiscal problems can lead to suburbanization, and suburbanization can contribute to 

central-city fiscal problems. 

25 Bradbury, Kathernine, Anthony Downs, and Kenneth Small. Urban Decline and the Future of American Cities. 

Washington, DC: Brookings Institution, 1982. 

26 Frey, W.H. “Central City White Flight: Racial and Non-Racial Causes.” American Sociological Review. 44: 1979. pp. 

425-88. 

27 Habitat impacts from human development have been documented extensively. See: Dobson, A. P., J. P. Rodriguez, 

W. M. Roberts, D. S. Wilcove. “Geographic Distribution of Endangered Species in the United States,” Science, Volume 

275, Number 5299. January 24, 1997. pp. 550-553; LaRoe, Edward T., Gaye S. Farris, Catherine E. Puckett. Our 

Living Resources - A Report to the Nation on the Distribution, Abundance, and Health of U.S. Plants, Animals, and 

Ecosystems. U.S. Department of the Interior, National Biological Service, Washington, 1995; Noss, Reed F. and Robert 

L. Peters. Endangered Ecosystems: A Status Report on America’s Vanishing Habitat and Wildlife. Washington, DC: 

Defenders of Wildlife, December 1995; Peters, Robert L., Evan Frost, and Felice Pace. Managing for Forest Ecosystem 

Health: A Reassessment of the ‘Forest Health Crisis. Washington, DC: Defenders of Wildlife, April 1996; Boucher, 

Norman, “Species of the Sprawl,” Wilderness. Defenders of Wildlife, Washington, DC, Summer 1995. pp. 10-24; Soule, 

Michael E. “Land Use Planning and Wildlife Maintenance - Guidelines for Conserving Wildlife in an Urban Landscape.” 

Journal of the American Planning Association. Volume 57, Number 3, Summer 1991. pp. 313-323. 

12 
Our Built and Natural Environments 

80 percent or more of the species listed under the 

federal Endangered Species Act; more than 95 

percent of listed species are endangered to some 

extent by habitat loss or alteration.28 

In many metropolitan areas, development has 

consumed thousands of acres of woodlands, 

wetlands, and other natural habitats. The 

Washington, DC, region, for example, lost 211,062 

acres of farmland, forest, wetlands, and other open 

space in the 1980s, according to a study by the 

National Center for Resource Innovations.29 The 

Occoquan Reservoir, which supplies water to much 

of northern Virginia, is being developed so rapidly 

that if current trends continue, a full 40 percent of 

the watershed’s open space will be lost by 2020.30 

And though habitat destruction may be most 

obvious in cities experiencing explosive growth, 

such as Phoenix, Las Vegas and Atlanta, it is 

certainly not limited to these areas. For example, in 

the lower 10 kilometers of Cape May, New Jersey, 

40 percent of the habitat critical to migratory and 

residential wildlife has been lost to development 

over the last 20 years31. Habitat in even our most 

Impact of Low Density Development 

on Wildlife: A Case Study 

From 1958 to 1990, low-density development in 

the Tuckahoe Creek, Virginia watershed 

dramatically increased, resulting in severe impacts 

on local warm water fish species. The acreage of 

low-density development increased from 7% to 8% 

(seemingly small, but an increase of 300%); the 

number of houses in the watershed increased from 

7,800 to 28,000 (an increase of 250%); roadstream crossings increased from 43 to 85; and 

road miles increased by 137%, with the area of 

road pavement increasing by 155%. 

Effects on local fish species were startling. Six 

indigenous fish species became locally extinct, 

while the population of the remaining fish species 

declined 80%. Fish species diversity (a measure of 

resilience or a species’ ability to rebound from 

adverse conditions) declined significantly. 

Source: Weaver, L. Alan, and Greg C. Garman. 

“Urbanization of a Watershed and Historical Changes in a 

Stream Fish Assemblage.” Transactions of the American 

Fisheries Society. Vol. 123, 1994, pp. 162-172. 

protected open spaces is endangered by development. A recent study by the Missouri State 

Department of Natural Resources indicated that the biological integrity of 27 state parks is 

currently threatened by increasingly dispersed patterns of development.32 

Another impact—one that frequently follows the destruction and/or degradation of habitat—is the 

invasion of non-native species. Disturbed habitat is readily invaded by exotic plants. For example, 

in the Florida Everglades, the Australian melaleuca tree invades after water diversions. In Florida, 

the non-native melaleuca, Brazilian pepper, and Australian pine now dominate many land areas. In 

addition, the exotic hydrilla and water hycinth have adversely affected many aquatic communities 

by outcompeting native species and altering water chemistry. In California, introduced European 

grasses have altered the structure of fragile coastal dune communities and invaded almost all 

28 Flather, C.H., L.A. Joyce, and C.A. Bloomgarden. 1994. (as cited in: Noss, Reed and Fobert Peters. Endangered 

Ecosystems: A Status Report on America’s Vanishing Habitat and Wildlife. Washington, DC: Defenders of Wildlife, 

December 1995. p. 46) 

29 Washington Post. “As the Economy Grows, the Trees Fall.” March 23, 1997. p. A1. 

30 Baley, Janet and Glenn Besa. Sprawl Costs us All. Sierra Club, 1997. 

31 Department of Environmental Protection, State of New Jersey. New Jersey’s Environment 1988. 

32 Etling, Kathy. “Of Owls and Interstates.” Missouri Conservationist. Volume 54, Number 11, Nov. 1993. pp. 6-9. 

13 
Our Built and Natural Environments 

remaining valley grasslands. Cases like these are common; more than 50 percent of federally 

listed species are affected by interactions with non-native species.33 

Housing developments, roads, and associated infrastructure have the potential to destroy existing 

forests and vegetation and also cause fragmentation of natural habitats. Road infrastructure in the 

United States is extensive and growing. In 1997, public roads occupied an estimated 17,345 square 

miles of land, a 2 percent increase over the road area in 1990.34 Fragmentation negatively affects 

wildlife in a number of ways. It interferes with wildlife travel, decreases habitat size, and reduces 

interaction with other wildlife communities. Fragmentation produces declines in both the number of 

species (diversity) and populations (abundance).35 For example, a study of the influence of narrow 

forest-dividing corridors (small roads and power lines) on forest-nesting birds in southern New 

Jersey revealed that, although not generally viewed as sources of forest fragmentation, such 

corridors measurably affect the diversity and abundance of birds in ways that are associated 

typically with the effects of forest fragmentation.36 

HABITAT FOCUS: WETLANDS 

In recent decades, it has become increasingly clear that wetlands, once viewed as unproductive, 

perform important ecological functions. Wetlands mitigate flooding and damage from erosion, 

wind, and waves; they facilitate sediment replenishment; provide habitat for water life, waterfowl, 

mammals, and reptiles, many of which are economically important; and improve water quality by 

removing excess nutrients and some chemical contaminants. For this reason, the United States has 

given particular priority to saving wetlands.37 

More than half (53 percent) of the wetlands in the lower 48 states were lost between the late 

1700s and the mid-1980s.38 In recent years, urban development and road construction has been a 

33 Flather, C.H., L.A. Joyce and C.A. Bloomgarden; Scemske, D.W., B.C. Husband, M.H. Ruckelshaus, et al. 1994. (As 

cited in: Noss, Reed and Fobert Peters. Endangered Ecosystems: A Status Report on America’s Vanishing Habitat and 

Wildlife. Defenders of Wildlife, December 1995. p. 47.) Also see Bright, C. “Understanding the Threat of Bioinvasions.” 

State of the World. Ed., Starke. New York: W.W. Norton & Co., 1996. pp. 95-113; Groves, R.H. and J.J. Burdon, eds. 

Ecology of Biological Invasions. Cambridge, UK: Cambridge University Press, 1986. 

34 Estimate based on data from U.S. Department of Transportation, Federal Highway Administration. “Highway 

Statistics:” 1990, 1995, 1996, and 1997. Table HM-60. 

35 Tolley, R.S. and B.J. Turton. “Transport Systems, Policy, and Planning: A Geographic Approach.” Longman 

Scientific and Technical. 1995. There is a large literature on urban wildlife, critical habitat, and wildlife corridors. See: 

Beatley, Timothy. Habitat Conservation Planning and Urban Growth. Austin, Texas: University of Texas Press, 1994; 

and Leedy, Daniel and Lowell Adams. “Wildlife in Urban and Developing Areas: An Overview and Historical 

Perspective.” in Integrating Man and Nature in the Metropolitan Environment: Proceedings of a National Symposium 

on Urban Wildlife, 4-7 ed. Adams and Leedy. Columbia, MD: National Institute for Urban Wildlife, November 1986. 

36 Rich et al., 1994. 

37 The U.S. Environmental Protection Agency adopted a No Net Loss of Wetlands goal (U.S. Environmental Protection 

Agency. EPA Strategic Plan. Washington, DC: 1997.), and the Federal Highway Administration has adopted a strategic 

goal of a 50 percent increase in net wetland acreage resulting from Federal-aid highway projects in 10 years (U.S. 

Department of Transportation, Federal Highway Administration. FHWA National Strategic Plan. 1997.) 

38 Dahl, T.E. and C.E. Johnson. Status and Trends of Wetlands in the Conterminous United States. Washington, DC: 

U.S. Department of the Interior, U.S. Fish and Wildlife Service, 1991. 

14 
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major source of wetland loss. Of the 12 states that listed wetland losses, six reported that they 

suffered significant losses due to highway construction, and 10 reported that they had significant 

wetland losses due to residential growth and development.39 

Wetland loss has been significant in many states. Ninety-one percent of California’s original 

5 million acres of wetlands have been drained or filled, and it is estimated that wetlands in 

California continue to be lost at a rate of almost 5,000 acres per year.40 

In a report on the status and trends of wetlands in the United States between 1970 and 1980, the 

U.S. Department of the Interior concluded: “Road construction in and across wetlands has led to 

direct and indirect wetland loss through the effects of filling, fragmentation, and alteration of 

hydrology.” Constructed highways also have indirectly led to wetland loss by enabling or inducing 

secondary development.41 Another report concludes that “between approximately 310,000 and 

570,000 acres of wetlands could potentially have been lost due to the construction of Federal Aid 

Highway Program roads between 1955 and 1980, at a cost to replace of between $153 million and 

$6 billion.”42 

Compensatory mitigation efforts attempt to address wetland loss due to highway projects. 

However, a 1992 Federal Highway Administration study evaluated 23 highway wetland mitigation 

projects and found that true one-for-one replacements were executed for only a few sites.43 

Degradation of Water Resources and Water Quality 

Many watersheds are rapidly becoming developed. For example, urban land use in the Occoquan 

watershed in northern Virginia is projected to increase from 7.3 percent in 1977 to 55.7 percent in 

2020. Impervious cover—the imprint of land development on the landscape, composed of the sum 

of roads, parking lots, sidewalks, rooftops, and other impermeable surfaces—in the watershed is 

expected to grow from 11 percent of the basin in 1995 to 20 percent in 2020.44 This development 

has serious environmental consequences. Stormwater runoff has been identified as one of the 

major contributors to ongoing water quality problems in this country. According to EPA’s National 

39 U.S. Environmental Protection Agency. National Water Quality Inventory: 1996 Report to Congress. 1998. 

40 Noss, Reed and Robert Peters. Endangered Ecosystems: A Status Report on America’s Vanishing Habitat and 

Wildlife. Washington, DC: Defenders of Wildlife, 1995. p. 28. 

41 LaRoe, Edward T., Gaye S. Farris, Catherine E. Puckett. Our Living Resources - A Report to the Nation on the 

Distribution, Abundance, and Health of U.S. Plants, Animals, and Ecosystems. Washington, DC: U.S. Department of 

the Interior, National Biological Service. 1995. 

42 Apogee Research, Inc. “Quantifying the Impacts of Road Construction on Wetlands Loss - Final Report.” Bethesda, 

MD: September 1997. 

43 U.S. Department of Transportation, Federal Highway Administration. Evaluation of Wetland Mitigation Measures, 

Volume I: Final Report. 1992. 

44 Baley, Janet and Glenn Besa, Sprawl Costs us All, Sierra Club, 1997. 

15 
Our Built and Natural Environments 

Water Quality Inventory: 1996 Report to Congress, impacts of increased imperviousness due to 

development on rivers include the following:45 

� 38 percent of assessed estuary miles are impaired, with approximately 46 percent of 

impairment attributable to urban runoff. 

� 36 percent of the total river miles recently assessed by states are still impaired, with urban 

runoff causing about 12 percent of the problem. 

� 39 percent of assessed lake acres are impaired, with urban runoff causing 21 percent of 

the impairment. 

� 13 percent of assessed ocean shorelines are impaired, with approximately 55 percent of 

impairment due to urban runoff through storm sewers. 

Development significantly alters water balance and water quality in a number of ways, including 

impacts stemming from the following: 

� Changes in hydrology (and reduced groundwater recharge)46 

� Increased water pollution and nutrients 

� Increased acidity 

� Higher water temperature 

These effects are discussed below. 

CHANGES IN HYDROLOGY 

Development in a watershed changes natural drainage patterns. Increases in impervious areas 

associated with development increase the volume and the rate of surface water runoff. In a study 

of 40 runoff monitoring sites across the nation, a 1-acre parking lot was found to produce a runoff 

volume almost 16 times as large as the runoff volume produced by an undeveloped meadow. Peak 

discharge, velocity, and time of concentration of storm water runoff were also found to be much 

greater. Furthermore, transportation-related impervious surfaces seem more often to exhibit a 

greater runoff volume than disconnected rooftop-related imperviousness of the same surface area. 

45 Note that the percentage of assessed water resources is quite low and differences in the resources being surveyed 

preclude making historical comparisons based on prior inventory reports. Source: U.S. Environmental Protection 

Agency, Office of Water. National Water Quality Inventory: 1996 Report to Congress. Washington, DC: 1998. 

46 U.S. Environmental Protection Agency. Environmental Impacts of Storm Water Discharges - A National Profile. 

(EPA 841-R-92-001) Washington, DC: June 1992. 

16 
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Channelization projects, such as concrete retention walls or lining along stream beds, channel 

realignment, and diversion of streams through culverts, also increase flow velocities. 47 

Increased peak discharges and shorter lag times between storms and the resultant runoff lead to 

larger and more frequent incidents of local flooding.48 Because the faster runoff prevents 

percolation of water that would normally feed regular stream flow, floods are followed by longer 

periods of below-normal stream levels. Lower flows during periods between storms may affect 

the aquatic habitat and the ability of a stream to dilute toxic spills.49 Higher flows often result in 

stream bank erosion, increased sedimentation in the channel, and decreased stability. Streams may 

widen to two to four times their predevelopment width if stormwater from developed areas is 

uncontrolled. Research models suggest that a threshold for urban stream stability exists at about 

10 percent site imperviousness.50 

Sediment pollutant loads created by increased erosion can cause a broad range of impacts in 

receiving waters, including reduced water storage capacity, impaired dissolved oxygen for aquatic 

organisms, decreased light penetration, increased need for dredging, increased costs for water 

treatment, accumulation of pollutants, and adverse effects on fish and shellfish. 

When runoff increases in volume and velocity, soils have less opportunity to absorb stormwater. 

This loss of groundwater recharge can reduce residential and municipal water supplies, decrease 

base flow into stream channels during dry weather, and threaten the health of local wetlands that 

rely on groundwater to maintain wet conditions during dry periods of the year.51 

INCREASED WATER POLLUTION AND NUTRIENTS 

Stormwater is often polluted by pesticides and fertilizers from homes, farms, heavy metals, 

antifreeze, lead and partially oxidized hydrocarbons from gasoline- and diesel-fueled vehicles, oil, 

urban debris, and spillage from accidents. Pollutants accumulate on impervious surfaces. These 

pollutants are quickly washed off during storms and delivered through pipes and ditches to 

47 Schueler, Tom. Environmental Land Planning Series: Site Planning for Urban Stream Protection. Center for 

Watershed Protection. Publication No. 95708. Washington, DC: Metropolitan Washington Council of Governments, 

December 1995. pp. 21-23. 

48 Field, Richard, Hugh Masters, and Melvin Singer. “Porous Pavement: Research, Development, and Demonstration.” 

Transportation Engineering Journal of the American Society of Civil Engineers. Vol. 108, No. TE3, May 1982. 

pp. 244-258. 

49 Bellevue NURP Report. In 1978, the EPA began The National Urban Runoff Program (NURP) to comprehensively 

study 28 project locations across the county, acquiring data on the receiving water impacts of urban runoff and 

evaluating management practices. The NURP studies provided the first national assessment of the increased levels of 

contaminants such as sediment, heavy metals, oil and grease, phosphorus, nitrogen, chemical oxygen demand, bacteria, 

viruses, and solid wastes in storm water. 

50 Schueler, Tom. Environmental Land Planning Series: Site Planning for Urban Stream Protection. Center for 

Watershed Protection. Publication No. 95708. Washington, DC: Metropolitan Washington Council of Governments, 

December 1995. pp. 23-24. 

51 Harbor, Jonathon M. “A Practical Method for Estimating the Impact of Land–Use Change on Surface Runoff, 

Groundwater Recharge, and Wetland Hydrology.” Journal of the American Planning Association. Volume 60, Number 

1, Winter 1994. 

17 
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streams, lakes, and estuaries.52 Monitoring and modeling studies have shown consistently that 

urban pollutant loads increase with watershed imperviousness.53 

Natural nutrient cycles may be altered by land use activities within a watershed. Excessive 

nutrients overstimulate the growth of aquatic plants, which may result in low oxygen levels in 

water, accelerate eutrophication, cause unsightly conditions, interfere with navigation and 

treatment processes, and cause unpleasant tastes and odors. Eutrophic conditions are evidenced 

by surface algal scums, reduced water clarity, odors, and dense algal growth on shallow water 

substrates.54 Algal blooms block the light needed by submerged aquatic vegetation, removing 

habitat for juvenile fish and shellfish. After blooms or at the end of a growing season, the 

decomposition of dead vegetation may cause reduced oxygen levels. Lower oxygen levels may, in 

turn, cause the death of fish and mass mortality of benthic organisms. 

INCREASED ACIDITY 

Nitrogen oxides (NOx) and sulfur dioxide (SO2) are the primary air pollutants that result in acid 

rain and highly acidic stormwater. Acid rain occurs when SO2, emitted primarily by electric utilities 

fired by coal, and nitrogen oxides (NOx), emitted primarily by transportation sources and utilities, 

are deposited in the form of wet or dry deposition. 

Several aspects of urbanization tend to create local conditions that may make receiving waters 

susceptible to impacts from acidity. High levels of airborne SO2 and NOx in large urbanized areas 

increase the acidity of the rainfall to levels above those typically found in the region. Runoff from 

paved surfaces and other impervious surfaces may have little or no opportunity to contact soils 

that could buffer the acidity of the rainfall. In urbanized areas with acidic rain, higher runoff 

volumes and rates associated with urban development can increase the acidity of receiving 

streams rapidly and lead to high peak acidity levels. 

HIGHER WATER TEMPERATURE 

High volumes of runoff from hot paved surfaces and rooftops may cause a rapid increase in 

surface water temperatures. Discharges from stormwater management devices, which retain 

collected runoff in unshaded ponds, also may increase stream temperatures. 

Increased temperature can harm fish and other aquatic life. Water holds less oxygen as it 

becomes warmer, which may affect habitat and make the water more susceptible to oxygendemanding pollutants. Sustained water temperatures in excess of 70°F are considered stressful or 

lethal to many cold water fish species and stream insects. The availability of food, attendant life 

cycle chemistry, and water quality changes are all affected by water temperature. 

52 Center for Watershed Protection. Blueprint to Protect Coastal Water Quality. Land Ethics, Inc. p.3. 

53 Schueler, Tom. Environmental Land Planning Series: Site Planning for Urban Stream Protection. Center for 

Watershed Protection. Publication No. 95708. Washington, DC: Metropolitan Washington Council of Governments, 

December 1995. p.24. 

54 Schueler, Tom. A Framework for Evaluating Compliance with the 10% rule in the Chesapeake Bay Critical Area. 

Annapolis, MD: Maryland Critical Area Commission, 1987. 

18 
Our Built and Natural Environments 

2.4 SUMMARY 

The size of virtually every metropolitan area in the United States has expanded dramatically in 

recent decades. As automobile ownership became increasingly common, residential development 

spread farther from urban centers. Although the sources of development dispersal are numerous 

and include demographic and socio-economic factors, public policies on highway construction, 

taxes, and land use also play an important role in shaping urban form. 

The direct environmental consequences of dispersed development patterns can be divided into two 

broad categories: habitat loss and fragmentation, and adverse effects on water resources. Housing 

developments, roads, and associated infrastructure have destroyed existing forests and vegetation, 

and displaced or eliminated wildlife populations. Road construction has fragmented natural 

habitats, causing a decline in the diversity and abundance of populations. In the span of only about 

three human generations, we have witnessed the extinction of nearly 1 percent of the nation’s 

flowering plants, more than 3 percent of our birds, and about 12 percent of the U.S.’s freshwater 

mussels.55 Wetlands, in particular, have been affected substantially. Wetlands trends on nonfederal 

lands indicate a loss rate of between 70,000 and 90,000 acres annually, most of which is due to 

increases in development.56 

Water resources and water quality are also threatened. Increasing acres of impervious surfaces 

lead to higher runoff volumes, larger and more frequent incidents of local flooding, and longer 

periods of below-normal stream levels. Development patterns also lead to reduced groundwater 

recharge and various negative effects like increased sedimentation, increased water acidity, and 

higher water temperatures. Water quality in many of our nation’s rivers, lakes, and estuaries is 

degraded to a point where those water bodies can no longer support basic uses such as fishing and 

swimming, and cannot be relied on as sources of clean drinking water. According to EPA’s most 

recent National Water Quality Inventory, approximately 40 percent of surveyed bodies of water in 

1996 were too polluted for basic uses.57 

Clean water and healthy habitat are national priorities, so it is important to understand the impacts 

that development patterns can have on our natural resources. 

55 Sierra Club. “The State of Disappearing Species and Habitat.” (http://www.sierraclub.org/habitat/speciesloss.asp) 

56 U.S. Environmental Protection Agency. Office of Water wetlands webpage: 

http://www.epa.gov/OWOW/wetlands/vital/status.html. 

57 U.S. Environmental Protection Agency. “National Water Quality Inventory: 1996 Report to Congress.” 1996. This 

report summarizes information submitted by 58 states, American Indian tribes, territories, Interstate Water 

Commissions, and the District of Columbia in their 1996 Section 305(b) reports. 

19 
Our Built and Natural Environments 

3. Vehicle Travel: Recent Trends and Environmental 

Impacts 

Vehicle travel has increased substantially in recent decades. Factors contributing to this trend are 

numerous. Although some new travel can be attributed to shifting demographics and market 

characteristics, substantial evidence suggests that much of the increase is a direct result of 

changing development patterns. 

As development becomes more dispersed, with increasing numbers of families living on large lots 

at the urban fringes, and as jobs and housing become increasingly segregated from one another, 

distances between destinations have increased. Further, people are forced to make more trips by 

car, since the distances between destinations are often too great to walk or bike. 

Rapid increases in vehicle travel have negatively affected the environment in numerous ways. 

The growth in travel degrades air quality, impairs water quality, and increases traffic noise. 

3.1 TRENDS IN VEHICLE TRAVEL 

Vehicle travel increased substantially in recent decades. Total vehicle miles of travel (VMT) in the 

United States increased 63 percent between 1980 and 1997. VMT has more than doubled since 

1970.57 As shown in Figure 3-1, the rate of 

growth in VMT has exceeded the rate of 

population growth significantly over the last 

decade. VMT growth also outpaced 

employment growth and economic growth. 

VMT is projected to grow considerably into 

the future. FHWA projects that light-duty 

VMT will grow at an annual rate of 

approximately 2.16 percent over the next 20 

years, resulting in a 53 percent increase in 

VMT.58 (See Figure 3-2.) As a result, FHWA 

has projected significant increases in annual 

travel delay times through 2005. 

Figure 3-1: Growth in Vehicle Miles 

of Travel and Population, 1980-1997 

1.6 

1.5 

1.4 

1.3 

1.2 

1.1 

1 

0.9 

1980 1982 1984 1986 1988 1990 1992 1994 1996 

1.7 

VMT 

Population 

3.1% annual 

growth 

1.0% annual growth 

Scale: 1980 value = 1.0 

Sources: U.S. Department of Transportation, Federal Highway 

Administration. Highway Statistics (Summary to 1995, and 

annual editions, 1996 and 1997), Washington, DC. 

57 U.S. Department of Transportation, Federal Highway Administration, Highway Statistics (Summary to 1995, and 

annual editions, 1996 and 1997.) Washington, DC. 

58 U.S. Department of Transportation, 1997 Status of the Nation’s Surface Transportation System: Condition & 

Performance Report to Congress, 1998. p. 60. 

