For at least the past four millennia, human development has been intertwined with the development of roads. Two millennia ago, the Roman culture made dramatic leaps in technological innovation to facilitate transportation and expand its empire. The Romans built straight, narrow roads to provide a stable base for moving humans, animals, and vehicles. Their roads extended long distances, creating one of the first networks of roads. With the development of roads and road networks, a variety of interactions and ecological effects occurred. Their techniques of excavation (including the removal of mountainsides), construction of bridges, and changes in water flows affected the environment and wildlife (and human) mortality. Many of these effects persist today.
The past century has seen a transformation in the magnitude and the scale of paved roads in the world in general and in the United States in particular. This transformation parallels the production and use of motorized vehicles. Roughly a century ago, the first fossil-fueled vehicles were being driven on a road system where the patterns on the landscape were largely in place but undeveloped. In 1900, an extensive network of roads existed, but only about 4% were paved (FWHA 1979). In the twentieth century, the system about doubled in length (Forman et al. 2003) to a recent estimate of 4 million linear miles or about 0.8% of the land surface area in the United States (Cook and Daggett 1995). As the system was getting longer, it was also becoming more structurally complex to