19 
Our Built and Natural Environments 

Figure 3-2: Projected Growth in VMT 

5

4

3

2

1

0 

1980 1985 1990 1995 2000 2005 2010 2015

Historical 

Projected 

Source: U.S. Department of Transportation. 1995 Status of the Nation’s Surface Transportation System: 

Condition & Performance Report to Congress, October 1995. 

When contrasted against population growth, the VMT increase is particularly high in specific 

metropolitan areas, as shown in Table 3-1.59 

Table 3-1: Growth in Daily Vehicle Miles of Travel Exceeds Population Growth, 1982-1996 trillions of miles 

Urbanized 

Area 

Population Growth, 

1982-96 

VMT Growth on 

Freeways and 

Principal Arterials, 

1982-96 

Atlanta, GA 

Boston, MA 

Charlottee, NC 

Chicago, IL-IN 

Houston, TX 

Kansas City, MO-KS 

Miami-Hialeah, FL 

Nashville, TN 

New York, NY-NJ 

Pittsburgh, PA 

Portland-Vancouver OR-WA 

Salt Lake City, UT 

San Antonio, TX 

Seattle-Everett, WA 

Washington, DC-MD-VA 

53% 

6% 

63% 

11% 

28% 

23% 

18% 

25% 

3% 

7% 

26% 

32% 

29% 

35% 

28% 

119% 

31% 

105% 

79% 

54% 

79% 

61% 

120% 

40% 

54% 

98% 

129% 

77% 

59% 

78% 

Source: Texas Transportation Institute, Urban Roadway Congestion, Annual Report 1998. Tables A-6 and A-7. 

59 U.S. Department of Transportation, Federal Highway Administration, Highway Statistics, 1990 and 1997. 

20
Our Built and Natural Environments 

3.2 SOURCES OF VMT GROWTH 

VMT growth can be attributed to a variety of factors, including the following: 

� Demographic and market changes that allow more families to own multiple cars and lead 

more individuals to drive on a regular basis 

� Development patterns that lead to increases in the number and average distance of trips 

� The ability of increased road capacity to encourage additional travel—“induced travel” 

Demographic and Market Changes 

VMT growth has occurred in response to a number of factors, including recent changes in the 

profile of the workforce and in general demographics. As the baby boom generation came of age, 

large numbers of people were thrown into the driving population at one time, causing VMT 

numbers to escalate. Female participation in the labor force has increased dramatically in recent 

decades, putting more drivers on the road in general, and during peak commute times in particular. 

According to the Nationwide Personal Transportation Survey (NPTS), women workers as a 

percentage of women aged 16 or older grew from 37 percent to 59 percent between 1969 and 1995. 

By 1995, women were driving approximately 60-70 percent as many miles as men do. This gap 

continues to close as women’s participation in the workforce increases.60 

A combination of rising incomes and falling fuel prices also affected VMT. As household incomes 

increased and fuel prices fell over the past several decades, families became better able to afford 

one or more cars. In 1969, 48 percent of all households owned one vehicle. By 1995, the figure 

dropped to 32 percent, while the proportion of households with two and three cars increased.61 

Development Patterns 

Observed VMT growth can be only partially attributed to demographic factors; changing 

development and commuting patterns also play a significant role. The 1990 NPTS found that 

between 1983 and 1990, only 36 percent of VMT growth was associated with demographic 

change. The remainder can be attributed to changes in land use patterns that have led to increases 

in average trip distances (38 percent) and in the number of trips made (25 percent). 62 

As discussed in Chapter 4 of this report, numerous studies have found a direct relationship between 

development patterns and the amount of driving done. On a national level, from 1983 to 1995, the 

average length of work trips increased by 36 percent (from 8.5 to 11.6 miles), reflecting the fact 

that jobs and housing have become increasingly segregated from one another in recent years.63 

60 U.S Department of Transportation, Federal Highway Administration. “1995 Nationwide Personal Transportation Survey.” 

September, 1997. 

61 Ibid. 

62 U.S. Department of Transportation, Federal Highway Administration, “1990 Nationwide Personal Transportation Survey”. 

63 United States Department of Transportation, Federal Highway Administration. Our Nation’s Travel: 1995 NPTS 

Early Results Report, 1997. Note that some of the increase in work trip lengths could be associated with trip chaining 

or other methodological issues associated with measuring trip lengths. 

21 
Our Built and Natural Environments 

Induced Traffic and VMT Growth 

“Induced traffic” is a term for traffic growth produced by the addition of highway capacity. The 

theory behind induced travel is that of supply and demand. Adding highway capacity (supply) 

reduces the cost of vehicle travel, particularly the costs associated with travel time. Demand is 

inversely related to cost. When cost goes down, demand goes up. As travel time and monetary 

costs fall, people travel more. 

Expansion in road capacity can have multiple effects on behavior. An increase in capacity may 

result in changes in travel route, timing, vehicle occupancy, or mode choice for any given trip. It 

may also result in changes in trip frequency and switches to alternative destinations. Of these 

effects, increases in trip making and mode switches clearly contribute to induced travel. Other 

effects, such as switching routes or changing travel times, may occur as well. Although these 

effects may reduce some of the expected improvement in traffic flow and savings in travel time 

associated with the road project, they do not constitute new vehicle travel. 

Different types of induced traffic are believed to occur in the short term and long term. In the 

short term, people may make more trips or switch from transit or carpools to driving alone 

because of improved traffic conditions. In the long term, reduced travel costs encourage more 

dispersed land use patterns that, in turn, can increase trip lengths and vehicle dependency. 64 

A REGRESSION ANALYSIS ON IMPACTS OF NEW LANE-MILES 

Probably the best-known quantification of induced travel using U.S. data is a study by a 

University of California-Berkeley team led by Mark Hansen. Using time-series data and multiple 

regression, Hansen et al. estimated the auto traffic effects of changes in road capacity. Hansen 

studied relatively long-run time-series data—up to 16 years—and cross-sectional data to 

overcome difficulties in other studies that used only cross-sectional data and limited time 

periods.65 The peer-reviewed results are statistically robust and quite clear: induced travel can 

occur and can absorb all new capacity. 66 

According to the study, vehicle miles traveled on state highways increase, on average, by 0.6 to 

0.7 percent at the county level for each 1 percent increase in highway miles, and by 0.9 percent at 

64 See discussion in section 2.2 of this paper. Also refer to: Litman, Todd. “Incorporating Generated Travel in 

Transportation Economic Analysis.” Paper presented at the 75th Annual Meeting of the Transportation Research Board. 

Washington, DC: January 1996; Cohen, H., “Review of Empirical Studies of Induced Traffic.” Expanding 

Metropolitan Highways: Implications for Air Quality and Energy Use. Transportation Research Board Special Report 

245, Washington, DC: National Academy Press, 1995. 

65 Hansen et al.’s 1993 study has been widely cited. He and his team have since improved on it, and we present here the 

updated results from Mark Hansen and Yuanlin Huang, “Road Supply and Traffic in California Urban Areas,” 

Transportation Research-A, 31:3 (1997), pp. 205-218. The Hansen team used data from 30 California urban counties. 

“The populations of the regions ranged over two orders of magnitude, from 150,000 to 15 million.” Because of the 

wide variety of urban sizes and types included in the sample, the findings are applicable beyond California. Because the 

1997 journal article was much shorter than the 1993 report, the article did not go into as much detail about the 

mechanisms of induced travel. In some cases we have quoted and cited the 1993 report to fill in detail. 

66 The explanatory power of the models is quite high, at R2

 = .92 - .99. Unless noted otherwise, we report here only 

statistically significant results. 

22 
Our Built and Natural Environments 

the metropolitan level. The full increase in VMT materializes within five years of the change in 

road supply. 67 

New road capacity does not simply affect travel on the new road or new lanes. It may also affect 

traffic outside its own corridor. People might use the new road rather than an older, more 

congested route. People may choose new destinations. A decision to use the new road probably 

means a decision to use a road connecting to it. Thus, capacity increase can lead to travel growth 

on other roads as well as on new roads highway lanes. 

Hansen found that: 

...adding lane miles in a given county increases VMT throughout the wider 

region. This will occur if, for example, increasing the capacity of a highway in a 

given county induces commuting to or through that county from other counties in 

the region. 68 

Hansen found that capacity additions have different impacts in different metropolitan areas. An 

additional lane mile in San Francisco, Los Angeles, or San Diego metro areas produces roughly 

12,000 additional daily VMT. In smaller Stockton, just over 8,000 additional daily VMT are 

produced per new lane mile. In much smaller Redding, roughly 3,000 additional VMT occur per 

new lane mile. “Greater quantities of induced traffic are predicted for larger urban regions 

because such regions have higher ratios of VMT to state highway lane miles.”69 

Hansen does acknowledge that other factors (specifically, population growth, income level, and 

gasoline prices) also affect VMT. However, his findings call into question whether communities 

can relieve congestion through new road construction. Transportation decisions can produce 

travel changes over a wide area, and local capacity additions can increase VMT system-wide. 

LITERATURE REVIEW AND SYNTHESIS OF EVIDENCE ON INDUCED TRAFFIC 

Goodwin’s paper, “Empirical Evidence on Induced Traffic: A Review and Synthesis,” is useful in 

surveying additional evidence for induced traffic. Where Hansen used a traditional multiple 

regression approach in his study, Goodwin assembled evidence for induced travel from studies in 

several fields.70 

Goodwin drew the evidence from studies on several subjects: the cost of car use, travel time 

budgets, the value of time, and other multiple regression studies. Together, those studies 

supported “an elasticity of traffic volume with respect to travel speed of about –0.5 in the short 

term and up to –1.0 in the long term.” That is, in the short term, a 1 percent decrease in travel 

time will result in a 0.5 percent increase in vehicle travel. Over the long term, a 1 percent 

improvement in travel time will cause a 1 percent increase in VMT. 

67 Hansen investigates whether and how the effect grows over time, using lagged and unlagged models. He finds that 

the effect does grow. For further discussion, see both the 1993 report and the 1997 article. 

68 Hansen, Mark and Yuanlin Huang. “Road Supply and Traffic in California Urban Areas,” Transportation Research, 

Volume 31, Number 3. Great Britain: Elsevier Science Ltd, 1997. p. 213. 

69 Hansen, Mark and Yuanlin Huang. “Road Supply and Traffic in California Urban Areas,” Transportation Research, 

Volume 31, Number 3. Great Britain: Elsevier Science Ltd, 1997. p. 217. 

70 “The approach taken is that inferences are to be judged on the basis of ‘the balance of probability’ rather than 

conclusive rejection of a formal hypothesis.” Goodwin 1996. 

23 
Our Built and Natural Environments 

Goodwin’s review led in part to a significant change in British national policy on the evaluation 

of road projects. The British government announced in December 1994 that “there is likely to be 

a significant proportion of schemes where there is a real possibility of extra traffic,” and ordered 

re-review of construction proposals to take into account the induced travel. 

OTHER STUDIES OF INDUCED TRAVEL 

Hansen and the U.S. Transportation Research Board (TRB), a unit of the National Academy of 

Sciences, have, like Goodwin, reviewed various empirical studies of induced travel. Our Built 

and Natural Environments does not analyze the empirical studies in the level of detail that 

Goodwin did, but our review of the studies found estimated elasticities of VMT with respect to 

road supply ranging from 0.1 to 0.8. Most of the U.S. studies collected were older than those 

reviewed by Goodwin. Nonetheless, the two reviews are fundamentally in agreement in finding 

consistent evidence for induced travel in response to road improvements. 

Transportation demand forecasting models that estimate the likelihood of choosing a specific 

mode for a trip, such as transit or motor vehicle, typically use a measure of travel cost as a 

determinant of mode choice. The statistical significance of travel cost in these models suggests 

that changes in travel costs associated with highway development typically will result in mode 

shifts from transit to personal vehicle use. There is a wide literature on travel price elasticities.71 

Stated preference surveys also have found that people plan to make more auto trips in response to 

added capacity. 72 

A recent statistical study analyzing the issue of how additions of highway lane miles can increase 

vehicle miles of travel found elasticities within the ranges of previous research, with short-run 

elasticities of about 0.5 and long-run elasticities of about 0.8. 73 

The 1995 TRB Report, Expanding Metropolitan Highways: Implications for Air Quality and 

Energy Use, stated: 

The evidence from the studies reviewed here supports the view that highway 

capacity additions can induce new trips, longer trips, and diversions from transit.74 

The study suggests that land use and pricing policies may be more effective than efforts to 

restrain the growth in highway capacity to achieve long-run improvements in air quality. 

71 Many of these studies estimate elasticities based on fuel prices. See: Dahl C., and T. Sterner. “Analyzing Gasoline 

Demand Elasticities: A Survey.” Energy Economics 13, 1991, No. 3:203-210. Dahl, C.A. 1986. “Gasoline Demand 

Survey.” The Energy Journal 7, no. 1:67-82. Gately, D. 1990. “The U.S. Demand for Highway Travel and Motor 

Fuel.” The Energy Journal 11, no 3:59-73. Greene, D.L. “Vehicle Use and Fuel Economy: How Big is the Rebound 

Effect?” The Energy Journal 13, 1992, No. 1:117-143. Southworth, F. “VMT Forecasting for National Highway 

Planning: A Review of Existing Approaches.” Oak Ridge, TN: Center for Transportation Analysis, Oak Ridge National 

Laboratory, 1986. 

72 Dowling, Richard G. and Steven Colman. “Effects of Increased Highway Capacity: Results of a Household Travel 

Behavior Survey.” Transportation Research Board paper No. 95-0409, presented at the 74th Annual TRB meeting, 

January 22-28, 1995. 

73 Noland, Robert. “Relationships between Highway Capacity and Induced Vehicle Travel.” Paper presented at the 78th 

Annual Meeting of the Transportation Research Board, January 1999. 

74 Transportation Research Board, Expanding Metropolitan Highways: Implications for Air Quality and Energy Use, 

TRB Special Report 245, Washington, DC: National Academy Press, 1995, p. 162. 

24 
Our Built and Natural Environments 

Trends in Vehicle Travel – Conclusion 

Literature on trends in vehicle travel indicate that numerous factors including demographic and 

market shifts, contributed to recent increases in VMT. Studies also show that increases in VMT 

cannot be entirely explained by those factors and that changes in development patterns have had a 

particularly significant impact on VMT growth. Furthermore, because additional road capacity 

can be absorbed quickly by induced traffic, adding capacity alone is not likely to solve the 

problem of rapidly rising VMT. 

Section 3.3 discusses how these increases in vehicle travel affect our natural environment. 

3.3 ENVIRONMENTAL CONSEQUENCES OF VEHICLE TRAVEL 

Although vehicle travel produces benefits in terms of mobility, convenience, and flexibility, it 

also creates unintended environmental consequences. Outcomes include: 

� Degradation of air quality 

� Impairment of water quality associated with deposition of air pollutants 

� Greenhouse gas emissions and global climate change 

� Increased traffic noise 

� Upstream impacts from activities associated with vehicle use, such as oil spills and water 

quality impacts from road de-icing. 

Many of these consequences have severe effects on environmental quality. 75 The following 

subsections review these problems. 

Degradation of Air Quality 

Motor vehicles emit pollution through fuel combustion (exhaust) during operation and fuel 

evaporation during and between periods of operation. EPA established National Ambient Air 

Quality Standards (NAAQS) to protect public health, including the health of sensitive populations 

such as children and the elderly, from adverse effects of poor air quality. Pollutants covered by 

NAAQS (so-called “criteria pollutants”) include carbon monoxide (CO), nitrogen dioxide (NO2), 

ozone (O3), sulfur dioxide (SO2), particulate matter less than or equal to 10 micrometers in diameter 

(PM10), particulate matter less than or equal to 2.5 micrometers in diameter (PM2.5), and lead (Pb). 

Volatile organic compounds (VOC) and oxides of nitrogen (NOX) are precursors to the formation of 

ozone. Motor vehicles emit each of these pollutants, and contribute a large portion of CO and ozone 

precursors in particular. Vehicle travel also kicks up large quantities of particulate matter from roads 

(especially on unpaved roads in rural areas). Table 3-2 presents emissions from motor vehicles in 

1997, and Figure 3-3 shows the share of air pollutants emitted by motor vehicles. 

75 For complete data on transportation’s impacts on the environment, see: U.S. Environmental Protection Agency, 

Office of Policy. Indicators of the Environmental Impacts of Transportation: Highway, Rail, Aviation, and Maritime 

Transport. October, 1999, EPA 230-R-99-001. 

25 
Our Built and Natural Environments 

Table 3-2: Motor Vehicle Emissions, 1997 

Pollutant Quantity Emitted 

(thousand short tons) 

Carbon monoxide (CO) 50,257 

Nitrogen oxides (NOX) 7,035 

Volatile organic compounds (VOC) 5,230 

Sulfur dioxide (SO2) 320 

Particulate matter (PM 10): 

From exhaust 268 

From road dust 14,820 

Lead (Pb) 0.019 

Source: U.S. Environmental Protection Agency. National Air Pollutant Emissions Trends, 1900-1997. 1999. 

Figure 3-3: Highway Share of Air Pollutants Emitted, 1997 

CO Emissions 

57% 

From 

Vehicles 

NOX Emissions 30% 

From 

Vehicles 

VOC Emissions 27% 

From 

Vehicles 

SO2 Emissions 2% 

From 

Vehicles 

PM-10 Emissions 

44% 

From 

Road 

Dust 

1% From 

Vehicles 

Lead Emissions 

0.5% 

From 

Vehicles 

Note: percentages are based on anthropogenic emissions, except for PM-10, which includes natural emissions. 

Source: U.S. Environmental Protection Agency. National Air Pollutant Emissions Trends, 1900-1997. 1999. 

Since 1970, per mile motor vehicle emissions have been decreasing as a result of vehicle emissions 

control systems and cleaner fuels. However, increasing VMT threatens to reverse this trend in the 

near future for carbon monoxide, sulfur dioxide, and particulate matter (PM10). The share of total 

emissions attributable to on-road mobile sources varies greatly by location. The share of NOx can 

range from 20 to 60 percent of total emissions (including biogenic) in most ozone nonattainment 

areas, and on-road VOC emissions can range from 10 to 40 percent of the total.76 

Motor vehicles also emit hazardous air pollutants (HAPs), sometimes referred to as air toxics. 

HAPs are pollutants known or suspected to cause cancer or other serious human health effects or 

ecosystem damage. Persistent air toxics are of particular concern in aquatic ecosystems, as toxic 

levels can magnify up the food chain. Compared with the criteria pollutants, less information is 

available concerning the health and environmental impacts of individual HAPs. According to 

EPA’s 1993 National Toxics Inventory (NTI), mobile sources released about 21 percent of the 

76 Apogee Research, Inc. and Sierra Research. “Strategic Analysis of Mobile Source Options for Air Quality: Regional 

Differences and Implications for Ozone Policy.” Prepared for the Federal Highway Administration. August 1996. 

26 
Our Built and Natural Environments 

8.1 million tons of air toxics released nationwide.77 EPA also compiled an interim 1990 emissions 

inventory of 30 proposed urban HAPs that pose the greatest threat to public health in urban areas. 

Of these, about 40 percent of emissions come from mobile sources, as shown in Figure 3-4. 

Figure 3-4: Mobile Source Contribution to HAPs, 1993 

National Total HAP Emissions 

based on 1993 NTI 

Mobile Sources 

21% 

National Emissions of 

30 Draft Urban Area HAPs, 1990 

Mobile Sources 

40% 

Source: U.S. Environmental Protection Agency. National Air Quality and Emissions Trends 

Report, 1997, December 1998. 

Table 3-3 shows the quantity and share of individual toxic emissions that were emitted by motor 

vehicles in 1993. 

Table 3-3: Toxic Emissions Due to Highway Vehicle Operations, 1993 

Pollutant 

Quantity Emitted 

(1993, metric tons) Percent of Total Emission 

Benzene 

1,3 Butadiene 

Formaldehyde 

158,149 

27,972 

73,874 

60% 

56% 

33% 

Source: U.S. Environmental Protection Agency. Motor Vehicle-Related Air Toxics Study, April 1993. 

IMPACT: HEALTH PROBLEMS 

In 1997, approximately 113 million people lived in counties that had failed to attain the NAAQS 

for at least one criteria pollutant.78 As shown in Table 3-4, these pollutants are associated with 

numerous public health problems. 

77 EPA National Toxics Inventory. 1993. 

78 U.S. Environmental Protection Agency, Office of Air Quality Planning and Standards. National Air Quality and 

Emission Trends Report, 1997. Research Triangle Park, NC, December 1998. 

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Our Built and Natural Environments 

Table 3-4: Health Impacts of Criteria Pollutants 

Pollutant Health Impacts 

CO Interferes with the absorption of oxygen by hemoglobin in the blood. Lack of oxygen 

impairs the cardiovascular and nervous system, with symptoms including chest pain, 

headaches, dizziness, nausea, fatigue, and slower reflexes. In addition, impairs visual 

perception, work capacity, manual dexterity, learning ability and performance of 

complex tasks. Affects fetal growth and tissue development. Results in mortality at 

extremely high concentrations. 

Ozone May cause temporary lung irritation, minor eye irritation, coughing, pain upon 

inhalation with short-term exposure. Heavy exercise becomes difficult. Long-term 

exposure to ambient ozone may cause structural lung damage leading to chronic lung 

disease, lung cancer, and increased susceptibility to respiratory infections, such as 

bronchitis and pneumonia. May interfere w/ the immune system. May be agent for 

infectious disease since produces more receptors for viruses. Exacerbates allergies. 

Particulate 

Matter 

May cause coughing, lung tissue damage, alteration in immune system, and respiratory 

and cardiovascular diseases. Effects on breathing and respiratory symptoms, 

aggravation of existing respiratory and cardiovascular disease, alterations in the body’s 

defense systems, damage to lung tissue, carcinogenesis. Raises risk of cancer (since 

particulates adhere to carcinogens). 

SO2 Constricts bronchial passages and alters the lungs’ defenses, with symptoms including 

effects on breathing, asthma, and respiratory illness. People with pre-existing chronic 

lung and heart diseases are at increased risk of acute illness or premature death during 

episodes of combination of SO2 and particulate matter. Also important since it 

contributes to particulate formation by reacting in the atmosphere to form sulfates, 

which are believed to be a significant portion of PM-10 in terms of both volume and 

effect on humans. 

Lead May cause increased blood pressure and heart disease. Impairment of children’s mental 

functioning. Neurological impairments, such as seizures, mental retardation, and/or 

behavioral disorders. 

Source: Apogee Research, Inc. Incorporating Additional Effects into the HERS Model for National 

Highway Investment Analysis. Prepared for the Federal Highway Administration. January 1996. Based 

on reviews from various health studies. 

There is strong evidence that air pollution from highways causes a significant number of public 

health problems. A detailed analysis of the costs of motor vehicle travel concluded that in 1991 

motor vehicle pollution was responsible for the following health problems:79 

� Roughly 50-70 million respiratory-related restricted activity days, of which 

approximately 43-60 million can be attributed to particulate matter alone 

� About 852 million headaches from CO 

� Approximately 20,000-46,000 cases of chronic respiratory illness (chronic cough, 

phlegm, wheezing, chest illness, and bronchitis) 

� An estimated 530 cases of cancer from air toxins (estimates of cancer risk, however, are 

highly uncertain) 

� An estimated 40,000 premature deaths in the United States. 

79 McCubbin, D. and M. Delucchi. Health Effects of Motor Vehicle Air Pollution. 1995. 

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IMPACT: IMPAIRMENT OF WATER QUALITY FROM AIR POLLUTANTS 

Air pollution can significantly affect water quality, as illustrated by the following statistics:80 

� Estimates of atmospheric nitrogen input to water bodies such as the Chesapeake Bay and 

other major East Coast estuaries range from 5 percent to 50 percent of the controllable 

load of nitrogen (most estimates are in the range of 30 percent). The error in such 

estimates, however, is cited as at least plus or minus 20 percent and up to a factor of two 

or three, depending on location and pollutant considered. 

� Atmospheric loadings of metals to water bodies such as the Chesapeake Bay may range 

from more than 95 percent of total loadings in the case of lead to about 10 percent in the 

case of cadmium. 

� Annual fluxes from wet deposition reported at various coastal locations range from under 

5 mg per square meter for copper, nickel, and lead to 15-30 mg per square meter for iron 

and zinc. 

� Wet deposition of various polycyclic aromatic hydrocarbons such as benzo[ghi]perylene 

(including some carcinogenic products of incomplete combustion) is in the range of 1-10 

micrograms per square meter per year. 

EPA and others are in the process of modeling the source contribution of each pollutant for the 

Chesapeake Bay and the Great Lakes. 

IMPACT: ECONOMIC COSTS 

In addition to causing health problems, air pollution causes damage to building materials, 

agriculture, and visibility. These impacts have large costs to society. A comprehensive study of air 

pollution from motor vehicles estimated annual costs of $28.7 to $531 billion in health damage, 

$2.5 to $4.6 billion in crop damage, and $6.0 to $43.54 billion in damage to visibility. 81 Another 

study estimated $36.6 billion (1990 dollars converted to 1999 dollars) in annual air pollution 

health and property costs due to roadway transportation. 82 A 1993 study calculated air pollution 

health costs as ranging from $146 to $271.6 billion (1990 dollars converted to 1999 dollars) and 

building damage to cost another $365.6 million (1990 dollars converted to 1999 dollars).83 

Greenhouse Gas Emissions and Global Climate Change 

Greenhouse gases from human sources threaten to alter Earth’s atmosphere, since the planet’s 

ecosystems cannot absorb such elevated levels of these gases. Carbon dioxide (CO2) is one of the 

primary greenhouse gases emitted by humans. 

80 Air deposition data from AQCG/STAC 1994/95, and Valigura et al., 1994/1995. 

81 Delucchi, M. et al. The Annualized Social Cost of Motor-Vehicle Use in the U.S., based on 1990-1991 Data, June 

1997. Converted into 1999 dollars. (UCD-ITS-RR-96-3). 

82 Ketcham, B. and C. Komanoff. Win-Win Transportation: A No-Losers Approach to Financing Transport in New 

York City and the Region. 1992. 

83 Miller, P. and J. Moffet. The Price of Mobility: Uncovering the Hidden Costs of Transportation. Natural Resources 

Defense Council, 1993. 

29 
Our Built and Natural Environments 

The accumulation of greenhouse gases in the atmosphere can lead to global climate change (also 

called “global warming,” since one outcome is an increase in the average atmospheric temperature). 

The results of global climate change are potentially dramatic. Increases in atmospheric and oceanic 

temperatures might raise sea levels and alter associated weather patterns, which in turn could 

increase the frequency and severity of extreme weather events worldwide. Such changes might alter 

current patterns of land use and human activity, as well as ecosystems and natural habitats.84 

Even an increase of a few degrees can lead to dramatic changes in climate. The total global 

warming since the peak of the last ice age—18,000 years ago—was only about 5°C. In 1990 EPA 

estimated that a doubling of atmospheric levels of CO2 would lead to an increase in average 

temperatures of anywhere from 1.5 to 5.5°C.85 The Intergovernmental Panel on Climate Change 

(IPCC) in 1995 predicted an increase of about 2 to 3.5°C between 1990 and 2100. 86 

Transportation is a significant source of greenhouse gas emissions. In 1997, for example, the 

transportation sector emitted 32 percent of U.S. CO2 emissions from fossil fuels, or 473.1 million 

metric tons of carbon. As indicated by Figure 3-5, carbon emissions from transportation have 

increased significantly over time.87 

Figure 3-5: US Carbon Dioxide Emissions from Transportation 

470 

450 

430 

410 

390 

370 

473.1 470.7

458.5

450.4

437.5 431.4 424.5

432.7

432.1

427.5 

411.1

399.1 

384.4 379 

1984 1986 1988 1990 1992 1994 1996 

The transportation sector is projected to be the fastest growing contributor to carbon emissions in 

the next 20 years. Carbon emissions from transportation are projected to grow by 47.5 percent 

over the period 1996-2020. Projected carbon emissions from transportation (based on modeling 

by the U.S. Department of Energy) are shown in Table 3-5. 

84 U.S. Congress Office of Technology Assessment. Preparing for an Uncertain Climate. Washington, DC, 1993. 

85 U.S. Environmental Protection Agency. Policy Options for Stabilizing Global Climate: Report to Congress. 

Washington, DC, December, 1990. 

86 Intergovernmental Panel on Climate Change. Second Scientific Assessment of Climate Change, Summary and Report. 

World Meteorological Organization/U.N. Environment Program. Cambridge, MA: Cambridge University Press, 1995. 

87 U.S. Department of Energy, Energy Information Administration. Emissions of Greenhouse Gases in the United 

States, 1997. October 1998. 

30 
Our Built and Natural Environments 

Table 3-5: Projected Carbon Emissions from Transportation 

Year Million metric tons per year 

1996 est. 

1997 est. 

2000 

2005 

2010 

2015 

2020 

472.8 

475.3 

515.8 

572.8 

626.3 

662.3 

697.3 

Source: U.S. Department of Energy, Energy Information Administration. Annual Energy Outlook 1999: With 

Projections to 2020. DOE/EIA-0383(98), December 1998, Table A-19. Reference Case Forecast. 

In addition to carbon emissions, vehicle travel contributes to emissions of two other greenhouse 

gases, methane and nitrous oxide. Emissions for these gases for 1997 are tabulated in Table 3-6: 

Table 3-6: Greenhouse Gas Emissions Due to Highway Vehicle Travel 

Pollutant Quantity Emitted (1997, thousand metric tons) 

Methane (CH4) 

Nitrous Oxide (N2O) 

213 

205 

Source: U.S. Department of Energy. Emissions of Greenhouse Gas in the US 1997, October 

1998, Table 26. 

Global climate change may have severe consequences for ecosystems and economies around the 

globe. IPCC models predict a rise in sea level over the next 100 years of 20 to 86 centimeters, with 

the most likely case of a rise of 50 centimeters.88 EPA predicts a median estimate of 45 centimeters. 

Such a rise would inundate wetlands and lowlands, accelerate coastal erosion, worsen coastal 

flooding, threaten coastal structures, raise water tables, and increase salinity of rivers, bays, and 

aquifers.89 Low-lying coastal areas would be the hardest hit, since a small sea level rise could put 

large areas under water. EPA estimated that a 50-centimeter sea level rise would inundate 5,000 

square miles of dry land and 4,000 square miles of wetlands in the United States.90 Total monetary 

losses caused by a 1-meter rise are estimated to be between $270 and $475 billion, not including 

future development.91 

The rises in global average temperature predicted by EPA, the IPCC, and the U.S. Congress Office 

of Technology Assessment could increase average global precipitation by as much as 7 to 

15 percent. Predictions suggest that precipitation would increase at high latitudes and decrease at 

low to middle latitudes, increasing the potential for more severe and longer-lasting droughts.92 

88 Intergovernmental Panel on Climate Change, Second Scientific Assessment of Climate Change, Summary and Report. 

World Meteorological Organization/UN Environment Program. Cambridge, UK: Cambridge University Press, 1995. 

89 Titus, J. and V. Narayanan. “Greenhouse Effect and Sea Level Rise: The Cost of Holding Back the Sea,” Coastal 

Management. Volume 19, 1991. 

90 Gardiner, David. “Global Climate Change Negotiations.” Testimony before the House Commerce Committee, 

Subcommittee on Energy and Power. June, 1996. 

91 Titus, J. and V. Narayanan. “Greenhouse Effect and Sea Level Rise: The Cost of Holding Back the Sea,” Coastal 

Management. Volume 19, 1991. 

92 U.S. Office of Technology Assessment. Preparing for an Uncertain Climate Volume 1. Washington, DC: U.S. 

Government Printing Office, 1993. 

31 
Our Built and Natural Environments 

Human health also could be significantly affected by global climate change, due to extended heat 

waves and a marked increase in vector-borne diseases such as malaria (due to the extension of the 

hospitable geographical range and seasons for these organisms).93 Although costs of mitigating 

these problems are uncertain, they may be in the billions of dollars per year. Two 1992 studies 

estimated the annual social costs of greenhouse gas emissions to the United States to be $25 

billion (1990 dollars) to $27 billion (1989 dollars).94 

Noise 

Automobile travel creates noise from engine operations, pavement-wheel contact, and wind noise. 

As a result, increased vehicle travel is likely to cause increased noise disturbances to communities. 

Because noise diminishes with distance from its source, the most serious transportation noise 

problems are experienced along major transportation corridors. The passage of the federal Noise 

Control Act of 1972 marked the recognition of noise as a major problem in urban living. As shown 

in Table 3-7, an estimate for 1980 indicated that 37 percent of the U.S. population was exposed to 

noise from road use great enough to cause annoyance—defined at Leq greater than 55 dB (A).95 

Table 3-7: Percent of U.S. Population Exposed to Road Transportation Noise 

(Outdoor Sound Level in Leq [dB(A)]) 

>55 dB(A) 

Annoyance 

>60 dB(A) 

Normal Speech 

Level 

>65 dB(A) 

Communication 

Interference 

>70 dB(A) 

Muscle/Gland 

Reaction 

>75 dB(A) 

Changed Motor 

Coordination 

37.0% 18.0% 7.0% 2.0% 0.4% 

Source: Organization for Economic Cooperation and Development. Indicators for the Integration of 

EnvironmentalConcerns into Transport Policies, OECD Publications, 1993. 

Numerous research projects for Organization for Economic Cooperation and Development (OECD) 

countries on the effects of noise and its wider repercussions indicate that an outdoor sound level of 

65 dB(A) is “unacceptable,” and an outdoor level of less than 55 dB(A) is desirable.96 Studies have 

estimated the annual social costs of vehicle-related noise to be anywhere from $2.7 to $9 billion. 97 

Other Environmental Impacts of Motor Vehicle Use 

Activities associated with the use of motor vehicles can create pollution that affects water quality 

and damages ecosystems. Upstream impacts associated with fuel production and distribution, in 

particular, are associated with adverse impacts to the environment, such as oil spills. About 1.6 

93 McMichael, Antony. “Global Health Watch: Monitoring Impacts of Environmental Change,” The Lancet. Volume 

342, December, 1993. 

94 Lower estimate from Ketcham and Komanoff (Win-Win Transportation: A No-Losers Approach to Financing 

Transport in New York City and the Region. 1992). Upper estimate from MacKenzie, et al. (The Going Rate: What it 

Really Costs to Drive. Washington, DC: World Resources Institute, 1992). 

95 Leq stands for Equivalent Sound Level and is a measure of a steady sound that has the same sound energy as an 

amplitude-varying sound of the same duration. Sound pressure levels are expressed in decibels (dB). 

96 Organization for Economic Cooperation and Development. Indicators for the Integration of Environmental Concerns 

into Transport Policies. OECD Publications. 1993. 

97 Lower estimate from Miller and Moffet. The Price of Mobility: Uncovering the Hidden Costs of Transportation. 

Natural Resources Defense Council, 1993. Upper estimate from MacKenzie, et al. (The Going Rate: What it Really 

Costs to Drive. Washington, DC: World Resources Institute, 1992). 

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Our Built and Natural Environments 

million gallons of oil were spilled in U.S. navigable waters from vessel incidents during 1996. 98 

One study estimates that large oil spills from vehicles transporting fuel cause water pollution that 

costs society $2.36 to $5.9 billion annually. 99 Leaking underground storage tanks at gas stations 

and other facilities can cause groundwater contamination with associated environmental and 

health costs estimated at $0.12 to 0.59 billion annually. 100 

Motor vehicle manufacture is associated with environmental impacts including air pollutant 

emissions, toxic releases, and the generation of various solid and liquid wastes. Highway 

maintenance to support vehicle travel also involves activities that can adversely affect the 

environment, such as road salting, use of solvents, and pesticides. Highway de-icing can 

adversely affect roadside vegetation, soil structure, drinking water supplies, and aquatic life. 

Highway de-icing is estimated to cost society $0.826 to $2 billion annually. 101 

3.4 SUMMARY 

Over the past several decades, improvements in automobile-related infrastructure (highways, 

roads, parking lots), greater separation between jobs and housing, greater distances between 

destinations, and induced traffic (or additional travel prompted by road capacity expansions) have 

led to increases in vehicle travel. 

The environmental consequences of vehicle travel and vehicle dependency pose a potential threat 

to long-term community and environmental health. Highway emissions cause chronic health 

problems, affect water quality, and impose economic costs stemming from crop damage, building 

and property damage, and damage to visibility. Transportation is also a generator of noise and a 

major contributor to global climate change. 

Communities are also realizing that adding new road capacity no longer generates the same 

economic benefits it may have at one time. Studies have indicated that new highway 

development, which was often viewed as necessary to economic development in the past, offers 

increasingly fewer economic benefits at the state and national levels. As the national road 

network nears completion, the benefits of additional network construction decrease drastically. 

New roads may offer fewer benefits on the local level, too; although they may appear to spur 

growth, they often simply shift economic activity away from other areas.102 

Communities across the country realize that adding new road capacity is not by itself a viable longterm solution to traffic congestion problems. As they also realize that adding new capacity offers 

fewer economic benefits than previously assumed, they are beginning to examine the relationship 

between development patterns, travel patterns, and their environmental consequences. Communities 

98 U.S. Department of Transportation, Bureau of Transportation Statistics. National Transportation Statistics 1998. 

Table 4-42. 

99 Delucchi, M. et al. The Annualized Social Cost of Motor-Vehicle Use in the U.S., based on 1990-1991 Data, June 

1997. Converted into 1999 dollars. (UCD-ITS-RR-96-3). Converted to 1999 dollars. 

100 Delucchi, M. et al. The Annualized Social Cost of Motor-Vehicle Use in the U.S., based on 1990-1991 Data, June 

1997. Converted into 1999 dollars. (UCD-ITS-RR-96-3). Converted to 1999 dollars. 

101 Delucchi, M. et al. The Annualized Social Cost of Motor-Vehicle Use in the U.S., based on 1990-1991 Data, June 

1997. Converted into 1999 dollars. (UCD-ITS-RR-96-3). Converted to 1999 dollars. 

102 Boarnet, Marlon G. “Highways and Economic Productivity: Interpreting Recent Evidence.” Journal of Planning 

Literature. Vol. 11, No. 4. May, 1997. 

33 
Our Built and Natural Environments 

are recognizing that in order to meet federal air quality standards and protect other aspects of 

environment and community, they must turn toward more land use-based solutions to transportation 

challenges. We examine those solutions in Chapter 4. 

34 
4. Effects of Different Development Types on the 

Environment 

Problems with air quality, water quality, and biodiversity are serious enough that Congress has passed 

national legislation setting timetables for improvement. The best known of these timetables is the 

Clean Air Act, which sets deadlines for states to clean their air to levels that protect vulnerable 

populations. 

In order to establish a framework for future actions, EPA recently developed a strategic plan that 

identifies the agency’s goals for implementing its mission “to protect human health and to safeguard the 

natural environment.” Five strategic goals with strong connections to aspects of the built environment 

are listed in Table 4-1, along with a description of how urban form affects the attainment of each goal.103 

Table 4-1: EPA Strategic Goals with Built Environment Linkages 

Health or Environmental Goal 

Is attainment of these goals affected 

by current land-use trends and increased VMT? 

1. Clean Air Yes, from tailpipe and evaporative emissions 

2. Clean and Safe Waters Yes, from road and pavement runoff, and from deposition of airborne 

emissions 

3. Preventing Pollution and Reducing Risk in 

Communities, Homes, Workplaces, and 

Ecosystems 

Yes, from habitat and ecosystem fragmentation and destruction 

4. Better Waste Management, Restoration of 

Contaminated Waste Sites, and 

Emergency Response 

Yes, to the extent that communities abandon or fail to redevelop 

brownfields 

5. Reduction of Global and Cross-Border Yes, from greenhouse gas emissions from motor vehicles 

Environmental Risks 

Although land use and transportation development affect EPA’s environmental goals, historically 

national efforts have relied principally on tailpipe or smokestack solutions. As Congress passed 

legislation to address environmental and human health threats, EPA responded with regulations aimed 

largely at controlling the most obvious risks, such as pollution from large industries. Although the 

logic and efficiency of managing risks in this manner are well recognized, it has become increasingly 

clear that there are limitations to this approach and that these strategies alone will not achieve our 

national goals. In particular, changes in the built environment threaten various environmental goals 

and may require new approaches. 

Not all development affects the environment in the same ways. As communities examine how to grow, 

they are looking for strategies that will protect the environment while accommodating new growth. This 

section examines how various types of development patterns and practices affect the environment. 

103 EPA’s strategic plan was completed in September 1997 and addresses the Government Performance and Results Act’s 

direction for each executive department and agency to develop a strategic plan that outlines the goals that will provide the 

framework for planning and resource allocation. EPA’s strategic plan includes 10 goals. The other five are: 6) Safe Food; 

7) Expansion of Americans’ Right to Know about their Environment; 8) Sound Science, Improved Understanding of 

Environment Risk, and Greater Innovation to Address Environmental Problems; 9) A Credible Deterrent to Pollution and 

Greater Compliance with the Law; and 10) Effective Management. 

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Our Built and Natural Environments 

Although the evidence is ample that different types of development patterns can affect the 

environment in different ways, the understanding of these effects is not yet complete. The direct 

effects in terms of habitat consumption and disruption are well documented and widely accepted. By 

contrast, the effects of highway investment on vehicle travel and the effects of specific development 

patterns on travel and emissions are somewhat less well understood, and the exact magnitude of these 

effects is subject to some debate. This section of Our Built and Natural Environments synthesizes 

findings from several researchers, noting where greater or less certainty exists about linkages between 

causes and effects and the relative magnitude of effects. Where previous sections described broad 

land use and transportation trends and impacts, this section focuses on impacts of current 

development versus more environmentally sensitive development. 

Certain characteristics of the built environment are associated with beneficial environmental results. 

This chapter discusses six of these features: 

1) Compact development 

2) Reduced impervious surfaces and improved water detention 

3) Safeguarding of environmentally sensitive areas 

4) Mixed land uses 

5) Transit accessibility 

6) Support for pedestrian and bicycle activity and other micro-scale urban design factors 

These elements are not entirely separable from each other. For example, encouraging compactness 

through infill or brownfields redevelopment often facilitates mixed-use development and provides 

support for transit use, and walking and cycling. Safeguarding environmentally sensitive areas often 

involves developing more compact or clustered development. In similar ways, other elements listed 

above may work most effectively in combination with each other rather than individually. 

Incorporating one beneficial element without others could have minimal effects or possibly prove 

detrimental to environmental goals. For example, increasing density without taking care to safeguard 

environmentally sensitive areas or to improve transit access could result in increased water quality 

impacts, traffic congestion, or air quality problems. The enhanced benefit of incorporating multiple 

aspects of design into communities is often referred to as “synergy.” Synergy is discussed in Section 4.7. 

Because most practices work synergistically with one another isolating the effects of one from another 

can be difficult. In general, the studies presented in the first six sections of this chapter were chosen 

because they attempt to isolate the effects of particular strategies. However, studying one technique in 

isolation is often nearly impossible, and the utility of doing so is somewhat limited, as most practices 

are used in combination with others. Nonetheless, although findings may differ on the magnitude of the 

effects of different practices, the evidence is overwhelming that some developments yield better 

environmental results than others.104 

104 For evidence of consensus on the view that some development practices yield better environmental results than others, 

see Burchell, Robert W. et al. The Costs of Sprawl – Revisited. Transit Cooperative Research Program (TCRP) Report 39. 

Washington, DC: National Academy Press, Transportation Research Board, 1998. Burchell et al provide a synthesis and 

critical assessment of the literature on sprawl. 

36 
Our Built and Natural Environments 

4.1 COMPACT DEVELOPMENT 

Compact metropolitan development generally means that the 

space needs of a population can be satisfied with less land 

area. Compact development can take various forms. From a 

regional perspective, metropolitan areas may limit the extent 

of development so that it does not extend too far into rural 

areas. New development can be targeted to specific areas, 

such as redevelopable areas within established communities. 

Types of Compact Development 

Communities can develop more compactly by using three 

techniques: 

� Infill development 

� Brownfields redevelopment 

� Cluster development 

INFILL DEVELOPMENT 

Infill development occurs in locations where some 

development has already taken place and infrastructure is 

already in place. In urban areas, infill development is typically 

executed by converting old buildings and facilities into new 

uses (redevelopment) or by filling undeveloped space within 

these areas. Figure 4-1 shows an example of infill 

development in an urban area, where parking lots are replaced 

by buildings, parks, and/or garages. 

Compact Development in 

Practice 

Some metropolitan areas have decided to preserve the scenic open 

space that attracted their residents in 

the first place, as a means of preserving the continuing desirability of 

their communities. A number of communities have chosen to target development and place limits on the geographic extent of regional development. In particular, Portland, Oregon, 

chose to implement an urban growth 

boundary as a means of preserving 

open space. Montgomery County, 

Maryland, established an agricultural 

reserve in the western half of the 

county. Other areas have elected to 

preserve greenway corridors, establish state or city parks, and purchase 

easements on land as ways of preserving open space. Portland plans to 

accommodate all market-demanded 

housing types within its urban growth 

boundary, which is periodically adjusted to ensure a 20-year supply of 

developable land. 

Figure 4-1: Infill Development in an Urban Area 

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Our Built and Natural Environments 

Infill development is a way to accommodate regional growth using relatively less suburban and rural 

greenspace. The potential for infill is considerable. In 1994, researchers at the University of California 

at Berkeley’s Institute of Urban and Regional Development surveyed planning directors in 1,200 

political jurisdictions nationally and concluded that “existing urban areas have substantial capacity for 

new residential development.” This conclusion was reached based on a conservative estimate: The 

survey asked only about currently undeveloped land, not about underutilized properties that are 

important land resources in most older urban areas.105 

BROWNFIELDS REDEVELOPMENT 

As a particular kind of underdeveloped land, brownfields have received significant attention as both a 

problem and a potential source of multiple urban benefits. Brownfields are “abandoned, idled, or 

underused industrial and commercial facilities where expansion or redevelopment is complicated by real 

or perceived environmental consequences.”106 Brownfields redevelopment has potentially strong 

repercussions for environmental quality and community life since undeveloped brownfield sites may be 

a health threat or a discouragement to further investment in established urban areas. 

The General Accounting Office estimates that the nation has between 13,000 and 450,000 

brownfields.107 Brownfields, like infill sites, have the potential to absorb significant amounts of 

development. A 1982 study of three urban areas found that vacant land within developed areas could 

accommodate from two-thirds to 100 percent of the projected 10-year housing need.108 That capacity 

has apparently not diminished significantly in the past 15 years. A 1996 study found that brownfields in 

Detroit, Chicago, Milwaukee, and Cleveland could absorb 1 to 5 years of residential development, 10 to 

20 years of industrial development, or 200 to 400 years of office space.109 These estimates are 

conservative because, due to cost constraints, only a subset of existing known brownfields was 

examined. Significant amounts of vacant land in each of these metropolitan areas have not been 

inventoried. 

Brownfield sites are different from other urban infill sites because of uncertainties about environmental 

liability and clean-up costs. Site owners, developers, and lenders often avoid investing in brownfields 

because of fear of contamination and the costs associated with it. Clean-up costs of brownfields vary 

widely depending on site size, the intensity and type of contamination, and the nature of the remediation 

required. Rather than developing brownfields, firms and investors instead turn to surrounding areas and 

undeveloped greenfields or relatively untouched and uncontaminated land.110 

105 Pendall, Rolf. “Land Availability and Zoning: Indications from a National Survey.” On the Ground. Fall, 1994. pp. 19-

20. 

106 U.S. Environmental Protection Agency, Office of Solid Waste and Emergency Response. Brownfields Initiative. (Quick 

Reference Fact Sheet) April, 1996. 

107 U.S. General Accounting Office. Community Development: Reuse of Urban Industrial Sites. (GAO/RCED-95-172) 

Washington, DC: US Government Printing Office, June 1995. 

108 Real Estate Research Corporation. Infill Development Strategies. Washington, D.C.: ULI-the Urban Land Institute and 

American Planning Association, 1982. 

109 Simons, Robert, Planning and Development, Cleveland State University. Brownfields Supply and Demand Analysis for 

Selected Great Lakes Cities. Prepared for EPA, 1996. 

110 “What is a Brownfield?” http://www.brownfield.org/#Brownfield. 

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Our Built and Natural Environments 

Local government officials frequently cite brownfields as one of their biggest environmental 

problems.111 Based on a national survey of communities, the U.S. Conference of Mayors reports that 

“brownfields are a major problem for cities large and small and the lack of funds to clean up these 

sites was the most frequently identified obstacle in recycling these lands.”112 Consequently, local, 

state, and federal agencies are targeting attention and resources to brownfields. 

Redevelopment also can help assure that no stigma (of either perceived or real contamination) 

impedes investment in the community. As a result, clean-up and redevelopment of brownfield sites 

may encourage additional redevelopment in surrounding areas. 

Despite the numerous barriers associated with brownfield redevelopment, productive reuse is 

becoming increasingly feasible and common. 

CLUSTER D EVELOPMENT 

In newly developed areas, clustering development into concentrated areas can protect natural habitat. 

Cluster developments are built at gross densities comparable to conventional developments but leave 

more open space by reducing lot sizes.113 Square footage of buildings and residential and commercial 

capacity may remain the same, but compact clusters reduce the dimensions and geometry of individual 

lots and shorten road lengths, as shown in Figure 4-2. In the large-lot development, private lots take up 

the entire area of the subdivision, while in the compact development, private lots take up only a portion 

of the total land area, allowing more than half the land area to remain in its natural state. 

Clustering has a number of advantages in addition to the environmental benefits discussed below. One 

of the main advantages of cluster development as a conservation tool is that it does not take 

development potential away from developers, since it changes the arrangement but not the number of 

units permitted on a property. It also can reduce costs for developers—by requiring fewer miles of 

roads and, if applicable, water and sewer lines. Furthermore, cluster development does not require 

large public expenditures to purchase development rights. 

111 Snyder, Robin, EZ/EC EPA Programmatic Contact – Personal communication, 1997 

112 U.S. Conference of Mayors. “Recycling America’s Land: A National Report on Brownfields Redevelopment, Volume 

II.” April 1999. 

113 Measures of density depend on the geographic area being examined. Perceptions of density are influenced by many factors, 

including the design of buildings, site layout, and mix of buildings and green space. Cluster developments are not necessarily 

perceived as denser than “large lot” developments, especially since they often contain larger tracts of continuous green space. 

39 
Our Built and Natural Environments 

Figure 4-2: Large-Lot Development Versus Compact Development 

Direct Environmental Effects of Compact Development 

Compact development has some clear effects on the environment that are a direct result of reduced 

use of land and the nature of development or redevelopment: 

� Reduced disruption and fragmentation of habitat 

� Reduced impervious surfaces, resulting in improved water quality 

� Clean-up of contaminated waste through brownfield redevelopment, which can reduce water 

pollution and community environmental risks 

REDUCED DISRUPTION AND FRAGMENTATION OF HABITAT 

Compact development minimizes land conversion to urban use, and maximizes retained natural habitat. 

Infill development of all kinds, including brownfields redevelopment, reduces development in more 

pristine areas since an acre built in town is often at least an acre less that is developed on a greenfield. 

In addition, the acre developed in an existing urban area usually requires less supporting space, 

especially roads and parking lots, because it can take advantage of urban transit and existing streets and 

parking facilities. When existing infrastructure is renovated, infill development can spare natural 

resources such as the wood and metal that would be needed for the construction of new facilities. 

Several studies document losses of fragile lands due to development. Numerous growth management 

plans also have evaluated how alternative development patterns would affect fragile lands. For 

example, a study comparing managed growth to a continuation of past trends in Orlando, Florida, 

projected that managed growth would result in a loss of 20 percent fewer acres of wetlands and 

floodplains.114 Studies of alternative development patterns have found that infill and compact 

114 Orlando, Florida. Urban Area Growth Management Plan. 1991. 

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Our Built and Natural Environments 

development maximize bird species abundance and diversity compared with a lower-density 

development of the same number of units spread over the same area.115 

Several analyses of development impacts on fragile lands have been conducted by Burchell et al. for 

New Jersey; Lexington, Kentucky; the Delaware Estuary; and Michigan.116 These studies generally 

find that planned versus trend development would reduce consumption of fragile environmental lands 

by almost one-fifth. The range of savings varied from 12 to 27 percent, depending on the starting 

level and location. Similar studies conducted in the San Francisco Bay area by Landis found even 

larger land savings under a compact growth scenario.117 

Large-tract, low-density development is usually characterized by plantings of lawns, flowers, shrubs 

and trees, some of which may offer habitat for certain songbirds and other human-tolerant creatures. 

But the diversity of native species can be reduced significantly. Large tracts of continuous 

development associated with cluster development allow preservation of more natural wildlife 

habitats, less affected by human disturbance. 

Compact development allows preservation of open space, including wetlands, farmland, and forests. 

Maintaining the integrity of wetlands is beneficial to water quality in many ways. (See Chapter 2.) 

Open space—farmlands and forests—all contribute to the economic, recreational, and ecological 

value of a community. 

REDUCED IMPERVIOUS SURFACES 

Infill development accommodates new growth with significantly less impervious surfaces per unit of 

development than development on undeveloped land does. As shown earlier in Figure 4-1, infill 

development may result in minimal changes in impervious surfaces by redeveloping parking lots or 

sites with abandoned buildings into new buildings. The infill site has roughly the same amount of 

impervious surface area (since parking lots or old rooftops are now new rooftops or other structures) 

before and after development and little net loss of vegetative cover occurs. Since locating the 

buildings on undeveloped land would result in new impervious surface cover (i.e. new roads and 

parking facilities), development at the infill site—that uses existing infrastructure—reduces overall 

impervious cover. Particularly in comparison to the alternative of building on a greenfield site— 

undeveloped land at the urban periphery—infill development can reduce impervious surfaces by 

decreasing the need for new roads and parking lots. Infill also can promote mixed-use development 

and help support transit usage, discussed below in Sections 4.4 and 4.5. 

115 Landis and Pendall, pp. 14-15. Also see: Johnston, Robert and Tomas de la Barra. Comprehensive Regional Modeling for 

Long-Range Planning: Linking Integrated Urban Models to Geographic Information Systems. Resubmitted to 

Transportation Research: A, 1998. 

116 Burchell, Robert and David Listokin. Fiscal Studies. Report to the Governor’s Commission on Growth in the Chesapeake 

Bay Region. Annapolis, MD: 2020 Commission, 1990; Burchell, Robert et al. Impact Assessment of the New Jersey Interim 

State Development and Redevelopment Plan. Trenton, NJ: New Jersey Office of State Planning. 1992.; Burchell, Robert and 

David Listokin. The Economic Impacts of Trend versus Vision Growth in the Lexington(Kentucky) Metropolitan Area. 

Report prepared for Bluegrass Tomorrow, Lexington, KY: January 1995; Bruchell, Robert and Harvey Moskowitz. Impact 

Assessment of DELEP CCMP versus Status Quo on Twelve Municipalities in the DELEP Region. Report prepared for the 

Local Governments Committee of the Delaware Estuary Program. Philadelphia, PA: August 1995; Burchell, Robert et al. 

Fiscal Impacts of Alternative Land Development Patterns in Michigan: The Costs of Current Development Versus Compact 

Growth. Southeast Michigan Regional Council of Governments. 1997. 

117 Landis, John. “Improving Land Use Futures: Applying the California Urban Futures Model.” Journal of the American 

Planning Association. 61, 4 (Autumn), 1995. pp. 438-457. 

41 
Our Built and Natural Environments 

By reducing the need for impervious surfaces, compact development significantly reduces runoff and 

water pollution. The Center for Urban Policy Research’s New Jersey Impact Assessment concluded 

that the state’s compact development plan would produce 40 percent less water pollution than more 

dispersed development patterns would. The smaller impervious areas would produce 30 percent less 

runoff, and concentrating this development in areas served by sewers would reduce its impact on the 

environment by another 10 percent.118 

In the Chesapeake Bay watershed, a study compared compact and dispersed developments on tracts 

of land of the same size. Compact development consumed one-third as much land as a dispersed 

development consumed and included about half as much impervious surface. As a result, the compact 

development pattern resulted in 43 percent less runoff than the more dispersed development.119 

Studies show that the impervious surface area of a clustered development site is often 10 to 15 percent 

less than that of more dispersed development, depending on the size and configuration of each 

individual project and the original lot size and road network. The greatest reduction in impervious 

surface area is found when cluster development is applied on large lots. Reductions in impervious 

surface area are primarily due to the shorter length of road network needed to serve lots.120 With 

impervious surfaces, the transportation component—roads, sidewalks, and parking lots—usually 

constitutes a larger share of impervious cover than rooftops do, both in residential and commercial 

areas.

121 

Although individual lots in low-density developments are generally more permeable (e.g., lawns, 

gardens) than higher density developments, the greater total amount of land affected by low-density 

development (its “footprint”), plus the greater amount of land devoted to roads and parking lots, often 

results in greater water quality impacts.122 In addition, low-density residential developments that are 

served by septic systems, rather than by sewers, can cause nitrogen and pathogen contamination of 

groundwater and surface waters if they are not properly sited, designed, installed, and maintained. 123 

In addition, less stormwater runoff and pollutant loads are found in cluster developments, due to 

reductions of impervious cover. By clustering development, 30 to 80 percent of the entire site may be 

left undisturbed, without reducing the number of lots on a site.124 Cluster zoning seems to be an 

effective method of preserving a site’s existing landscape character, forested areas, aquatic and 

terrestrial habitat, and watershed resources, and protecting these sensitive areas from the secondary 

impacts typically associated with new development. 

118 CUPR, and Landis and Pendall. The Real Estate Research Corporation in its 1974 report The Cost of Sprawl also found 

less runoff from compact communities. 

119 Chesapeake Bay Foundation. A Better Way to Grow. 1996. p.7. 

120 Schueler, Tom. Environmental Land Planning Series: Site Planning for Urban Stream Protection. Center for Watershed 

Protection. Publication No. 95708. Washington, DC: Metropolitan Washington Council of Governments, Dec. 1995. p. 61. 

121 Schueler, Tom. Environmental Land Planning Series: Site Planning for Urban Stream Protection. p. 19 

122 Arnold, Chester L. Jr., C. James Gibbons. “Impervious Surface Coverage - The Emergence of a Key Environmental 

Indicator.” Journal of the American Planning Association. Volume 62, Number 2, Spring 1996. pp. 243-258. 

123 U.S. Environmental Protection Agency. Guidance Specifying Management Measures for Sources of Nonpoint Pollution 

in Coastal Waters. Chapter 4, Section VII: Onsite Sewage Disposal Systems. 1991. 

124 Schueler, Tom. Environmental Land Planning Series: Site Planning for Urban Stream Protection. pp. 63-4. 

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Our Built and Natural Environments 

Conventional urban fringe and suburban development—with large lot sizes, wide streets, and substantial 

parking—can produce storm runoff almost 50 percent greater than more compact development.125 

Watersheds containing less than 10 percent impervious surface maintain healthy streams, thus providing 

habitat for sensitive species. At more than 10 percent imperviousness, most watersheds show signs of 

impairment, and watersheds with more than 30 percent imperviousness are seriously degraded.126 

A case study of Remlick Hall Farm, a planned subdivision outside Washington, DC, illustrates the 

importance of development patterns for the watershed. 127 The conventional plan contains a total of 84 

residential lots spread out fairly evenly over the site. Assuming a 20-foot road width, approximately 

20,250 linear feet of roadway are required to serve the development pattern. Road and driveways alone, 

under this scenario, comprise about 10.8 acres of new impervious surface area. Including rooftops and 

other hard surfaces, the total impervious surface cover reaches 26.3 acres. It should be noted that this 

estimate is quite conservative, considering that conventional road width for suburban developments is 

actually approximately 32 feet, with 50 feet not uncommon in many new suburban developments. 

In contrast, an alternative clustered subdivision plan clusters 52 individual residential sites to preserve 

farmland, shoreline areas, and forests. Roads are narrowed from 20 to 18 feet in width, while a 53 

percent reduction in road length is achieved under the cluster plan. Total impervious surface cover 

comes to 15.37 acres. Reductions in road infrastructure lead to an estimated $525,000 savings in 

development costs, as well as major savings in polluted stormwater runoff. Runoff in the conventional 

subdivision was estimated at 79 acre feet per year, while for the cluster development estimated runoff 

amounted to only 46 acre feet per year. 

CLEAN-UP OF CONTAMINATION THROUGH BROWNFIELDS REDEVELOPMENT 

Past industrial activity bequeathed a legacy of soil and water pollution at formerly industrialized sites. 

Thousands of these brownfields are located in densely populated, urban areas where residents tend to 

congregate and children play. Many of these sites are located near rivers and streams, which once 

served as valuable transportation corridors. The juxtaposition of toxic chemicals, human activity, and 

sensitive environmental habitats can lead to a range of problems, including compromised human and 

environmental health. 

Even in areas where individual properties are not highly contaminated, the collective effect of a 

number of mildly contaminated properties in one area can be significant. The clean-up and 

redevelopment of these sites can lead to substantial environmental and human health benefits. The 

benefits of brownfields clean-up are dependent on several factors, including the type and severity of 

contamination; the geographic distribution of the contamination and its proximity to communities, 

water resources, and biologically sensitive areas; and the clean-up standard employed. 

Potential public health benefits of brownfields clean-up include reduced blood lead levels, decreased 

risk of cancer, and minimized respiratory problems. The encapsulation (“capping”) of contamination 

on brownfields can reduce runoff of toxics into nearby bodies of water. This reduced runoff can lead 

to improvements in overall water quality and, consequently, improvements in habitat. 

125 Schueler, Tom. Site Planning for Urban Stream Protection. Center for Watershed Protection, Silver Spring, Maryland: 1995. 

126 Arnold, Chester L. Jr., C. James Gibbons. “Impervious Surface Coverage - The Emergence of a Key Environmental 

Indicator.” Journal of the American Planning Association. Volume 62, Number 2, Spring 1996. p. 246. 

127 Chesapeake Bay Foundation. A Better Way to Grow. 1996. pp. 28-31. 

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Our Built and Natural Environments 

Brownfields redevelopment also benefits the environment; removing toxics from the site can result in 

cleaner soil and improved water quality. In addition, many brownfields are located in urban areas that 

are already served by existing sewer lines, roads, and other 

infrastructure. Redeveloping these properties can 

consequently minimize greenfield development. 

Indirect Environmental Effects of

Compact Development:

Reduced VMT and Emissions

In addition to direct effects on habitat, water quality, and 

brownfields, compactness in a metropolitan area also affects 

travel activity. Increased compactness is viewed as a means to 

reduce vehicle travel because it affects travel demand in the 

following ways: 

� Trip lengths – Compactness increases the number of 

activities accessible within a given area. By locating 

activities closer together, compact development can 

reduce travel distances. 

� Mode choice – Locating activities closer together 

allows trips to be made by walking and bicycling 

rather than motor vehicle. In addition, density 

provides the “mass” needed for mass transit and 

carpooling. Compactness reduces the costs of 

providing transit services since shorter trips and trip 

times allow transit operators to provide the same 

frequency of service with fewer vehicles and fewer 

driver hours.128 As a result, dense areas tend to have 

greater access to transit, which reduces the relative 

costs of using transit, and creates greater 

opportunities for shared rides during commuting. 

Efficient Use of Infrastructure 

Existing public infrastructure often is 

inadequate for serving new low-density 

development. Frequently, jurisdictions or 

developers are forced to build new 

infrastructure, even while existing capacity 

is underutilized. For example, in 

metropolitan Minneapolis-St. Paul, school 

enrollment fell by 81,000 between 1970 

and 1990 as the growth in the baby boom 

population ended. Approximately 130 

schools were closed in the central city and 

immediate suburbs, yet 50 new schools 

were opened on the metropolitan fringe.128 

Some school closings were probably 

justified in terms of physical obsolescence. 

However, many schools were still 

adequate, meaning that the investment in 

suburban schools was inefficient. 

One advantage of compact development is 

that it makes more efficient use of 

infrastructure. Development on infill sites 

can usually tap into existing infrastructure, 

including roads, water lines, sewer 

systems, and schools. Even when new 

infrastructure is necessary, compact 

development is still less costly because the 

community can be adequately served by 

fewer schools, fewer miles of roads, and 

fewer new water pipes than a low-density 

community might require. 

� Vehicle ownership – Dense areas reduce the need for vehicle travel for personal mobility, so 

people are less likely to own as many vehicles. Dense areas also tend to be associated with 

limited parking availability and higher cost, as well as increased transit accessibility, all of 

which are associated with reduced vehicle ownership. 

EVIDENCE FROM SIMULATION MODELS 

Several regional models of development patterns and transportation investments find that more 

compact development results in less vehicle travel and fewer emissions of air pollutants than 

128 Parsons Brinckerhoff Quade and Douglass, Inc. Transit and Urban Form. “Part I: Transit, Urban Form, and the Built 

Environment: A Summary of Knowledge.” Transit Cooperative Research Program Report 16. Washington, DC: 

Transportation Research Board, 1996. 

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Our Built and Natural Environments 

dispersed development patterns do.129 It should be noted that simulation models typically have a 

margin of error of 5-10 percent and that their results may not be exact in terms of the magnitude of 

VMT reductions. Taken together, however, these models clearly indicate that compact development 

does have an effect on VMT, even if consensus on the magnitude of that effect has not been 

reached.130 

An analysis of changing allocations of new employment and residential growth over the period 1990 to 

2020 in the Puget Sound, Washington, region found that patterns of compact development reduced 

VMT compared with more dispersed patterns. The regional metropolitan planning organization (MPO) 

examined three alternative land-use patterns. In the “Major Centers” alternative, new employment 

growth was concentrated in a few major centers, higher density residential development within walking 

distance of major transit access points was encouraged, and transit investments were emphasized. The 

“Multiple Centers” alternative concentrated new employment and housing growth in a relatively large 

number of centers, with a balance of jobs and housing within each center’s area of influence, with high 

transit emphasis. Finally, the “Dispersed Growth” alternative dispersed employment and housing 

growth into newly developing areas. The modeling suggested that the Major Centers alternative would 

reduce VMT by 4 percent from the baseline, and the Multiple Centers alternative would reduce VMT by 

1 percent from the baseline. The Dispersed alternative meanwhile was projected to increase VMT 3 

percent above the baseline level. 131 

The estimate from the Puget Sound model may understate the benefits of planning that increases 

regional concentrations of development. Travel impacts were estimated using traditional four-step 

Urban Transportation Planning System-based models. These traditional models are not sensitive to 

land use characteristics when predicting vehicle ownership, mode choice, or trip frequency. In 

addition, these models do not account for non-motorized trips and so fail to account for trips that may 

be diverted to walking and bicycling in dense urban areas. 

The Region 2040 process in Portland, Oregon, employed more sophisticated travel demand models 

that included sensitivity to land uses. Portland used the models to examine land-use scenarios. In the 

base case, the urban area expanded by more than half its current size. In the Growing Up alternative, 

urban growth was maintained inside the current urban growth boundary. The simulations suggest that 

129 Four regional simulation studies sponsored by the Federal Highway Administration are profiled in: DeCorla-Souza, Patrick. 

The Impacts of Alternative Urban Development Patterns on Highway System Performance. Presentation to ITE Conference on 

Transportation Engineering in a New Era. 1992. Although these models predict the future, they are based on the observed 

behavior of people reacting to real choices. Other simulations that demonstrate a relationship between the compactness of 

development and travel behavior include: Rodier, Caroline A. and Robert A. Johnston. “Travel, Emissions and Welfare Effects 

of Travel Demand Management Measures.” Transportation Research. Record 1598. 1997; Johnston, R.A., and R. Ceerla. 

“Land Use and Transportation Alternatives,” in D. Sperling and S. Shaheen (eds.), Transportation and Energy, ICEEE, 1995. 

130 Some researchers have suggested that vehicle hours traveled (VHT) have greater impact on air quality than vehicle miles 

traveled (VMT) do—a view implying that investments that ease congestion and increase vehicle speed may be more 

effective in reducing pollution than policies that aim to change land use patterns. Even if the relationship between VHT and 

air quality is a strong one, research on the relationship between development patterns, vehicle travel, and air quality 

indicates a direct relationship between VMT and air quality, suggesting that reductions in VMT will lead to improved air 

quality. In any case, reductions in VMT from additional capacity are likely to be short-term as induced demand returns 

congestion to its original levels. See Chapter 3. 

131The Multiple Centers alternative was expected to reduce delay and congestion more than any of the other alternatives, 

presumably because reduced VMT would more than offset potential increases in local congestion around the activity centers. 

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Our Built and Natural Environments 

the Growing Up scenario would double the regional transit mode split to 6 percent from 3 percent in 

the base case and would reduce regional VMT by 16.7 percent compared with the base scenario.132 

REGIONAL LOCATION OF DEVELOPMENT 

Recent research indicates that location of development within a metropolitan region is a significant 

factor in determining the vehicle travel and emissions generated by the development. One recent 

simulation study compared alternative locations for a proposed development located in the San 

Diego, CA; Montgomery County, MD; and West Palm Beach, FL, metropolitan areas.133 Using fourstep transportation models and a geographic information system-based analysis tool, the travel and 

emissions impacts of location in a central infill site in each case versus location at the regional 

periphery (a greenfield site) were compared. 

Four-step transportation models use regional data, including data on local and regional transportation 

patterns, transportation behavior of residents, and roadway and transit networks, to estimate the 

number of trips by purpose, mode, and travel times. The first step, trip generation, estimates the total 

number of trips produced by households and total trips attracted by employment centers, recreational 

facilities, etc. In the second step, trip distribution, the model allocates trips generated in the first step 

to specific origin-destination movements. The third step, mode split, determines the share of trips 

made by mode of travel (driving, transit, walking, etc.). Traffic assignment, the final step, estimates 

traffic volumes by mode for each link in the transportation system. 

In each of the cases modeled, the infill site generated less VMT per capita and reduced emissions of 

most air pollutants and greenhouse gases, as shown in Table 4-2. 

Table 4-2: Travel and Emission Indicators for Infill Site versus Greenfield Site 

Case Study 

Per capita daily VMT, 

infill as percentage of 

greenfield 

Emissions, infill as 

percentage of greenfield 

San Diego, CA 52% 

CO: 88% 

NOX: 58% 

SOX: 51% 

PM: 58% 

CO2: 55% 

Montgomery County, MD 42% 

CO: 52% 

NOX: 69% 

SOX: 110% 

PM: 50% 

CO2: 54% 

West Palm Beach, FL 39% 

CO: 75% 

NOX: 72% 

SOX: 94% 

PM: 47% 

CO2: 50% 

Source: Allen, E, Anderson, G, and Schroeer, W, “The Impacts of Infill vs. Greenfield Development: A Comparative 

Case Study Analysis,” U.S. Environmental Protection Agency, Office of Policy, EPA publication #231-R-99-005, 

September 2, 1999. 

132 Metro. Metro 2040 Growth Concept. Portland, OR: December 8, 1994. 

133 Allen, E, Anderson, G, and Schroeer, W, “The Impacts of Infill vs. Greenfield Development: A Comparative Case Study 

Analysis,” U.S. Environmental Protection Agency, Office of Policy, EPA publication #231-R-99-005, September 2, 1999. 

46 
Our Built and Natural Environments 

The sources of the projected emissions reductions 

differed among the sites. In West Palm Beach, the 

estimated travel reduction was projected to come from 

reductions in average trip length, with minimal changes 

in mode share, due to the relative auto-dependence of 

the region. In contrast, in Montgomery County, outside 

Washington, DC, more significant changes in mode 

share were projected to occur. In this case, the infill site 

was located near a major transit center, well-served by 

buses and heavy rail. Although projected road 

congestion results varied across the case studies, in 

general the shorter trip distances made up for slower intown travel speeds and total vehicle trip times were 

lower for the infill sites. 

Despite some potential increases in localized 

congestion, the three site comparisons suggest that 

infill development would reduce motor vehicle 

emissions compared with a greenfield site. Even when 

infill development cannot take advantage of regional 

transit, infill tends to reduce air pollution because 

regionally accessible, centrally located sites require 

shorter average trip distances than do sites along the 

regional periphery. 

A follow-up study in Atlanta, GA, examined the 

transportation and emissions impacts of locating a large 

new development at an infill site formerly occupied by 

the former Atlantic Steel mill, compared to suburban 

alternatives.134 Results suggest that vehicle miles of 

travel associated with a suburban site could run as 

VMT at Infill versus Suburban 

Development Sites: 

The Atlantic Steel Case 

EPA recently conducted an analysis to determine 

the transportation and emissions impacts of 

locating a new development at an infill site 

(formerly used by Atlantic Steel) compared with 

several suburban sites. EPA used Atlanta’s 

regional travel model and EPA’s MOBILE 5 

emissions model to analyze the likely effects of 

developing each site with the same amount and 

mix of development. The study concluded that, 

depending on which suburban site is considered, 

development on the infill site would result in the 

following savings: 

� VMT savings of 15-52 percent 

� NOX emissions savings of 37-81 percent 

� VOC emissions savings of 293-316 percent 

Transit share of work trips were projected to be 

significantly higher at the Atlantic Steel site: 27 

percent of work trips made by transit compared 

with the regional average of approximately 8 

percent and the 0-13 percent transit share that 

would result from development at the suburban 

alternatives. 

Source: U.S. Environmental Protection Agency. 

November 1, 1999. “Transportation and Environmental 

Analysis of the Atlantic Steel Development Project.” 

Prepared by Hagler Bailly. 

much as 52 percent higher, while NOX emissions could be 81 percent higher than would result from 

placing the same amount of development at the Atlantic Steel site. 

EVIDENCE FROM EMPIRICAL STUDIES 

Increasing regional compactness generally involves increasing density in specific targeted areas of the 

region. A large number of empirical studies (based on comparisons of actual communities) have 

found that population density is associated with reduced vehicle travel.135 

134 U.S. Environmental Protection Agency. “Transportation and Environmental Analysis of the Atlantic Steel Development 

Project.” Prepared by Hagler Bailly, November 1, 1999. 

135 Boyce, D.E., M.C. Romanos, B.N. Janson, P. Prastacos, M. Ferris, and R.W. Eash. Urban Transportation Energy 

Accounts. September 1981.; University of Toronto/York University Joint Program in Transportation, Data Management 

Group. The Transportation Tomorrow Survey: Travel Survey Summary for the Greater Toronto Area. June 1989; Harvey, G. 

Relation of Residential Density to VMT per Resident: Oakland. Metropolitan Transportation Commission. 1990. As cited by 

Holtzclaw, John. “Explaining Urban Density and Transit Impacts on Auto Use.” Presented to State of California Energy 

Resources Conservation and Development Commission, January 1991. 

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Our Built and Natural Environments 

One well known study by Holtzclaw found that a doubling of residential densities is associated with a 

decrease of 20 to 30 percent in VMT per capita.136 His analysis, which used odometer readings from 27 

California communities, suggested that residential density and access to public transportation were the 

two urban form factors that most reliably predict household auto travel behavior. Communities with 

higher residential density and transit service had the lowest rates of auto ownership and vehicle mileage 

per capita. Hotzclaw concludes that if a region’s population doubles wholly by infill, its vehicle miles of 

travel will likely increase by only 40 to 60 percent, rather than the 100 percent if the population grew at 

its present density. Doubling population at low density (as is common in many new suburbs) would 

likely increase average auto mileage by 150 to 186 percent. 

An earlier study, based on data for 105 U.S. metropolitan areas, including the New York City region, 

also found a correlation between density and mode share, and reported threshold densities at which 

transit use increases.137 According to the study, at densities between 1 and 7 dwellings per acre, transit 

use is minimal, while at densities above 60 dwellings per acre, more than half the trips tend to be 

made by public transportation. The authors conclude that a minimum density of 7 dwellings per acre 

appears to be a threshold above which transit use increases sharply. 

The highest density areas (e.g., Manhattan) tend to have the most transit service and the tightest 

parking supply. Recent evidence suggests that the effects of density on household travel behavior are 

complex and may be more related to characteristics typically associated with dense development than 

to population density itself. High-density areas tend to be associated with higher parking costs, 

limited parking, increased transit service, and a mix of land uses. Dense areas also tend to be located 

toward the center of a metropolitan region rather than on the periphery, and as a result are more 

accessible. Separating these effects is difficult, so the statistical significance of density in studies that 

fail to account for other urban form factors may be due in part to the strength of density as a proxy for 

other difficult-to-observe variables that affect travel behavior. 138 

Population density also tends to be correlated with income, a primary source of differences in travel 

behavior. A number of studies have attempted to control for income and other household 

characteristics to identify the independent effects of urban form on travel behavior. These studies 

generally find that urban form factors do have a discernible effect on travel behavior, even after 

accounting for household characteristics. For example, a study using data from the 1985 American 

Housing Survey performed a regression analysis to simultaneously test the effects of various factors 

on components of travel demand. The study found that vehicle ownership and vehicle mode share 

decline with neighborhood density. The study, which controlled for household income, household 

size, adequacy of public transit service, and location within a central city, found that households in an 

area of high-rise apartments were likely to own 0.42 fewer vehicles than households in a nearby 

136 Holtzclaw, John. “Explaining Urban Density and Transit Impacts on Auto Use.” Presented to State of California Energy 

Resources Conservation and Development Commission, January 1991. 

137 Pushkarev, B. and J. Zupan. Public Transportation and Land Use Policy. Bloomington, IN: Indiana University Press, 1977. 

138 For example, a study by Kockelman, K. (“Travel Behavior as a Function of Accessibility, Land Use Mixing, and Land 

Use Balance: Evidence from the San Francisco Bay Area.” Submission to the 76th Annual Meeting of the Transportation 

Research Board, January 1997), on the effect of urban form on VMT, automobile ownership, and mode choice found that 

under all but the vehicle ownership model, the effect of population density was minor after controlling for accessibility. 

Work by Cambridge Systematics et al.(Making the Land Use Transportation Air Quality Connection: Model Modifications. 

Vol. 4. Prepared for Thousand Friends of Oregon, May 1996) on the Portland regions’ travel models also found that 

residential density was not statistically significant in explaining vehicle ownership; rather, a measure of land use mix, transit 

accessibility to jobs, and the pedestrian environment were statistically significant in predicting vehicle ownership levels. 

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Our Built and Natural Environments 

neighborhood with single-family detached units. Holding transit service and number of vehicles 

constant, the probability of automobile commuting was greater in a neighborhood of single-family 

units or low-rise apartments than in a neighborhood with mid- or high-rise buildings. For example, 

the probability that a person in a one-car household commutes by transit was almost 30 percent if that 

household lived in a mid- to high-rise multifamily neighborhood in the central city but was less than 

10 percent if that person lived in a mostly single-family neighborhood in the city. 139 

A multivariate regression analysis of mode choice based on travel data from the Puget Sound region 

found that employment density at trip origins and destinations was significant in predicting the 

percentage of travel by each mode—more significant, in fact, than population density, after 

accounting for various control variables such as household type, proportion with a driver’s license, 

and vehicle availability. Modal shifts from single occupant vehicles (SOV) to transit and walking 

occurred between 20 and 75 employees per acre (causing SOV percentage to drop from about 90 

percent to 60 percent), and again with more than 125 employees per acre.140 

4.2 DESIGN FOR REDUCED IMPERVIOUS SURFACES AND IMPROVED WATER 

DETENTION 

As described in Chapter 2, impervious surfaces have substantial environmental impacts. Impervious 

surfaces increase peak discharges, pollutant loads, and volumes and velocity of runoff. In areas with 

large paved surfaces (such as parking lots), high volumes of storm runoff are carried out through 

storm drains into watercourses, starting soon after the storm begins and continuing during the 

duration of heavy rainfall. During periods of heavy rainfall, widespread coverage by impervious 

surfaces can increase the likelihood of serious flash flooding. Absorbing runoff where it originates 

helps reduce flooding and maintain the water table, wells, and creeks.141 

Techniques for Reducing Impervious Surfaces and Improving Water 

Detention 

Compact development often minimizes or reduces impervious land area, as described in Section 4.1. 

Compact development devotes less land area to roads and may also devote fewer acres to buildings if 

residential or commercial space is built up vertically rather than out horizontally. In addition to 

compact or cluster development, other variations in built environment designs can reduce impervious 

cover and improve stormwater infiltration and detention. This section describes those designs. 

Techniques for reducing impervious surfaces and improving water detention include: 

� Modification of street standards and parking requirements 

139 Cervero, Robert. “Mixed Land Uses and Commuting: Evidence from the American Housing Survey.” Transportation 

Research. Vol. 30, No. 5, 1996. pp. 361-377. 

140 Frank, L. and G. Pivo. “Impacts of Mixed Use and Density on Utilization of Three Modes of Travel: Single-Occupant 

Vehicle, Transit, and Walking.” Transportation Research Record 1466, 1994, pp. 44-52. 

141 Thelen, Edmund, and L. Fielding Howe. Porous Pavement. The Franklin Institute Press. 1978. 

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Our Built and Natural Environments 

� Use of porous surfaces rather than concrete and asphalt 

� Use of open and natural drainage systems 

� Landscaping that helps retain soil moisture and conserve water usage 

MODIFYING STREET STANDARDS AND PARKING REQUIREMENTS 

Surfaces used for transportation infrastructure account for much of the imperviousness at a 

development site and in urban areas. In areas with dispersed development patterns, for example, roads 

and parking lots account for as much as 70 percent of the total impervious surface.142 Impervious 

areas may be reduced in size, while retaining their utility. 

Reduced street widths (to widths typical in many existing communities) have been found to handle 

traffic and emergency response equipment adequately while significantly reducing the impervious 

surface area.143 Reducing street widths provides benefits beyond reduced imperviousness: Less 

clearing and grading is required, neighborhood traffic is slowed (traffic calming), and construction 

and maintenance savings accrue.144 In fact, livability issues, pedestrian-friendliness, and traffic 

calming are major reasons reduced street widths have been proposed. Portland, Oregon, for example, 

implemented its Skinny Streets Program in 1991 to reduce street widths and calm traffic. Reducing 

street widths has helped to preserve livability in communities, lessen stormwater runoff, reduce 

construction costs of new streets, and limit the impact of grading on slopes.145 

Existing roads also can be retrofitted to reduce impervious surface area. For example, cul-de-sacs 

may be retrofitted with vegetated islands designed to retain and infiltrate stormwater, and vegetated 

grass swales may be used in place of concrete curbing to capture and infiltrate runoff from paved 

streets.146 Median islands can be added with grass, flowers, or other vegetation. 

Another way to reduce impervious cover is modifying or downsizing parking areas. Studies suggest 

that parking is greatly oversupplied in both residential and commercial areas. Zoning codes typically 

require between three and five spaces per 1,000 gross square feet of office building area, with four 

spaces per 1,000 square feet often used as a rule of thumb. 147 Parking utilization surveys, however, 

typically show peak demand levels of between two and three spaces per 1,000 square feet.148 A case 

142 Chesapeake Bay Foundation. A Better Way to Grow. 1996. p.6. 

143 Federal Highway Administration. Flexibility in Highway Design. 1998; Institute of Transportation Engineers. Traffic 

Calming in Practice. Landor Publishing. 1994. 

144 Schueler, Tom. Environmental Land Planning Series: Site Planning for Urban Stream Protection. p.148. 

145 Southworth, Michael, and Eran Ben-Josaph. Streets and the Shaping of Towns and Cities. McGraw Hill, 1997. p.34. 

146 See Institute of Transportation Engineers (ITE). Traditional Neighborhood Development Street Design Guidelines. (Pub. 

No. RP-027). 1997. for more information about street designs. 

147 Bergman, David. “Off-Street Parking Requirements.” APA Planning Advisory Service Report 432. Chicago: American 

Planning Association. 1991; Gruen Gruen + Associates. Employment and Parking is Suburban Business Parks: A Pilot 

Study. Washington, DC: Urban Land Institute, 1986; Shoup, Donald. “Cashing Out Employer-Paid Parking.” FTA-CA-11-

0035-92-1. Washington, DC: U.S. Department of Transportation. 1993. 

148 Cervero, Robert. America’s Suburban Centers: The Land Use Transportation Link. Winchester, MA: Unwin Hyman, 

1989; Gruen Gruen + Associates. Employment and Parking is Suburban Business Parks: A Pilot Study. Washington, DC: 

Urban Land Institute, 1986. 

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Our Built and Natural Environments 

study of suburban office buildings in southern California found that parking was oversupplied by a 

factor of almost two in the typical sites studied—the amount of parking supplied averaged 3.8 spaces 

per 1,000 square feet, while only 2.1 spaces were used during peak times.149 Similarly, a recent 

survey of parking demand at neighborhood commercial lots in Iowa City, Iowa, during the 10-day 

period before Christmas found that the highest parking occupancy rate was 74 percent; over the count 

period and all sites, the average occupancy rate was 36 percent. The parking supply at the sites 

closely matched parking required in the city zoning ordinance. Based on these data, Iowa City 

reduced its parking requirements for neighborhood commercial lots by 33 percent.150 

Conventional design standards for parking areas may be revised to reduce the total parking area by 

several means. Parking standards can be developed for average parking needs rather than for those of 

a single peak day, such as holiday shopping periods. Research shows that a 10 to 15 percent reduction 

in total impervious cover can be achieved through either: (a) substituting parking stalls designed for 

compact cars for 30 percent of all spaces, (b) sharing parking arrangements (in which two businesses 

agree to share parking spaces, enabled by different peak times for parking demand), and (c) using the 

smallest allowable dimensions for regular parking stalls. If all three measures are used in 

combination, a 30 percent reduction in total impervious cover could be attained.151 

USE OF POROUS SURFACES 

Porous surfaces allow the soil to absorb precipitation, reducing runoff and replenishing the water 

table compared with impervious surfaces. Two types of paved surfaces allow soil to absorb water: 

� Surfaces that allow some precipitation to seep into the soil, such as lattice blocks, bricks set 

in sand, stones bonded with epoxy, perforated cast concrete slabs, steel grids, and wood slabs 

or logs (corduroy) — Surfaces such as lattice blocks and bricks set in sand can be used in 

place of concrete and asphalt for certain purposes such as sidewalks, walkways, driveways, 

parking areas, and low-volume roads. They usually do not lend themselves to projects 

requiring large pavement areas or heavy traffic volumes.152 

� A porous pavement, which involves a porous asphalt layer and underground stone reservoir.

153 

— Precipitation passes through the pavement and is collected and stored in stone voids beneath 

the top surface. The stored water then gradually infiltrates into the subsoil. To the extent that 

stormwater can be absorbed using porous pavement, stormwater collector systems and 

treatment plants can be minimized. Pervious pavement may be designed to retain rainfall 

completely with no runoff or to retain enough precipitation so that runoff flow is delayed and 

149 Willson, Richard W. “Sububan Parking Requirements: A Tacit Policy for Automobile Use and Sprawl.” Journal of the 

American Planning Association. Vol. 61, No. 1 Winter 1995. pp. 29-42. 

150 Shaw, John. “Minimum Parking Requirements in Midwestern Zoning Ordinances.” Paper presented at 76th Annual 

Meeting of the Transportation Research Board. January 1997. 

151 Schueler, Tom. Environmental Land Planning Series: Site Planning for Urban Stream Protection. p.168. 

152 Thelen, E., and L. F. Howe. Porous Pavement. The Franklin Institute Press, 1978. 

153 See: Schueler, Thomas, Peter Kumble, and Maureen Heraty, Anacostia Restoration Team, Metropolitan Washington 

Council of Governments. “A Current Assessment of Urban Best Management Practices: Techniques for Reducing NonPoint Source Pollution in the Coastal Zone.” Prepared for U.S. Environmental Protection Agency. March 1992. Field, R., H. 

Masters, and M. Singer. “Porous Pavement: Research; Development; and Demonstration,” Transportation Engineering 

Journal of the American Society of Civil Engineers. Volume 108, Number TE3, May 1982. pp. 244-258. 

51 
Our Built and Natural Environments 

peak demand on storm sewers is reduced.154 The use of porous pavement is highly constrained, 

requiring deep and permeable soils, limited traffic, and suitable adjacent land uses. 

Since porous pavements can replicate natural drainage patterns, they are useful for pavement in areas 

where the natural character of the land should be preserved and in areas where surfacing will be only 

temporary. If removed later, the land is returned to its natural state.155 

Over time, porous pavement sites have a high failure rate due to partial or total clogging of the facility 

that occurs when porous asphalt is clogged by sediment and oil. 156 In addition, porous surfaces should 

not be exposed to spillage of toxic chemicals and intermediates and so should not be used for 

highways and heavy-duty streets.157 

OPEN AND NATURAL DRAINAGE SYSTEMS 

In a traditional closed drainage system, stormwater is removed as fast as possible with curbs and 

gutters, catch basins, underground pipers, culverts, and/or lined channels. An open drainage system, 

in contrast, detains stormwater for short periods in swales and filter strips, and for longer periods in 

ponds and wetlands..158 Extended detention lowers peak discharge rates and gives physical, chemical, 

and biological processes time to work on pollutants. 

Figure 4-3 demonstrates how the inclusion of a detention pond in a new development can spread the 

discharge period out over time. After development, because of impervious surfaces, the total volume 

and peak flow of stormwater is much greater than before urbanization. A detention pond does not 

reduce the amount of runoff but spreads the flow to minimize erosion and the potential for flash 

flooding. Stormwater is detained in the basin and released at a constant rate. 

Open drainage systems can be more cost-effective than storm sewer systems. In addition, wetlands and 

wet ponds have amenity value that can be captured in the price of adjacent lands.159 

154 Thelen, E. and L. F. Howe. Porous Pavement. The Franklin Institute Press, 1978. 

155 U.S. Environmental Protection Agency , Office of Research and Monitoring. Investigation of Porous Pavements for 

Urban Runoff Control. March, 1972. 

156 Ways to improve longevity include routine vacuum sweeping, restriction of use to low-intensity parking areas, 

restrictions on access by heavy trucks, and restrictions of use of de-icing chemicals. Source: Schueler, Thomas, Peter 

Kumble, and Maureen Heraty, Anacostia Restoration Team, Metropolitan Washington Council of Governments. “A Current 

Assessment of Urban Best Management Practices: Techniques for Reducing Non-Point Source Pollution in the Coastal 

Zone.” Prepared for U.S. Environmental Protection Agency. March 1992. 

157 Thelen, E. and L. F. Howe. Porous Pavement. The Franklin Institute Press, 1978. 

158 Field, R., H. Masters, and M. Singer. “Porous Pavement: Research; Development; and Demonstration.” p.244-258. 

159 Ewing, R. Best Development Practices. Joint Center for Environmental and Urban Problems. Florida Atlantic 

University/Florida International University. May 1995. p.106. 

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Our Built and Natural Environments 

Figure 4-3: Impact of Urbanization and Mitigation on Runoff Rates 

FLOW RATE (DISCHARGE) 

TIME 

Source: Adapted from Harbor, J. “A Practical Method for Estimating the Impact of Land Use Change on Surface Runoff, 

Groundwater Recharge and Wetland Hydrology. Journal of the American Planning Association. 60: 1, Winter 1994. pp. 95-

108. 

LANDSCAPING 

Effective landscaping can retain soil moisture and conserve water usage in any developed area. Natural 

vegetation can reduce runoff, provide habitat for birds and other native wildlife, and reduce water 

consumption. Trees, shrubs, and other plants can help reduce runoff by absorbing precipitation. Locally 

adapted plants with large leaf surfaces and deep fibrous root systems absorb the most water.160 

Conversely, conversion of woodlands and undisturbed land affects storm runoff and groundwater 

discharge. An 11 to 100 percent loss of natural groundwater recharge, along with an 11- to 19-fold 

increase in stormwater, occurred at one site when its woodlands were converted to residential and 

commercial land use.161 

Landscaping in suburban areas can reduce water usage itself. All development requires water, but 

low-density development tends to require more water for lawns and gardens. Adequate water supply 

is an important issue in some regions of the country. Development in several states—most notably six FLOOD HEIGHT

of the eight mountain states—is leading to net water supply depletion.162 A water-conserving 

landscaping scheme applies mulches instead of turf to retain soil moisture. Vegetation species used in 

xeriscaping typically require minimal irrigation and less maintenance. Less maintenance implies 

fewer air emissions from lawn and garden equipment. 

160 Ewing, R. Best Development Practices. p.114. 

161 Harbor, J. M. “A Practical Method for Estimating the Impact of Land Use Change on Surface Runoff, Groundwater 

Recharge, and Wetland Hydrology.” Journal of the American Planning Association. Vol. 60, 1994. p. 95-108. 

162 U.S. Environmental Protection Agency, Office of Policy, Planning and Evaluation. Natural Resources for the 21st 

Century: An Evaluation of the Effects of Land Use on Environmental Quality. Washington, DC: June, 1989. p. 61. 

AFTER 

URBANIZATION 

AFTER URBANIZATION 

WITH A DETENTION BASIN 

BEFORE 

URBANIZATION 

Rainstorm 

53 
Our Built and Natural Environments 

Direct Environmental Effects of Reducing Impervious Surfaces and 

Improving Water Detention 

Reducing impervious surfaces and improving water detention can have positive effects on hydrology 

and water quality, plus vegetation and wildlife. 

Large parking lots are major contributors to runoff. A 1-acre parking lot produces 16 times as much 

runoff as does a 1-acre meadow.163 Parking lot runoff usually has a more detrimental effect on water 

quality than runoff from most other impervious cover because parking lots tend to be heavily polluted 

by leaks and drips from vehicles. Because smaller parking lots also decrease vehicle travel, decreasing 

the size of parking lots can reduce both runoff and associated pollutant loads.164 

Using porous surfaces is another strategy for reducing runoff and increasing groundwater recharge. In 

Figure 4-4, runoff from three types of pervious pavement are compared with an asphalt surface. All 

three pervious surfaces are shown to exhibit much lower coefficients of runoff than asphalt.165 

Figure 4-4: Comparison of Runoff from Various Surfaces 

Source: Adapted from Day, G.E. “Investigation of Concrete Grid Pavements.” In Downing, W.L. Proceedings of the 

National Conference on Urban Erosion and Sediment Control: Institutions and Technology. Great Lakes National Program 

Office, U.S. Environmental Protection Agency. Chicago. 1980, pp. 127-136. As presented in Ewing, R. Best Development 

Practices: Doing the Right Thing and Making Money at the Same Time. Prepared for the Florida Department of Community 

Affairs. May 1995. 

163 Chesapeake Bay Foundation. A Better Way to Grow. 1996. p.4. 

164 Higgins, Thomas, K.T. Analytics, Inc. “Parking Management and Traffic Mitigation in Six Cities: Implications for Local 

Policy.” Paper presented to Transportation Research Board, January 1989. U.S. Department of Transportation. Implementing 

Effective Travel Demand Management Measures: Inventory of Measures and Synthesis of Experience. September 1993. U.S. 

Environmental Protection Agency, Office of Air Quality Planning and Standards. Transportation Control Measure 

Information Documents. March 1992. 

165 Ewing, R. Best Development Practices. p.104. 

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Our Built and Natural Environments 

Porous pavement systems also have been shown to have high removal rates for sediment, nutrients, 

organic matter, and trace metals. The majority of the removal occurs as the result of the exfiltration of 

runoff into the subsoil and subsequent absorption or straining of pollutants within the subsoil. 166 

Open and natural drainage systems also can serve as effective pollutant removers. At one site near 

Orlando, Florida, an alternative stormwater treatment system of catch basins, wet ponds, and marsh 

areas was established. During larger storm events for 1993-94, pollutant levels were measured for 

stormwater flowing into, through, and out of the system. Table 4-3 shows average reductions in 

pollutant concentrations found with the natural drainage system:167 

Table 4-3: Pollutant Removal Efficiencies,

Lake Greenwood Stormwater Treatment System, Florida

Pollutant Removal Efficiency 

Total Solids 91% 

Total Lead 81% 

Total Zinc 59% 

Total Phosphorous 85% 

Total Nitrogen 64% 

Source: McCann, K. and L. Olson. Greenwood Urban Wetland Treatment Effectiveness. Florida Department of Environmental 

Protection. Tallahassee, FL, 1994, pp. 17 and 28. Cited in Ewing, Reid. Best Development Practices: Doing the Right Thing 

and Making Money at the Same Time. Prepared for the Florida Department of Community Affairs. May 1995. 

The natural drainage system proved effective in significantly reducing pollutant loads in runoff, with 

reductions of 59 to 91 percent in pollutant levels. 

When properly designed, open and natural drainage systems also can provide valuable habitat areas. 

The more natural the drainage system, the more valuable it will be for wildlife and water quality. 

Vegetated swales, stormwater ponds, marshes, and wetlands can serve as habitat for many creatures, 

including wetland birds and other waterfowl.168 

Porous pavement can be beneficial to native vegetation and wildlife as well, by allowing roadside 

vegetation to receive the water it needs to survive and grow. Porous pavement is particularly 

beneficial to water-starved street trees in urban areas.169 

166 Schueler, Thomas, Peter Kumble, and Maureen Heraty, Anacostia Restoration Team, Metropolitan Washington Council 

of Governments. “A Current Assessment of Urban Best Management Practices: Techniques for Reducing Non-Point Source 

Pollution in the Coastal Zone.” Prepared for U.S. Environmental Protection Agency. March 1992. 

167 Ewing, R. Best Development Practices. p.105. 

168 Franklin, T.M. “Use of Urban Stormwater Control Impoundments by Wetland Birds.” Wilson Bulletin. Vol. 97, 1985, pp. 

120-122; Adams, L.W. et al. “Design Considerations for Wildlife in Urban Stormwater Management.” Transactions of the 

51st North American Wildlife and Natural Resources Conference, 1986, pp. 249-259. 

169 U.S. Environmental Protection Agency , Office of Research and Monitoring. Investigation of Porous Pavements for 

Urban Runoff Control. March, 1972. 

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Our Built and Natural Environments 

4.3 DESIGN FOR SAFEGUARDING SENSITIVE AREAS 

Minimizing environmental impacts not only involves decisions about how much to build, but also 

where to build. Some locations lessen direct effects on habitat and water resources. Minimizing harmful 

environmental impacts may mean forestalling development in sensitive natural areas such as streams, 

wetlands, floodplains, steep slopes, mature forests, swamps, critical habitat areas, and shorelines. 

Environmentally sensitive areas have benefits beyond scenic value. Riparian buffers along rivers and 

streams, for example, are often critical habitats. One study indicates that nearly 70 percent of all 

vertebrate species use riparian areas in some significant way during their life cycles.170 

Techniques for Safeguarding Environmentally Sensitive Areas 

Different communities have different types of environmental assets that they want to protect, so 

techniques for safeguarding environmentally sensitive areas vary across communities. Three of the most 

commonly used techniques are wetlands protection, establishing riparian buffers, and use of greenbelts. 

WETLANDS PROTECTION 

Wetlands are an especially valuable resource. Reductions in wetland area as a percentage of an urban 

watershed are associated with increases in water pollutant loads, as shown in Figure 4-5. Each point on 

the graph represents a watershed, showing the pollutant load in relation to wetland area as a percentage 

of the watershed. Total nitrogen pollutant loads increase as the percentage of wetlands decreases. 

Figure 4-5: Increase In Pollutant Load with Loss of Wetland Area 

Source: Adapted from Oberts, G.L. “Impacts of Wetlands on Watershed Water Quality.” In B. Richardson (ed.), Selected 

Proceedings of the Midwest Conference on Wetland Values and Management. Freshwater Society, Navarre, MD: 1981, pp. 

213-226. Cited in Ewing, R. Best Development Practices: Doing the Right Thing and Making Money at the Same Time. 

Prepared for the Florida Department of Community Affairs. May 1995, p. 96. 

Although development of undisturbed areas inevitably will persist to some degree, well-designed 

communities and infrastructure can protect the most sensitive natural areas and minimize adverse 

170 Ewing, R. Best Development Practices. p.95. 

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Our Built and Natural Environments 

impacts. The best means of safeguarding sensitive areas is simple avoidance. Given the same site and 

development program, new development can stand back from wetlands or encroach into them. When 

encroachment on wetlands is unavoidable, cautious planning can minimize negative impacts. 

RIPARIAN BUFFERS 

Protection of sensitive areas can be improved by establishing buffers. Riparian buffers between land and 

water along shorelines, around wetlands, and adjacent to tidal guts, creeks, and streams may be applied 

systematically to create a “green corridor” along the banks of rivers or streams. Three primary aquatic 

areas qualify for buffers: the shoreline of a lake or estuary, a delineated wetland, or a stream channel. 

Buffer systems are typically inexpensive and simple to implement. Often a prescribed distance, usually 

between 25 and 100 feet from a water’s edge or mean high tide mark, is adopted by a local jurisdiction 

as the setback standard for a protected buffer area. Typically, construction, grading, dredging, and any 

other form of environmental disturbance are prohibited or severely restricted within the buffer area.171 

Usually only low-intensity uses such as recreation are allowed. Natural and cultural characteristics of 

the river basin determine correct buffer width. Relevant natural factors include seasonal water levels, the 

nature and extent of adjacent wetlands and floodplains, the steepness of adjacent topography, the nature 

of riparian vegetation, and the wildlife values of adjacent lands. Relevant cultural factors include 

riverfront parcel size and depth, traditional use patterns of the river and its adjacent lands, and existing 

development along the river. 

171 Mantell, M. A., S. F. Harper, and L. Propst. Creating Successful Communities: A Guidebook to Growth Management 

Strategies. The Conservation Foundation. Island Press, 1990. 

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Our Built and Natural Environments 

Multiple Species Conservation Plan, County of San Diego, California: A Case Study 

On March 18, 1998, the San Diego City Council approved a Multiple Species Conservation Plan (MSCP), a subregional 

development and conservation plan under California’s Natural Communities Conservation Program. The MSCP covers 

approximately 900 square miles in San Diego County and includes goals and criteria to conserve the habitats of more 

than 85 species in the area. 

conserved. To the greatest extent possible, the plan tries to use public lands to satisfy required conservation levels. 

Before the development of the MSCP, non-federal property owners wishing to build on or alter their land in ways that 

might result in the incidental “take,” or harm, of a listed species had to obtain an Incidental Take Permit (ITP) from the 

U.S. Fish and Wildlife Service. In order to obtain an ITP, the property owner had to develop a Habitat Conservation Plan 

(HCP) outlining ways in which the negative effects on the listed species would be offset. 

Because the new MSCP functions as a Habitat Conservation Plan for the region, it allows issuance of all ITPs for covered 

species. Political jurisdictions develop their own plans consistent with the MSCP, then obtain ITPs for the plan area. The 

jurisdictional possession of ITPs for the entire sub-area eliminates the need for private citizens to develop HCPs for activities 

that are already taken into consideration under the MSCP and, in most cases, the Environmental Impact Report for the 

entire MSCP will satisfy the requirements for project-specific Environmental Impact Reports. 

A regional conservation plan such as the MSCP is advantageous to all involved: Critical species and habitats are 

protected, and private landowners benefit from a simplified development process. 

Sources: Merrick, J. (1998). The San Diego Multiple Species Conservation Plan. In Improving Integrated Natural Resource Planning: Habitat 

Conservation Plans , [Web page]. National Center for Environmental Decision-making Research. Available: http://www.ncedr.org/casestudies/hcp/ 

sandiego.htm [1998, October 14].; County of San Diego, Department of Land Use and Planning. 

landuse/planning/mscp/ ; US Fish and Wildlife Service [Web page] http://endangered.fws.gov/hcp/index.html 

The regional plan designates those areas that may be developed and those that must be 

http://www.co.san-diego.ca.us/cnty/cntydepts/ 

GREENBELTS 

Greenbelts are areas of preserved open space, or areas of significantly reduced development, designed 

as buffers to protect areas of land or water resources from development impacts. The preservation of 

patches of high-quality habitat, connected by wildlife corridors, can preserve wildlife and ecosystems 

even in areas with significant adjoining development. Wildlife corridors can serve as “land bridges” 

between “habitat islands” and as dwelling habitats in their own right.172 

Greenbelts also may be used to preserve agricultural land, recreational areas, and natural resources in 

close proximity to a town or city. Greenbelts are achieved through zoning, creative development 

planning, or land acquisition, or a combination of these approaches. Since greenbelts are usually 

considered amenities, they have the potential to increase property values of adjacent land. 

Direct Environmental Effects of Protecting Environmentally 

Sensitive Areas 

Buffers, greenbelts, and other preservation tools have numerous environmental benefits. They can 

protect watersheds, guard animal habitats, and preserve existing vegetation. For example, buffers may 

reduce watershed imperviousness (by preventing development on land along the streambed), provide 

effective flood insurance (by keeping structures away from floodplains), protect stream temperatures 

172 Ewing, R. Best Development Practices. p. 95. 

58 
Our Built and Natural Environments 

(with their canopies), stabilize streambanks, and protect against streambed erosion (since trees and 

vegetation are more resistant to bank erosion than grass is). Buffers can protect sensitive areas from 

physical encroachment and scenic degradation, and preserve wildlife habitat for terrestrial and aquatic 

animals.173 

Since buffers are able to intercept and absorb runoff before it reaches the water resource, they have 

the potential to improve water quality by reducing runoff volume. Buffers act as a sort of natural 

scrubber, preventing excess nutrients and pollutants from entering waterways and wetland areas.174 A 

buffer’s ability to reduce pollutant loads depends on runoff velocity and site conditions such as soil, 

vegetation, buffer size, and slope.175 

4.4 MIXED LAND USES 

Standard zoning separates uses into distinct zones for residential, commercial, or industrial uses. In 

contrast, mixed-use development locates land uses with complementary functions close together. 

Complementary uses may include housing, shopping, offices, restaurants, and movie theaters—any 

destinations that people travel to on a regular basis. 

Techniques for/ Types of Mixed Use Development 

Mixed-use development can occur on a number of levels: site-specific, neighborhood, or subregional. 

On a site-specific basis, individual buildings or complexes can be designed to incorporate a variety of 

uses. For example, a single building might include apartments, offices, and retail. At the 

neighborhood level, mixed-use development refers to the arrangement of different uses across several 

blocks or acres of land so that they are not physically isolated from one another. Finally, at the 

subregional level, mixed-use often aims to balance jobs and housing so that people have the 

opportunity to live closer to their places of employment. 

Direct Environmental Effects of Mixed-Use Development: Habitat 

and Water Quality 

Mixing land uses can have direct effects on habitat loss and runoff since mixed-use developments 

have the potential to use surface parking lots and transportation infrastructure more efficiently, 

requiring less pavement. When office buildings also contain retail shops and restaurants, the 

infrastructure that supports the building—the roads and parking lots—is in use for more of the day. 

Office traffic arrives during rush hour and uses the parking lot during the day. That parking can be 

used in the evenings for restaurant and theater traffic. The alternative is two sets of roads and parking 

lots—one set serving office buildings and another that serves retail and entertainment areas. 

173 Schueler, Tom. Environmental Land Planning Series: Site Planning for Urban Stream Protection. Center for Watershed 

Protection. Publication No. 95708. Washington, DC: Metropolitan Washington Council of Governments, December 1995. p. 90. 

174 Center for Watershed Protection. Blueprint to Protect Coastal Water Quality. Land Ethics, Inc. Section 4-3. 

175 Jones, H., L..M. Maureen Heraty, and B. Jordan. Environmental Land Planning Series: Riparian Buffer Strategies for 

Urban Watersheds. Urban Watershed Planning Section. Washington, DC: Department of Environmental Programs. 

Metropolitan Washington Council of Governments. December, 1995. p.3. 

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Indirect Environmental Effects of Mixed-Use Development: 

Reductions in Vehicle Travel 

At any level—building, neighborhood, or regional—the travel-related environmental effect of mixing 

uses is similar. By encouraging people to walk, bike, and use transit rather than drive, mixed-use 

development patterns reduce VMT. Reductions in VMT lead to decreases in automobile emissions, 

thereby improving regional air quality. 

Mixing land uses can reduce VMT in several ways: 

� Trip lengths – By locating activities closer together, a mix of land uses can minimize travel 

distances and improve access to employment, services, or recreational opportunities. 

� Mode choice – Locating activities closer together allows trips to be made by walking and 

bicycling rather than by driving motor vehicles and increases the opportunity for non-auto 

trip chaining. Individuals can drive to one destination, for example, and then walk to others 

once they have parked their car. 

� Vehicle ownership – Access to employment and shopping by walking and bicycing reduces the 

need to own a motor vehicle for personal mobility. 

Land use mixing may influence travel demand in a number of ways, but its greatest impact is thought 

to be on mode choice.176 Mixed land use areas influence mode choice by enhancing the relative 

convenience of non-auto modes. For example, mixing employment and residential uses may reduce 

commute distances, thus making walking, bicycling, and transit more competitive with auto travel in 

terms of time. Alternatively, mixing employment or residential centers with retail and commercial 

establishments may increase the attractiveness of alternatives to single occupant vehicle driving by 

reducing the need for a vehicle to accomplish mid-day tasks or errands typically completed on the 

way to or from work. 

Evidence of the effects of land use mixing on travel patterns is organized around three geographic 

levels of mixing: 

� Mix (or balance) of jobs and housing within subregions 

� Mix of uses within neighborhoods or communities 

� Mix of uses at employment centers. 

176 Cervero, R. “Mixed Land Uses and Commuting: Evidence from the American Housing Survey.” Transportation 

Research. Volume 30, Number 5. 1996. p. 363. 

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MIXING USES WITHIN RESIDENTIAL NEIGHBORHOODS 

Several empirical studies have found that places with a mix of land uses and retail services located 

within walking distance of residences have higher levels of pedestrian travel and fewer vehicle trips 

compared with solely residential 

neighborhoods. For example, a comparison of 

travel diaries of residents in three Seattle 

mixed-use neighborhoods with those of 

residents of suburban, single-use 

neighborhoods in surrounding King County 

found that the average trip length was 

significantly lower in the mixed-use 

neighborhoods. In most cases, the average 

distance per trip driven by residents of mixeduse neighborhoods was half that of those 

living in the single-use neighborhood. For 

work trips, nonmotorized mode share was 

significantly higher in mixed-use 

communities—12.2 percent of trips in mixeduse Seattle neighborhoods compared with 3.9 

Figure 4-7: Mode Share for Work Trips 

in Seattle Mixed-Use Communities 

and King County 

12% 

10 Mixed-use 

Seattle Communities

8 

King County 

6 

4 

2 

0 

Pedestrian Bicycle 

percent of trips in the comparison communities, as Figure 4-7 shows.177 The higher numbers of auto 

trips lead to increased emissions. In the most compact mixed-use communities, the share of weekday 

trips by walking was as high as 18.1 percent. 

These findings reflect the fact that in mixed-use communities, more destinations are located near each 

other. In the survey, respondents of mixed-use Seattle communities reported making more than three 

times as many weekday trips to destinations within a mile of home as King County residents did. The 

results are summarized in Table 4-4. 

Table 4-4: Percent of Weekday Trip Stops by Distance from Household 

Location Distance of Stops from Household Location 

1.0 miles 1.5 miles 2.0 miles 

Mixed Use 

King County 

17.4% 

4.5% 

25.4% 

11.6% 

38.7% 

18.2% 

An analysis of American Housing Survey data demonstrates that locating retail activities within a 

short walk of homes is associated with increased levels of commuting by mass transit, walking, and 

bicycling. Households with grocery stores and other consumer services within 300 feet—generally a 

one- or two-block distance—of one’s residence were less likely to commute by automobile, 

177 Rutherford, G.S., E. McCormack, and M. Wilkinson. “Travel Impacts of Urban Form: Implications from an Analysis of 

Two Seattle Area Travel Diaries.” TMIP Conference on Urban Design, Telecommuting, and Travel Behavior. October 27-

30, 1996. The counter to this argument is that people self-select into areas that support their preferred behaviors. Those who 

prefer to avoid driving choose homes where they do not have to drive. Those who like driving choose suburbs. The 

implication that the latter group would drive at the rates they do now, even if for some reason they lived in dense, mixeduse, transit-served communities. 

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controlling for such other factors as land density, number of automobiles per household, and 

adequacy of public transit services.178 

Travel model improvements undertaken in the Portland region found that one measure of land-use 

mix—the number of retail jobs in a transportation analysis zone—was statistically significant in 

explaining residential automobile ownership and the choice between motorized and nonmotorized 

modes. The addition of a measure of employment density into the auto-ownership and mode choice 

models improved the fit of the models to actual survey data.179 

Locating parks and recreational facilities, small shops or grocery stores, schools, and religious 

institutions within residential neighborhoods also has been found to reduce VMT (and, therefore, 

emissions), even in places that are automobile-oriented. A comparison of work and nonwork travel 

among residents of six communities in Palm Beach County, Florida, found that the presence of 

shopping, recreation, and school facilities within communities can lower vehicle hours traveled per 

capita significantly even when transit is not widely available.180 This study suggests that locating 

facilities and services in communities reduces vehicle travel even in locations where the automobile 

for all intents is the only primary mode of transportation. Increased accessibility within the region 

also was associated with reduced vehicle hours of travel. Given that the presence of additional uses in 

a neighborhood cannot directly increase speeds, the decrease in VHT must be a function of shorter 

trips, which would, in turn, lead to decreased emissions.181 

MIXING USES AT EMPLOYMENT AND COMMERCIAL CENTERS 

Just as improving the mix of uses in residential communities has been shown to reduce vehicle travel, 

mixing uses within commercial and employment areas also has been identified as a way to increase 

transit share and reduce vehicle use. Locating stores close to workplaces makes stores accessible by 

foot during the workday and allows efficient linking of trips even if the stores are distant from the 

employees’ homes. As a result, employees may substitute mid-day pedestrian errands for after-work, 

vehicle-based trips. The presence of restaurants, shops, and consumer services at or near employment 

sites encourages transit use and ridesharing since many workers no longer need to have a car 

available for mid-day or after-work trips. 

Pedestrian connections are an important component in mixing land uses. Many larger commercial 

developments contain both office space and commercial development but still require a car to get 

around. These developments do not function as a mixed-use area, because most of the commercial 

178 Cervero, R. “Mixed Land Uses and Commuting: Evidence from the American Housing Survey.” Transportation 

Research. Vol. 30, No. 5, 1996. pp. 361-377. Cervero controlled for such factors as residential densities and vehicle 

ownership levels. His analysis also found that having retail shops beyond 300 feet yet within 1 mile of residences tended to 

encourage auto-commuting, perhaps because of the ability to link work and shop trips efficiently by car. In any case, the 

mixed-use neighborhoods were associated with shorter commuting distances. 

179 Cambridge Systematics et al. Making the Land Use Transportation Air Quality Connection: Model Modifications. Vol. 4. 

Prepared for Thousand Friends of Oregon. May 1996. 

180 VMT could not be easily derived from the travel survey data files. However, differences in VMT are almost certainly 

even more pronounced than differences in VHT are since vehicle travel speeds are highest in areas of high VHT. Ewing, R., 

P. Haliyur, and G. W. Page. “Getting Around a Traditional City, a Suburban Planned Unit Development, and Everything in 

Between.” Transportation Research Record 1466. 1994. pp. 53-62. 

181 Again, lower VHT is sometimes the product of higher vehicle speeds, which, if high enough, can increase emissions. But 

the decrease in VHT in this study must be from decreased trip lengths. 

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development is in large malls and shopping centers separated from office developments by wide 

highways lacking sidewalks. 

Several studies have demonstrated that developing a mix of uses at employment and commercial 

centers reduces the portion of trips made by personal vehicles and increases rates of transit use. In 

particular, several studies of suburban activity centers have found that office developments that have a 

mix of uses, even those in auto-oriented locations, have lower vehicle trip rates. A study of 57 large 

U.S. office developments found that each 10 percent increase in floor space devoted to retailcommercial uses was associated with a 3 percent increase in the share of transit and ridesharing 

commutes.182 A follow-up survey of commuting to six large suburban activity centers, including 

Perimeter Center north of Atlanta and Tyson’s Corner in the Washington, DC, area, found the 

existence of a retail component within a suburban office building was associated with an 8 percent 

reduction in vehicle-trip rates per employee. Buildings with mixed uses averaged 3 percent more 

commutes by transit than buildings containing only offices.183 Enabling workers to switch from 

automobile to transit commutes decreases VMT and associated emissions. 

A study of suburban centers in southern California suggested that at sites with travel demand 

management (TDM) incentives, areas with a substantially mixed land use had more than double the 

transit mode share of other site—6.4 percent share in centers with a substantial mix compared with 

2.9 percent in those with a limited mix. 184 In locations without demand management, the presence of 

mixed land uses increased commute trips by transit from 3.6 to 5.5 percent. People also bicycled and 

walked more in areas with a substantial mix of uses.185 

The same study also found that having convenience-oriented services, such as restaurants, banks, 

child care centers, dry cleaners, drugstores, and post offices, located near work sites significantly 

increased the portion of commuters using transit. Among sites using financial incentives, locations 

with convenience services had more than double the transit share of sites with limited services 

nearby—7.1 percent in sites with convenience services compared with 3.4 percent in areas without 

convenience services. Employment sites with a substantial level of nearby convenience services also 

had higher rates of bicycling and walking than other sites did. 

In a study of mixed-use sites in Colorado, the Colorado/Wyoming Section Technical Committee of the 

Institute of Transportation Engineers (ITE) found that average trip rates for individual shops in retail 

plazas and other mixed commercial settings were below the mean rates for freestanding stores published 

in ITE’s Trip Generation (1991) manual.186 The committee recommended adjusting trip rates downward 

by 2.5 percent to reflect the higher likelihood of linked walk trips, instead of separate vehicle trips 

182 Cervero, R. “Land Use Mixing and Suburban Mobility.” Transportation Quarterly. Volume 42, 1988. p. 429-446. 

183 Cervero, R. “Land Uses and Travel at Suburban Activity Centers.” Transportation Quarterly. Volume 45, 1988. p. 479-491. 

184 U.S. Department of Transportation, Travel Model Improvement Program. “The Effects of Land Use and Travel Demand 

Management Strategies on Commuting Behavior.” Prepared by Cambridge Systematics, November, 1994. 

185 The study also found that in locations with a substantial mix of uses, transit use increases with the introduction of 

financial incentives for transportation demand management. On the other hand, in areas with a limited land use mix, 

transportation demand management incentives appeared to shift trips from transit to rideshare, resulting in a lower transit 

mode share than if no incentives were offered. 

186 Colorado/Wyoming Section Technical Committee, Institute of Transportation Engineers. “Trip Generation for Mixed 

Use Developments.” ITE Journal, Volume 57, 1987. p. 27-29. 

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between establishments in mixed-use settings. Replacing short car trips with walk trips would decrease 

the total number of auto trips, and thus the emissions levels, generated at these settings. 

SUBREGIONAL BALANCE OF JOBS AND HOUSING 

On a larger scale, promoting a “balance” of employment and housing at the subregional level has 

been identified as a way to reduce commute distances. The idea is that if people could live closer to 

their employment sites, they would do so to reduce commute time and costs. Balanced communities 

offer affordable, high-quality housing that is close enough to employment sites that residents can 

avoid commuting long distances on congested highways to get to work. 

Measuring the jobs-housing balance is somewhat difficult since there is no nonarbitrary geographic 

scale at which to assess the match or mismatch. Regions as a whole are by definition “balanced,” 

while individual blocks or neighborhoods almost never are. Considerable debate continues over the 

effectiveness of the jobs and housing balance as a measure for reducing congestion or improving air 

quality. On the one hand, increasing the availability of housing near employment centers provides 

individuals with the option to live closer to work. On the other hand, some researchers have found 

little evidence suggesting that the balance or mismatch of jobs and housing alone has had a significant 

effect on commuting patterns.187 

Those promoting a jobs-housing balance suggest that increasing the housing opportunities near major 

employment centers allows workers to locate closer to their jobs and may reduce traffic. A number of 

studies suggest that this effect may occur. For example, an analysis in Toronto suggested that the 

effect of substantial new office construction between 1975 and 1988 on peak-hour work trips was 

offset by increased housing occupied by people working in the central city. 188 Another study based on 

1980-1990 U.S. journey-to-work data showed that “balanced” cities averaged 12 to 15 percent less 

work-trip VMT per employed residents than did “job-surplus” cities.189 

A simulation of regional development patterns in the Washington, DC area found that an alternative 

that promoted a closer balance between employment and housing growth within the region resulted in 

greater transit use and shorter average trip lengths due to greater proximity of housing to jobs. Vehicle 

trips per household were reduced by 5.0 percent. When combined with a 4.5 percent reduction in 

average trip length, the balanced alternative resulted in 9.2 percent less VMT per household.190 

Reducing vehicle trips and VMT can lead to similar reductions in vehicle emissions. 

A number of studies have found that accessibility to jobs is one of a number of determinants of 

vehicle ownership. An analysis of travel in the San Francisco region found that controlling for other 

187 From the analysis of the location of jobs and housing in a number of regions, Giuliano, G. and K. Small (“Is the Journey 

to Work Explained by Urban Structure?” University of California Transportation Center. Working Paper, No. 107, June 

1992) and Genevieve Giuliano and Kenneth Small (“Subcenters in the Los Angels Region.” Journal of Regional Science 

and Urban Economics. Vol. 21, pp. 163-182) concluded that a jobs-housing balance has a statistically significant but not 

very large influence on actual commuting times and therefore that policies attempting to alter the jobs-housing balance alone 

are likely to have little impact on commute patterns. 

188 Nowlan and Stewart. “Downtown Population Growth and Commuting Trips.” Journal of the American Planning 

Association. Vol. 57(2), 1991. pp. 165-182. 

189 Cervero, Robert. “Jobs-Housing Revisited.” Journal of American Planning Association. 1996. 

190 Metropolitan Washington Council of Governments. “Transportation Demand Impacts of Alternative Land Use 

Scenarios.” May 31, 1991. 

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land use and household factors, a doubling in accessibility results in a 7.5 percent decrease in the 

number of vehicles owned.191 Efforts to incorporate land use factors into the vehicle ownership model 

of Portland, Oregon, also suggest that the number of retail jobs in a transportation analysis zone is 

statistically significant in helping to explain residential automobile ownership. 192 As the number of 

retail jobs in a zone increases, vehicle ownership per household decreases, holding other factors like 

household income and size constant. The possibility of using transit to get to work is also found to 

reduce vehicle ownership rates, while residential density does not prove to be important on its own. 

By definition, “accessible” areas require shorter travel distances than “nonaccessible” areas. 

However, a strategy for improving the balance of housing and jobs reduces commute trip distances 

only if jobs-housing imbalances are a major source of long commute lengths and if people make 

location decisions at least in part on the basis of commute distances. Some researchers argue that a 

jobs-housing imbalance has little to do with long commutes.193 There are many reasons why the 

journey to work might play only a limited role in residential location decisions. Rapid job turnover 

and high moving costs may cause households to seek accessibility to an array of future jobs, twoworker households may be unable to find jobs close together, and the importance of nonwork trips 

may reduce the importance of travel time to work. In addition, other neighborhood characteristics, 

such as crime, school quality, and taxes, might overshadow time and travel costs. 

The degree to which different factors affect location decisions is widely debated and may vary in 

different circumstances. An analysis of travel and housing location in the Minneapolis region found 

that if one is considering communities representing commute differences of only 5 minutes, local 

service attributes and housing characteristics dominate residential choice. However, within an entire 

60-minute commute shed, commute time was found to be overwhelmingly the best explanatory 

variable of household location choice.194 The analysis suggested that jobs-housing balances and 

imbalances were particularly significant influences on residential location decisions for low- to 

moderate-income single worker households. A recent poll of San Francisco Bay area residents found 

that given a choice between houses that cost the same, 60 percent would opt for a smaller home closer 

to where they work rather than a larger one farther away.195 

Although various factors might prevent households from locating close to their places of work, a jobshousing balance provides more opportunities for living closer to work than a land-use pattern in which 

employment and residential areas are widely separated. As a result, communities that provide that 

balance are likely to require shorter commutes, which generate fewer VMT and associated emissions. 

191 Kockelman, K. “Travel Behavior as a Function of Accessibility, Land Use Mixing, and Land Use Balance: Evidence 

from the San Francisco Bay Area.” Submission to the 76th Annual Meeting of the Transportation Research Board. January 

1997. 

192 Cambridge Systematics et al. Making the Land Use Transportation Air Quality Connection: Model Modifications. Vol. 4. 

Prepared for Thousand Friends of Oregon. May 1996. 

193 See Giuliano, G. “Is Jobs-Housing Balancing a Transportation Issue?” Transportation Research Record 1305, 1991, pp. 

305-312; Giuliano, G. and K. Small. “Is the Journey to Work Explained by Urban Spatial Structure?” Urban Studies 30:9, 

1993. p. 1485-1500. 

194 Levine, J. (University of Michigan) “Land Use Solutions to Transportation Problems? Rethinking Accessibility and JobsHousing Balancing.” Paper submitted to the 75th Annual Meeting of the Transportation Research Board. 1996. 

195 San Francisco Chronicle. “Long Haul to American Dream.” March 18, 1997. p. A-1. 

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4.5 TRANSIT ACCESS 

Transit systems that are well designed and operated can reduce vehicle travel, resulting in reduced 

criteria pollutant and greenhouse gas emissions. A transit bus carrying 40 passengers requires only 

about one-sixth the energy consumption it takes to transport each person in a private vehicle.196 

Transit also helps to reduce traffic congestion. One full 40-foot bus is equivalent to a line of moving 

automobiles stretching six city blocks, and one full six-car heavy rail train is equivalent to a line of 

moving automobiles stretching 95 city blocks (assuming traffic operates at 25 mph).197 Transit 

provides mobility to individuals of all ages, income levels, and abilities. With an aging population and 

increased attention being paid to linking low-income families to jobs, improved accessibility and 

mobility are particularly important. 

Techniques for Improving Transit Access 

Shifting location of employment and housing centers within a region can render once-useful transit 

service obsolete. These changes have encouraged many cities across the country to rethink and 

improve transit access. Two general ways in which transit access can be improved are by expanding 

transit supply through construction or service improvements, and focusing development around 

existing transit (transit-oriented development). 

EXPANDING TRANSIT SUPPLY THROUGH CONSTRUCTION OR SERVICE 

IMPROVEMENTS 

Cities can expand transit supply through the construction of new transit systems or the expansion of 

existing facilities. Constructing new systems or expanding existing ones are capital projects with a 

significant price tag. Cities across the country, however, are determining that an increase in transit 

access is worth the investment. 

Salt Lake City, for example, is currently constructing new light rail and commuter rail systems. Other 

cities have chosen to expand or alter the mix of transit options provided to residents. The Chicago 

metropolitan area has proposed extending commuter rail lines to serve the suburbs on the far 

northern, southern, and western edges of the city. This expansion of service would remove cars and 

the associated emissions from the roadways and reduce congestion on the area’s roads. 

Cities can improve transit access by making existing transit services more responsive to the needs of 

the population they serve. Extending service hours, for example, could capture ridership from 

individuals who need to travel later or earlier than the existing service allows. Regions have found 

that reducing wait times at stations (by adding trains or buses to existing lines) and improving transitrelated signage (by making bus stops more obvious and posting schedules and route maps at each 

stop) can encourage transit use. 

196 Average U.S. energy consumption per vehicle mile for an automobile = 5,748 Btu; for a transit bus = 39,081 Btu. US 

Department of Energy. Transportation Energy Data Book. 15th Edition, Table 2.15, p. 2-25. 

197 A full 40-foot bus holds about 70 people including standees. At an estimated national average of 1.2 persons per 

automobile, one bus is equivalent to 58 automobiles. A full heavy rail car accommodates about 180 people; a train of six 

cars carries 1,080 people, replacing 900 automobiles. Ten city blocks per mile, an average auto length of 16 feet, and a onecar-length-per-each-10-mile-per-hour following length is assumed. (American Public Transit Association. 1994-1995 

Transit Fact Book .) 

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FOCUSING DEVELOPMENT AROUND EXISTING TRANSIT STATIONS (TRANSITORIENTED D EVELOPMENT) 

For some areas, the most cost-effective way to improve transit access may be to increase development 

around existing stations, rather than extend transit service. This type of development is typically 

referred to as transit-oriented development (TOD). The premise of transit-oriented development is 

that locating residential development and employment near transit stations increases the market for 

transit services and yields greater ridership than is achieved at stations (or bus stops) surrounded by 

low-density development. 

Figure 4-8 shows three different development patterns with varying levels of transit access for 640 

households in a 1 square mile zone. The first development alternative features a perfectly uniform 

density of households to 1-acre parcels. The second alternative increases the density of households to 

four per acre and locates them on the east and west sides of the zone, as if oriented to north-south 

arterial streets. The last alternative also has density of four households per acre, but the households 

are oriented to the side of the zone where transit service is available. Although the three alternatives 

have equal overall density, they feature very different levels of transit accessibility. 198 

Figure 4-8: Transit Access Distance and Local Land Use Organization 

Diagram excerpted from: Eash, R. “Incorporating Urban Design Variables in Metropolitan Planning Organizations’ Travel 

Demand Models.” Prepared for Conference on Urban Design, Telecommuting, and Travel Behavior, Travel Model 

Improvement Program. October 1996. 

As discussed below, results from a number of empirical studies and regional simulations indicate that 

increasing transit accessibility through transit-oriented development has the potential to increase the 

share of trips made by transit. 

198 Locating higher density development near transit does not necessarily mean increasing an area’s average density. Rather, 

it could simply rearrange land uses and densities so that more people are close to the transit lines. 

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Indirect Environmental Impacts from Improving Transit Access 

Effective transit systems require supportive land use patterns. Locating high-density commercial and 

residential development around transit stations is expected to reduce vehicle travel for two reasons: 

� Mode Choice – Providing increased transit access increases the likelihood that the average trip 

will be made by transit, irrespective of vehicle ownership rates. 

� Vehicle Ownership – Transit provides a potential substitute for vehicle travel and is expected 

to reduce the need to own personal vehicles for mobility. 

EVIDENCE FROM REGIONAL SIMULATIONS 

A number of metropolitan areas including Orange County (CA), Los Angeles, San Francisco, Boston, 

Seattle, Baltimore, Washington (DC), Dallas, and Denver have conducted simulations using regional 

travel demand forecasting models to identify the types of urban form that best support transit use and 

reduce dependence on vehicle travel. These analyses generally find that compact regions with a 

limited number of subregional centers linked by transit can support transit ridership and reduce VMT 

compared with other regional development patterns.199 

An analysis of alternative land-use/transportation plans in the Chicago region, using a sketchplanning network of the northeastern Illinois region, indicated that compact patterns of residential and 

employment development would reduce vehicle travel. In particular, focusing development on 

regional corridors was projected to yield an increase in transit use and a decrease in average trip 

length due to an improvement in the jobs-housing balance. Dense regional centers were projected to 

produce less VMT due to increased transit use, and produce even less VMT when transit service 

improvements are added. In addition, this development pattern was projected to reduce auto trip 

lengths due to the more compact nature of the region. 200 

Montgomery County, Maryland, performed a similar analysis, using computerized transportation 

models to examine alternative long-range development scenarios and their impact on VMT and traffic 

congestion. The study found that by clustering most new development near an expanded rail and busway system, making improvements to pedestrian and bicycle conditions, and equalizing commuter 

subsidies, the county could accommodate a doubling of households and employment over 30 years 

with acceptable levels of traffic congestion. The strategy could result in countywide VMT and traffic 

congestion levels comparable to those of the existing 2010 forecast, while accommodating 62 percent 

more houses and 29 percent more jobs in the county than the current forecast does.201 

199 Parsons Brinckerhoff Quade and Douglass, Inc. “Part I: Transit, Urban Form, and the Built Environment: A Summary of 

Knowledge.” Transit Cooperative Research Program Report 16. Transit and Urban Form. Washington, DC: Transportation 

Research Board, 1996. p.5. 

200 The sketch/aggregated planning approach did not include a trip-generation step, so auto ownership and auto occupancy 

were fixed parameters. These aspects suggest that the magnitude of effects may have been underestimated (Lupa, Mary, et. 

al., “Transportation Sketch Planning with Land Use Inputs.” Transportation Research Record 1499. 1995. p. 83-94.) 

201 Replogle, Michael. “Land Use/Transportation Scenario Testing: A Tool for the 1990s.” Silver Spring, MD: Montgomery 

County Planning Department. 1993. (Prepared for Presentation at Transportation Research Board 1993 Annual Meeting.) 

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EVIDENCE FROM EMPIRICAL STUDIES 

A number of empirical studies also show a relationship between transit accessibility and factors that 

affect air quality, such as mode choice or car ownership. Several studies, including some international 

case studies, describe how regional transit systems have promoted development around stations, 

helping to improve accessibility and reduce regional congestion. 202 Locating high-density commercial 

and residential development around transit stations improves accessibility to transit since more 

households are within walking distance of transit facilities. 

Several studies show that increased density around transit stations is associated with increased transit 

use.203 Light rail stations in higher density residential settings have higher ridership than those in lower 

density settings, holding constant other factors that influence ridership such as the distance between 

stations, the availability of feeder bus service, and the distance of the station from the Central Business 

District.204 Increasing densities depend on adequate transit service to provide access without excessive 

congestion. San Francisco’s higher density and better transit service shorten trip lengths sufficiently to 

allow 1 mile on transit to replace 8 miles of driving compared with trips in suburban Danville-San 

Ramon.205 Compared with households in Danville-San Ramon, the average household in Nob Hill spent 

nearly $14,000 less on autos, the average resident burnt 339 gallons less gasoline, and emitted 14 kg 

less hydrocarbons, 12 kg less nitrogen oxides, and 98 kg less CO per year. 

Increasing accessibility of housing to transit increases transit mode shares. For the Washington, DC 

Metrorail (heavy rail) system, for example, a 1987 survey of residential buildings within one-third mile 

of a suburban station found rail transit capture rates for work trips in the range of 18 to 63 percent, 

which is significantly higher than the regional work-trip average. Ridership was highest for projects 

closest to Metrorail stations and among station-area residents headed to central Washington, DC.206 

A more recent study of travel to regional shopping centers in California found that travel mode shares 

were best explained by the amount and regional coverage of public transit services and the density and 

proximity of the surrounding land uses. One shopping center in a suburban, low-density area with 

limited transit service had a 95 percent auto share, while the shopping center in an urban, high-density 

202 Parsons Brinckerhoff Quade and Douglass, Inc. “Part IV: Public Policy and Transit-Oriented Development: Six 

International Case Studies.” Transit Cooperative Research Program Report 16. Transit and Urban Form. Washington, DC: 

Transportation Research Board, 1996. pp. 37-70. 

203 High densities around transit stations may reflect high land values associated with desirable, accessible locations. 

204 For example, a study of 19 light-rail lines in 11 regions found that a 10 percent increase in residential density yields on 

average 5.9 percent more riders per station. Further, a 10 percent increase in CBD employment density increases light rail 

boardings at stations outside the CBD by about 4 percent holding other factors constant (Parsons Brinckerhoff Quade & 

Douglas, Inc. “Transit and Urban Form.” Transit Cooperative Research Program. Washington, DC: National Academy 

Press, 1996). The most useful studies for examining environmental implications focus on mode shares, since transit 

ridership does not provide information on vehicle travel reduction. It is possible that dense areas yield increased transit 

ridership but also yield increased driving because more trips are made than in low-density areas. 

205 The savings result from the increased convenience of higher density mixed-use areas. Holtzclaw, J. “Explaining Urban 

Density and Transit Impacts on Auto Use.” Presented to State of California Energy Resources Conservation and 

Development Commission. January 15, 1991. 

206 Station location also was found to play a role in transit ridership. Downtown offices averaged transit work trip modal 

shares of about 50 percent, compared with less than 20 percent for suburban office projects near rail stations. These 

differences may reflect differences in parking availability and price, pedestrian friendliness, or transit time compared to 

driving time. JHK and Associates. Development-Related Ridership Survey I. Washington, DC: Washington Metropolitan 

Area Transit Authority, 1987. 

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area with high transit service had the highest use of transit and walking—roughly 60 percent for the two 

modes combined. Modal share trends were consistent within demographic categories (e.g., age, income, 

household size), suggesting that site characteristics were an important factor in mode share.207 

A study of transit-oriented development in California found that developments near transit have 

significantly higher shares of trips made by transit than the regional average.208 For the 27 surveyed 

residential developments near transit, rail was used for 19 percent of work trips. For developments 

near Bay Area Rapid Transit (BART), rail was used for 33 percent of work trips, significantly higher 

than the 5 percent 1990 average for the three BART-served counties. In each Bay Area city served by 

BART, residents living near rail stations are about five times as likely to commute by rail as is the 

average resident-worker in the same city. Rail’s mode share falls linearly with distance from the 

station for the surveyed housing projects—on average, by about 0.85 of a percentage point for every 

100-foot increase in walking distance. 

In the same study, surveys of 18 office sites near rail found that the average rail modal split for work 

trips was nearly 9 percent. For worksites near BART, rail share was 17 percent, well above the Bay 

Area’s rail work trip share of 5 percent. For offices, the ridership gradient follows an exponential decay 

function. For non-BART sites, only offices within 500 feet of a station have as much as 15 percent of 

their workers commuting by rail; beyond 500 feet, no more than 10 percent of workers take rail to work. 

Proximity to transit is one of the most important factors in encouraging transit mode share. An 

analysis of transit-oriented developments in California concluded that barring serious problems like 

urban blight or high crime rates, the characteristics of the immediate surroundings (e.g., mix of land 

uses and pedestrian quality) were of minor importance once people were near stations. As long as 

development was geographically close and oriented toward a rail station, reasonable percentages of 

residents and workers traveled by rail. When both trip ends were clustered around a transit station, the 

odds of traveling by rail transit increased dramatically. 209 

Other analyses have found that distance to transit is the most significant factor in the decision whether 

to reach transit by foot. An analysis of pedestrian access to transit using a 1992 survey of BART 

riders found that distance to the transit station was the most significant factor in deciding whether to 

walk to the station, drive, or take a bus.210 Generally, individuals will walk up to about a half mile.211 

207 The suburban low-density, low-transit service sites consistently had higher shares of personal vehicle use than the urban 

high-density, high-transit service sites for shoppers within all income categories. JHK & Associates and K.T. Analytics. 

Analysis of Indirect Source Trip Activity: Regional Shopping Centers. Prepared for California Environmental Protection 

Agency, Air Resources Board. November 1993. 

208 Cervero, R. “Ridership Impacts of Transit-Focused Development in California.” University of California Transportation 

Center, Working Paper No. 176. 1993. 

209 Cervero, R. “Ridership Impacts of Transit-Focused Development in California.” 

210 This study found that walk trips accounted for the highest proportion of home-based access trips at stations located in 

dense, mixed-use areas of San Francisco—74.2 percent of trips at the 16th and Mission station and 67.2 percent of trips at 

the 24th and Mission station. At the opposite end of the spectrum, the Fremont station had a 3.9 percent pedestrian share and 

the Orinda station had a 2.8 percent pedestrian share. These stations are surrounded by large parking lots and low-density 

development. When individual characteristics, such as gender, age, and income, were taken into account, population density 

provided to be insignificant in the decision of whether to walk or take another mode to the transit station. Rather, the 

distance to the transit station was the most significant factor in mode choice (Loutzenheiser, David. “Pedestrian Access to 

Transit: A Model of Walk Trips and their Design and Urban Form Determinants Around BART Stations.” Presentation to 

76th Annual Meeting of Transportation Research Board, 1997.) 

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Transit-oriented communities often include mixed-use clusters of housing, office, and retail. By 

bringing these activities closer together, these developments also reduce the need to own a private 

vehicle. In the Portland, Oregon region, Metro incorporated a measure of transit accessibility–the 

number of employees within 30 minutes travel time by transit—into its vehicle ownership model. 

Based on statistical analysis in which the utility of owning zero, one, two, or three cars depends on a 

variety of factors, vehicle ownership drops as transit accessibility increases, independent of income 

and other household demographic factors. 

4.6 SUPPORT FOR PEDESTRIAN AND BICYCLING ACTIVITY/ 

MICROSCALE URBAN DESIGN FACTORS 

Currently we don’t build either land use or transportation systems for pedestrians and bikes, creating 

barriers that are dangerous, inconvenient, and aesthetically displeasing. 

Aspects of the built environment such as building orientation, street connectivity and design, and 

building design all contribute to the relative friendliness of that area to pedestrians and bicyclists, and 

to the general aesthetic appeal of an area. Together, these are often referred to as “microscale” urban 

design factors—small-scale elements that affect the safety, convenience, and desirability of living and 

working in areas of higher density and of using transit and nonmotorized modes of transportation. 

These design factors affect travel mode choice. In areas that do not include adequate bicycle and 

pedestrian facilities (sidewalks, bike lanes, and crosswalks), people are more hesitant to travel by foot 

or bike. More than 6,000 pedestrians die in collisions with cars each year, and 110,000 are injured. , 

According to federal statistics, pedestrians and bicyclists account for 15 percent of all “highway” 

fatalities.212 The risks fall disproportionately on senior citizens, who make up 13 percent of the 

population but account for 23 percent of pedestrian fatalities. “Long crosswalk distances and traffic 

signals timed for young adults present added difficulties, even life-threatening hazards, for older 

persons at intersections,” according to the American Association of Retired Persons.213 

Children face similar challenges; only 13% of trips to school are made by walking,214 a figure down 

from what some experts believe was more than 50% in the 1960s.215 

211 Data from the National Personal Transportation Study suggest that 70 percent of Americans walk 500 feet (one-tenth of a 

mile) for normal daily trips, 40 percent walk 1,000 feet (one-fifth of a mile) and 10 percent walk up to a half mile (Unterman, D. 

“Accommodating the Pedestrian: Adapting Towns and Neighborhoods for Walking and Bicycling.” Personal Travel in the 

U.S., Vol. II, A Report of the Findings from 1983-1984 NPTS, Source Control Programs. U.S. Department of Transportation. 

1990). A study in Montgomery County, MD, found that residents will walk one-quarter of a mile median distance to a bus and 

one-half of a mile to a rail stop (Replogle, M. Bicycles and Public Transportation. 1984. Cited by Holtzclaw, J. “Using 

Residential Patterns and Transit to Decrease Auto Dependence and Costs.” Natural Resources Defense Council. June 1994). 

212 In 1996, of 41,907 traffic fatalities, 5,412 were pedestrians and 761 were bicyclists. Source: U.S. Department of 

Transportation, Bureau of Transportation Statistics. National Transportation Statistics 1998. Table 3-19. 

213 Quote is from Jo Reed, of the American Association of Retired Persons. Figures were taken from: Witham, Drake and 

Bill Salisbury. “The Walking Wounded.” St. Paul Pioneer Press. April 14, 1997, B2. 

214 Calculations from 1995 National Personal Transportation Survey. 

215 Ulman, Marian, “A Healthy Start: Aiming to Revive the Walk to School.” Philadelphia Inquirer, March 28, 2000. 

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By increasing the relative desirability of walking compared with driving, urban design factors can 

encourage more walking or bicycling trips. Reductions in vehicle travel and emissions can occur if 

walking and bicycling trips replace vehicle trips. 

Techniques for Enhancing Micro-Design Features 

A community’s micro-design may be improved through pedestrian and bicycle facilities and street 

connectivity, as well as design and architecture. 

Pedestrian and Bicycle Facilities and Street Connectivity: Typical modern communities often contain 

a hierarchy of dead-end or cul-de-sac local streets that lead to collector streets and then to major 

arterials that connect communities to others via freeways. (See Figure 4-9.) 

Figure 4-9: Street Network Design 

Disconnected, Hierarchical 

Street Network 

Pedestrian-oriented Grid 

Street Network 

Many communities are bounded by walls, lakes, or other physical barriers, and often do not have 

sidewalks. This pattern makes pedestrian and bicycle travel difficult since circuitous routes and 

limited access increase the length of trips. Collector and arterial streets tend to be wide to allow 

vehicles to move faster and to handle the large traffic volumes that are channeled onto a few hightraffic routes. Wide streets are difficult and often dangerous for pedestrians and bicyclists to cross or 

to share with vehicles, especially if they lack facilities such as sidewalks and crosswalks. Such poor 

pedestrian environments encourage people to drive, even for short trips. 

Aside from well-connected street networks, features that improve the pedestrian environment include 

sidewalks, clearly marked crosswalks and walk signals, lighting, and other amenities, such as shade 

trees, benches, and streetscapes designed with the pedestrian in mind. Features that improve the 

bicycling environment include bicycle paths and lanes on streets, bicycle parking, clear signs, and 

facility design that improves accessibility. 

Design and Architecture: Just as street connectivity and bike and pedestrian facilities are important, the 

design and placement of buildings and the aesthetics of streetscapes also shape people’s attitudes toward 

travel. In residential areas, design for pedestrians includes making the street environment more 

attractive by placing porches and home entrances in the foreground, and garages and driveways more in 

the background. In commercial areas, pedestrian design means orienting stores to the street with 

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window displays and pedestrian entrances, rather than entrances through parking lots and garages. 

Narrow streets, shade trees, well-maintained sidewalks and traffic slowed through traffic calming 

measures (such as speed bumps, raised crosswalks, traffic circles, and median barriers) also improve the 

pedestrian environment. In less pedestrian friendly environments, structures are located without 

reference to neighboring buildings or properties. Some office parks lack provision for foot traffic, so 

that a walk from an off-site bus stop to an office might involve walking through large parking lots. 

Indirect Environmental Effects of Microscale Urban Design and 

Pedestrian and Bicycle Support: Travel Mode Choice 

The most significant benefit of good urban design is that it reduces auto travel by enabling other 

travel choices. 

EFFECT OF DESIGN FOR PEDESTRIANS AND BICYCLISTS 

Several studies have found that higher levels of pedestrian and bicycling activity occur in areas with 

street connectivity and pedestrian amenities than in those without those features. For example, a 

recent study compared two Puget Sound area neighborhoods that are similar in terms of gross 

residential density and intensity of commercial development: Wallingford in Seattle and Crossroads in 

Bellevue. The analysis found that Wallingford—the neighborhood with a high level of pedestrian 

network connectivity—had almost three times as much pedestrian activity as did Crossroads, which 

had a low level of pedestrian connectivity. 216 

An analysis conducted in Portland, Oregon (using data from a home interview survey, results from 

regional travel forecasting models, and land use information) found that the pedestrian environment is 

a significant factor in mode choice decisions and vehicle miles traveled. In particular, the analysis 

found that improving the quality of the pedestrian environment to a level comparable to that of 

Portland’s most pedestrian-oriented zones would result in a 10 percent reduction in VMT.217 A similar 

analysis by the Maryland National Capital Parks and Planning Commission, using its regional travel 

models, found that pedestrian and bicycle friendliness is quantifiable and is a significant predictor of 

work trip mode choice.218 

Neither the Portland nor the Maryland studies explicitly included safety in their analyses. Not 

surprisingly however, pedestrian safety is a factor that people typically consider when deciding 

whether to walk in an area. An analysis of employment sites in southern California, for example, 

216 This study, however, did not address effects on vehicle travel. Moudon, et al. “Effects of Site Design on Pedestrian Travel in 

Mixed Use, Medium Density Environments.” Submission to 76th Annual Meeting of the Transportation Research Board, 1997. 

217 A pedestrian environment factor (PEF), measured on a scale of 4 to 16, was developed based on street connectivity, 

sidewalk connectivity, ease of street crossing, and topography. A value was assessed for each zone in the travel demand 

forecasting model. The model found household vehicle trips and vehicle miles of travel declined with increases in the PEF. 

Multiple regression analysis suggested that each unit increase in the zonal PEF would reduce daily VMT per household by 

2.5 percent, or 0.7 miles. Cambridge Systematics et al. Making the Land Use Transportation Air Quality Connection: 

Analysis of Alternatives. Model Modifications. Vol. 4. Prepared for Thousand Friends of Oregon. May 1996. 

218 Replogle, M. Integrating Pedestrian and Bicycle Factors into Regional Transportation Planning Models: Summary of 

State-of-the-Art and Suggested Steps Forward. Environmental Defense Fund. July 20, 1995. 

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found that areas characterized as “safe” had higher levels of transit use and bicycling and walking 

than those that were characterized as “unsafe.”219 

A number of studies contrasting bicycling in European cities with biking in U.S. communities note that 

bicycling is thriving in those metropolitan areas that have adopted policies to make bike travel faster, 

safer, and more convenient. In Munich, Germany, for example, bicycle use has almost tripled since 

1976, with modal split rising from 6 percent to 15 percent. The increase is due in part to the fact that the 

length of the bikeway network was more than doubled during that period, all residential neighborhoods 

in the city have been traffic calmed, and bicycle parking facilities have been expanded.220 

The pedestrian environment also has been found to influence decisions about vehicle ownership. In 

the Portland, Oregon regional travel models, data on the pedestrian environment were found to 

improve the predictive ability to estimate the number of vehicles owned per household.221 Similarly, 

in a logit model for vehicle ownership developed by the Chicago Area Transportation Study, the 

pedestrian environment and auto work trip mode share were found to be statistically significant in 

predicting vehicle ownership rates. Less than 40 percent of these households in urban areas have two 

or more vehicles, while more than 90 percent in suburban areas have two or more vehicles.222 As 

these studies indicate, a pedestrian-friendly environment allows walking trips to substitute for vehicle 

trips and, as a result, reduces the need to own as many vehicles. 

A number of studies suggest that grid street patterns can reduce vehicle trip lengths. For example, a 

simulation study found that traditional grid circulation patterns reduce VMT by 57 percent compared 

with VMT in more conventional networks.223 A modeling analysis of two simple, hypothetical street 

patterns estimated that total morning peak hour vehicle travel would fall more than 10 percent when a 

conventional suburban street pattern is replaced by a grid network.224 The full VMT implications are 

unclear, however, since the simulations assume that trip frequencies are unaffected by the street 

patterns. Some researchers argue that increased accessibility associated with grid street patterns can 

219 In this study, sites were considered to have a higher level of safety if they were characterized by sidewalks, street 

lighting, pedestrian activity, and the absence of vacant lots. In locations that offered TDM incentives, the share of transit 

was 5.4 percent in safe areas compared with 3.6 percent in less safe areas. The share of bicycling and walking was 3.2 

percent in safe areas compared with 1.7 percent in less safe areas. U.S. Department of Transportation, Travel Model 

Improvement Program. “The Effects of Land Use and Travel Demand Management Strategies on Commuting Behavior.” 

Prepared by Cambridge Systematics. November 1994. p. 3-18. 

220 Pucher, John. “Bicycling Boom in Germany: A Revival Engineered by Public Policy.” Transportation Quarterly, Vol. 

51, No. 4. Fall, 1997. P. 41. 

221 Cambridge Systematics et al. Making the Land Use Transportation Air Quality Connection: Analysis of Alternatives. 

Model Modifications. Vol. 4. Prepared for Thousand Friends of Oregon. May 1996. 

222 Eash, R. “Incorporating Urban Design Variables in Metropolitan Planning Organizations’ Travel Demand Models.” 

Conference on Urban Design, Telecommuting, and Travel Behavior. Williamsburg, VA, October 1996. 

223 Kulash, Anglin, and Marks “Traditional Neighborhood Development: Will the Traffic Work?” Development, Vol. 21, 

July/August 1990, pp. 21-24. 

224 McNally and Ryan. “A Comparative Assessment of Travel Characteristics for Neotraditional Developments.” University 

of California Transportation Center. University of California at Berkeley. Working Paper No. 142. August 1992. 

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induce additional travel. The degree to which this travel occurs by walking or driving is unclear and 

may differ depending on specific circumstances.225 

EFFECT OF BUILDING DESIGN AND ARCHITECTURE 

A preponderance of research finds that urban design can affect mode choices. However, identifying 

discernible travel effects from individual design features has proven difficult. For example, a study of 12 

census tracts in the San Francisco Bay area found that most individual neighborhood urban design 

features alone were not useful for explaining mode choice. However, when considered as a bundle of 

attributes, the influence of urban design was apparent. Pedestrian design was found to increase the 

probability that a nonmotorized mode would be used for nonwork trips by about 10 percent.226 A 

separate analysis of four neighborhoods in the San Francisco area also found no evidence that individual 

residential design elements has an effect on travel. However, the research suggested that neighborhood 

design as a whole is an important determinant of whether residents perceive walking as an option and 

affects pedestrian activity in a community. 227 Although certain specific design treatments, such as 

building orientation, might appear trivial, their collective influences may be important.228 

4.7 SYNERGIES: COMBINING TECHNIQUES 

Many of the above patterns and practices—compact development, mixing of land uses, higher density 

development oriented around transit, and pedestrian and bicycle facilities—have demonstrated 

positive environmental implications—reducing infrastructure requirements, reducing vehicle travel, 

reducing land and habitat consumption, and reducing water consumption and pollution. 

The efficacy of these practices depends on how well they are implemented, and how they are 

combined with other programs. Some of the benefits of mixed-use development, for example, require 

225 Crane, R. “Cars and Drivers in the New Suburbs.” Journal of the American Planning Association 62: 1, Winter 1996. pp. 

51-65. A 1992 study of travel in four San Francisco neighborhoods found that residents of older, compact neighborhoods 

made 2.75 to 5.5 times as many shopping trips by walking as residents of more auto-oriented neighborhoods, but that 

residents of both types of neighborhoods made about the same number of auto trips to regional shopping centers. Handy, S. 

Regional Versus Local Accessibility: Implications for Non-Work Travel. Doctoral Dissertation, University of California, 

Davis. Institute of Transportation Studies. 1992. 

226 Transit Cooperative Research Program (TCRP). Influence of Land Use Mix and Neighborhood Design on Transit 

Demand. TCRP Project H-1. March 1996. 

227 Handy, Susan. “Understanding the Link between Urban Form and Travel Behavior.” Paper presented at the 74th Annual 

Meeting of the Transportation Research Board, January 1995. In different areas, different types of design elements may be 

appropriate or fit into the neighborhood environment differently. A specific element such as the absence or availability of 

trees and shrubs at sidewalks may not be effective in the absence of other important pedestrian enhancements. This analysis 

suggests the importance of comprehensive design rather than identifying a specific design element that works in all places. 

The overall pedestrian environment is the prime concern. 

228 Cervero, Robert. Suburban Gridlock. New Brunswick, NJ: Rutgers University Press, 1986. p. 61. A study conducted by 

Susan Handy in Austin, Texas, confirms this finding. In the study, Handy surveyed residents from six Austin neighborhoods 

that are similar in socio-economic profile but different in design characteristics. Handy found that respondents from 

Clarksville—the neighborhood in which the quality of the pedestrian environment is high and many households are within 

walking distance of commercial areas—walked to a store about six times per month, as compared with residents of the other 

neighborhoods, who walked to a store between 0.72 and 2.06 times per month. Handy, Susan. “Urban Form and Pedestrian 

Choices: Study of Austin Neighborhoods.” Transportation Research Record 1552. p. 142. 

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pedestrian amenities. The reverse is also true. Adding pedestrian facilities, such as sidewalks and 

crosswalks, may be most effective in reducing vehicle travel when a variety of shops and services are 

within a short walk. Provided together, land use characteristics produce synergy—enhanced benefits 

due to incorporating multiple beneficial aspects of design into communities.229 

Measuring the Effects of Combined Policies 

The previous sections examined the impacts of individual actions. It is nonetheless difficult to isolate 

the effects of specific aspects of urban form on travel. Certain land use attributes, such as high 

density, a mix of land uses, and a pedestrian-friendly environment tend to occur together.230 This 

section therefore reviews studies that look at the combined effects of density, transit, etc. 

In practice, isolating the effects of individual land use characteristics on travel behavior may not be 

extremely important. The evidence indicates that the location of development in a region is important 

in affecting trip distances and that a combination of urban design factors is important in influencing 

mode choice. There appear to be synergies that come from combining beneficial aspects of land use 

and thresholds in the travel effects of individual land use factors. Thresholds occur when the travel 

effects of changing one land use factor are limited unless other land factors also are altered. 

Several studies have found that changes in individual microscale aspects of urban form, such as adding 

sidewalks and street benches, may not be sufficient to achieve changes in vehicle ownership or mode 

choice if the region as a whole is oriented toward vehicle travel. For example, an analysis of travel in 

Palm Beach County, Florida, found that transit mode share was minimal in all communities examined 

despite differences in design, presumably because the entire region is relatively auto-dependent, with 

limited transit service. Similarly, other studies have found that in a low-density area characterized by a 

wide separation of distinct land uses, sidewalks and attractive landscaping are unlikely to prompt 

residents to walk to shops and stores.231 Some newly built neotraditional communities may not 

significantly reduce vehicle travel if they are not regionally accessible or transit-accessible.232 

229 A community that contains a mix of land uses but that does not provide a pedestrian-friendly environment would be 

unlikely to achieve the full benefits associated with a mix of uses, such as reduced vehicle trip-making. People may drive 

even short distances between employment, retail, and restaurants if the pedestrian environment is poor. Locating higher 

density development near transit and employment provides the mass necessary for local stores that rely on foot traffic to 

locate there. Although not all compact urban environments have a mix of uses or transit-supportive urban design, few lowdensity environments have meaningful land use mix or transit-oriented amenities (Parsons Brinckerhoff Quade & Douglass. 

“Transit, Urban Form, and the Built Environment: A Summary of Knowledge.” Transit Cooperative Research Program. 

Washington, DC: 1996. p. 25.) 

230 A number of researchers have found correlations between these variables. See: Dunphy, R. and K. Fisher. Transportation, 

Congestion and Density: New Insights. Urban Land Institute, August 1993. Holtzclaw, John. “Using Residential Patterns and 

Transit to Decrease Auto Dependence and Costs.” Natural Resources Defense Council. June 1994. Loutzenheiser, David. 

“Pedestrian Access to Transit: A Model of Walk Trips and their Design and Urban Form Determinants around BART Stations.” 

Submission to the 76th Annual Meeting of the Transportation Research Board. January 1997. Cambridge Systematics, Inc.; 

Parsons, Brinckerhoff, Quade & Douglas; S.H. Putman Associates, Inc. Making the Land Use Transportation Air Quality 

Connection: Model Modifications. Vol. 4. Prepared for Thousand Friends of Oregon. May 1996. 

231 Cervero, R. and K. Kockelman. “Travel Demand and the Three Ds: Density, Diversity, and Design.” Institute of Urban 

and Regional Development, University of California at Berkeley. July 1996. 

232 The Kentlands community in suburban Washington, DC, is a well-known example of a new neotraditional development. 

Although Kentlands has been cited for its character and quality of life, the community is not located in an inner suburb or 

along the region’s transit rail system. As a result, one would not expect significant differences in transit share or vehicle 

travel between Kentlands and other nearby suburban communities. 

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When travel options are available, however, urban form and design characteristics can affect travel 

behavior to a greater degree, particularly in terms of mode choices. Increases in the mix of land uses, 

improved transit access to employment, and enhancements to the pedestrian environment together can 

alter the relative utilities of different choices, under the right conditions. Density is generally viewed 

as important because without compactness, urban design and mix are often not sufficient to ensure a 

built environment in which transit, bicycle, and pedestrian travel can play an important role. More 

compact neighborhoods support a diversity of land uses since they require commercial activities, 

which may be located close enough to a large population to facilitate nonmotorized trips. Researchers 

have concluded that higher densities, diverse land uses, and pedestrian-friendly designs (the three Ds) 

must co-exist to a certain degree if meaningful transportation benefits are to accrue.233 

Evidence from Comparisons of Communities 

Household characteristics like income, household size, and employment characteristics affect travel 

behavior. Several studies that control for these factors suggest that urban design and land use patterns 

have an important effect on travel behavior regardless of demographics. In the San Francisco area, a 

comparison of two neighborhoods with similar socio-economic characteristics found that residents of 

the compact neighborhood were more likely to walk or bicycle than residents of the auto-oriented 

community. When income and auto-ownership were taken into account, households with one or more 

cars in the “traditional” Rockridge neighborhood were about twice as likely to use a nonautomotive 

mode for nonwork trips as households in the “suburban” Lafayette neighborhood. A member of a 

two-car household in Rockridge had a 19 percent probability of walking, bicycling, or riding transit 

for a nonwork trip compared with a 9 percent probability for a similar resident of Lafayette.234 

The San Francisco Bay area has been the subject of considerable study due to its wide variation in 

land use types and neighborhoods. An analysis of trip data from a 1980 regional travel survey of San 

Francisco Bay area households found that those in newer suburban communities have substantially 

higher vehicle trip generation rates, a higher proportion of drive-alone trips, and a lower percentage of 

public transportation trips than households in communities that have high-density, mixed land use, 

and interconnected street networks.235 For all trips, the rate of travel by transit in compact 

communities was more than double the rate in suburban communities (7 percent compared with 3 

percent) and the share of trips made by walking was 50 percent greater (12 percent compared with 8 

233 Cervero, R. and K. Kockelman. “Travel Demand and the Three Ds: Density, Diversity, and Design.” Institute of Urban 

and Regional Development, University of California at Berkeley. July 1996; Cervero, R. “Mixed Land Uses and 

Commuting: Evidence from the American Housing Survey.” Transportation Research. Vol. 30, No. 5: 361-377. 1996. 

234 Transit Cooperative Research Program (TCRP). An Evaluation of the Relationship between Transit and Urban Form. 

Research Results Digest, No. 7. June 1995. 

235 This analysis was based on data from a 1980 regional travel survey of San Francisco Bay area households. Communities 

were defined as suburban if they were developed since the early 1950s with segregated land uses, have a well-defined 

hierarchy of roads, concentrate site access at a few key points via arterial roadways, and have relatively little transit service. 

Survey zones were labeled traditional if they were mostly developed before World War II, have a mixed-use downtown with 

significant on-street parking, and have an interconnecting street grid and mixed land uses. (Friedman, Bruce, Stephen 

Gordon, and John Peers. “Effect of Neotraditional Neighborhood Design on Travel Characteristics.” Transportation 

Research Record 1466. p. 63-70.) 

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percent). Auto use per household was about 32 percent higher in the suburban areas (7.1 trips per day) 

than in the compact areas (5.3 trips per day).236 

Another study in the San Francisco Bay area found results similar to those cited above for nonwork 

trips: residents of older, compact neighborhoods make more trips by nonmotorized modes than 

residents of suburban neighborhoods do. 237 Neighborhood design may provide the most potential for 

reducing nonwork VMT, since shopping and recreational activities are accessible by walking and 

bicycling. Residents of older, compact neighborhoods made 2.75 to 5.5 times as many shopping trips 

by walking as residents of more auto-oriented neighborhoods do. A set of neighborhood case studies 

suggests that urban form is an important determinant of whether residents perceive walking as an 

option. Short distances, commercial areas designed for pedestrian access, and certain types of 

destinations (such as restaurants) are particularly conducive to pedestrians.238 

On the East Coast, trip data compiled for two older, compact neighborhoods in Portsmouth, New 

Hampshire, showed that trip rates in these neighborhoods, which have moderate residential densities, 

gridded street patterns, and local shops and services, were about 50 percent lower than those predicted 

by the ITE Trip Generation manual.239 

Finally, a study recently conducted for a proposed suburban “village center-style” development in the 

San Joaquin Valley, California, estimated that the project would produce about one-third less vehicle 

travel per household on average than a typical single-use, low-density suburban housing tract would. 

The study also projected that such suburban villages could reduce the number of automobile trips by 

about 13 percent per household when compared with the typical suburban development pattern.240 

An Evaluation of Synergistic Policies 

Like the strategies, individual policies may be effective but they are most valuable when pursued 

comprehensively. 

Portland, Oregon, recently adopted a regional Vision 2040 and now evaluates transportation and 

development projects according to how well they move the region toward that vision. The region 

expects to grow 60 percent by 2040, and its citizens are deciding how to meet today’s needs while not 

236 The analysis suggests that community design and urban form have a sign\ificant relationship to travel behavior. The 

researchers eliminated from analysis those in the lowest and highest income categories; however, differences in income, 

demographics, and other factors may explain some of the travel differences. 

237 Handy, Susan. “How Land Use Patterns Affect Travel Patterns: A Bibliography.” Built Environment. Winter/Spring 

1992. 

238 The researchers noted that given an opportunity to walk, it is likely that more residents will choose to walk. However, not 

all of these trips will be in place of driving; many of them will be in addition to driving trips. This increased accessibility 

enhances quality of life even when it does not reduce vehicle travel. The travel impact depends on what other choices are 

available and thus will be different in various contexts. (Handy, Susan. “Understanding the Link between Urban Form and 

Travel Behavior.” Paper presented at the 74th Annual Meeting of the Transportation Research Board. January 1995.) 

239 White Mountain Survey Company. “City of Portsmouth Traffic/Trip Generation Study.” Ossippee, New Hampshire: 

White Mountain Survey Company, 1991 (unpublished), cited by Robert Cervero (“Mixed Land Uses and Commuting: 

Evidence from the American Housing Survey.” 1996. p. 362). 

240 Fehr and Peers. “Effect of Stockton’s Proposed Suburban Village Center Development.” January 1992. As cited by 

Parker, Terry. California Air Resources Board. The Land Use-Air Quality Linkage: How Land Use and Transportation 

Affect Air Quality. 1994. p. 10. 

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damaging the city’s ability to meet the needs of citizens in 2040. The region has decided to 

accommodate growth on its West Side by targeting development in transit-oriented developments. 

The proposed Land Use, Transportation, Air Quality Connection (LUTRAQ) alternative includes 

transit-oriented development, mixed-use centers, transit system improvements, and market strategies 

(a daily parking charge for commuters who drive alone and free transit passes, at least partially 

funded by parking revenues). 

State-of-the-art analysis showed that the LUTRAQ alternative is superior to the more auto-oriented 

alternative on many fronts not just on many of the criteria discussed above related to sustainability, 

but also on those often used to judge standard highway projects:241 

� 22.5 percent fewer work trips made in single-occupant vehicles, saving congestion, fuel, 

emissions, and money 

� 27 percent more trips made on transit and by walking and biking 

� 18 percent less highway congestion with 10.7 percent fewer hours of vehicle travel during the 

afternoon rush hour 

� 21 percent greater access to jobs in the region, as measured by the percentage of the area 

within 30 minutes travel of 500,000 jobs 

� Reduced air pollution: hydrocarbons, -6 percent; NOx, -8.7 percent; and CO, -6 percent. 

� Reduced emissions of the greenhouse gas CO2: -7.9 percent, and 7.9 percent less fuel used. 

The projected benefits stem from the combination of land use measures and market-based 

mechanisms. These benefits are for the entire West Side. Benefits will be even larger for homes and 

businesses in the new transit-oriented developments. The new developments avoid wetlands and other 

sensitive areas, maximizing biodiversity and recreational opportunities. The plan also reduces paved 

areas and interferes with the hydrologic cycle as little as possible. 

4. 8 SUMMARY 

Research has shown that development decisions have both direct and indirect effects on the 

environment and that growth can be accommodated in ways that minimize negative impacts on 

human and natural environments and in some cases even improve environmental quality. Strategies 

that minimize negative environmental impacts include compact development, reduced impervious 

surfaces and improved water detention, safeguarding of environmentally sensitive areas, mixed land 

uses, transit accessibility, and support for pedestrian and bicycle activity. 

Used in combination, these practices can significantly reduce impacts to habitat, ecosystems and 

watersheds, and can reduce vehicle travel, which in turn reduces emissions of local, regional, and 

global concern. 

2411000 Friends of Oregon, “Making the Connections: A Summary of the LUTRAQ Project.” February 1997. 

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5. Conclusion 

Across the country, communities are concerned about the built environment not just for community 

and economic reasons, but also because of the effect that development has on human health, 

environmental resources, and natural habitats. Our Built and Natural Environments has reviewed 

evidence demonstrating that the built environment can significantly affect ecological and human 

health. As citizens and public officials have come to understand the relationships among land use, 

transportation, and the environment, they have begun to seek new ways to grow—ways that capture 

the benefits of protecting the environment, and the jobs, economic development, health, and quality 

of life that depend on the protection of air and water quality. 

Urban form affects attainment of national environmental goals in each of the following areas: 

� Habitat and Ecosystems—Development uses land space and modifies habitats and 

ecosystems. Land consumption rates in the United States are high and are rapidly 

increasing. More land was developed during the five-year period from 1992–1997 than 

during the 10-year period that preceded it. Over those five years, the national rate of 

development more than doubled to 3 million acres per year. Not only does development 

directly destroy areas of natural habitat, it can fragment habitat and lead to invasion of 

non-native species that severely alter ecosystem function and reduce biodiversity. 

Adverse impacts can be reduced by clustering development to preserve large areas of 

continuous natural habitat. Development that avoids sensitive and critical habitats, such 

as wetlands, greenways, and buffer zones around sensitive habitat, can preserve 

ecosystem integrity and can create amenities for adjacent neighborhoods. 

� Water Quality—Urban development affects water quality through alterations to the 

natural flow of water within a watershed, particularly by increasing impervious surfaces 

and channeling stormwater runoff. EPA estimates that 36 percent of the nation’s lakes, 

rivers, and estuaries are impaired by pollution, and approximately 21 percent of the lakes, 

12 percent of the rivers, and 46 percent of the estuaries are impaired due to urban runoff. 

As communities nationwide strive to comply with the Clean Water Act and Safe Drinking 

Water Act, which protect water resources both as natural habitat areas and as sources of 

clean drinking water, understanding the impact of development on water quality becomes 

increasing important. Development results in increased runoff volumes and peak period 

discharges, which in turn increase sedimentation and pollutant levels, increase water 

temperature, and reduce stream stability. Water quality is adversely affected by increased 

pollutant loads that are washed from surfaces in paved areas and deposited from air 

pollution. Water quality can be improved by minimizing impervious surfaces through 

more compact, mixed-use development, minimization of parking areas and street widths, 

use of porous or pervious pavements, and landscaping. 

� Air Quality—As regions seek to reach air quality attainment goals outlined in the Clean 

Air Act, the need to improve understanding of the relationship between air quality and 

development and transportation patterns becomes clear. Motor vehicles emissions 

currently account for a significant portion of many air pollutant emissions, contributing 

57 percent of all CO emissions, 30 percent of NOX emissions, 44 percent of PM-10 

emissions, and 27 percent of VOC emission. Per mile motor vehicle emissions have 

decreased since about 1970 as a result of vehicle emissions control systems and cleaner 

fuels, but increasing VMT threatens to reverse this trend in the future. 

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Our Built and Natural Environments 

Air quality is indirectly affected by urban form to the degree that development patterns 

affect travel behavior. There is significant evidence that compact, mixed-use development 

focused around transit can reduce vehicle travel and air pollution from motor vehicles. 

Infill development (including redevelopment of brownfields) generally means greater 

accessibility to existing transit services, which should reduce vehicle travel compared 

with development on the urban periphery. Enhancement of the pedestrian environment 

also can encourage people to walk rather than drive for short distances. In addition, 

pricing roads and parking so that drivers recognize the full costs of their behavior can 

work in tandem with changes in urban form to encourage use of transit, carpooling, 

walking, and bicycling. 

� Global Climate—Like air quality, global climate is indirectly affected by urban form to 

the degree that development patterns affect travel behavior. Combustion of motor vehicle 

fuel emits carbon dioxide, a greenhouse gas that helps trap heat within the atmosphere. 

Emissions of carbon dioxide from motor vehicles have been increasing over time, and 

transportation is projected to be the fastest growing source of carbon dioxide emissions of 

any sector. In 1997, the transportation sector emitted 32 percent of U.S. carbon dioxide 

emissions from fossil fuels, and carbon emissions from transportation are projected to 

grow by approximately 48 percent over the 1996-2020 period. Many communities have 

agreed that global warming is an issue of serious concern and are attempting to encourage 

practices that reduce greenhouse gas emissions. Examples include providing more 

transportation choices, reducing vehicle travel, and improving vehicle fuel economy 

through decreases in traffic congestion. 

� Contamination and Risk in Communities—Old abandoned industrial facilities in 

urbanized areas, potentially contaminated with hazardous or toxic waste, pose risks to 

communities. Redevelopment of brownfields provides the opportunity to clean up 

contaminated sites, reducing threats to water quality and human health. Brownfields 

redevelopment has numerous other benefits. It allows more efficient use of existing 

infrastructure, saving the money and time needed to construct new infrastructure such as 

schools, roads, and water systems, and it protects a community’s open space by placing 

new development in previously developed areas, rather than on greenspace. 

The extent to which some development practices can reduce the direct impacts of development on 

habitat and hydrology is relatively clear. The magnitude of the impact of land use practices that 

reduce indirect effects of urban form on air quality can be more ambiguous, due to uncertainties 

regarding travel behavior. Nonetheless, an overwhelming number of studies on this topic agree 

that urban form has undeniable implications for environmental goals. 

The information reviewed in Our Built and Natural Environments provides evidence that: 

� U.S. urban form, including its land use and transportation components, has changed 

significantly in recent decades. 

� These changes affect environmental quality over the short and the long run, and interfere 

with the ability of the United States to meet its health and environmental goals. 

� Current development patterns are not simply due to population growth and therefore are 

not inevitable. 

� Communities have choices in their development decisions. 

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� Communities can exercise that choice by developing the built environment in ways that 

contribute toward the attainment of health and environmental goals. 

There is ample evidence that the built environment matters to communities—not just for social 

and economic reasons, but also for environmental reasons of national concern. Issues related to 

our built environment are growing in importance and, if left unaddressed, will make it difficult to 

meet our nation’s environmental goals. Fortunately, communities, regions, and states are starting 

to find ways to expand that achieve better economic, community, and environmental outcomes. 

The U.S. Environmental Protection Agency plans to continue building knowledge about the 

relationships between land use, transportation, and the environment as it supports our nation in 

meeting its environmental and human health goals. 

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Our Built and Natural Environments 

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Bradbury, Kathernine, Anthony Downs, and Kenneth Small. Urban Decline and the Future of 

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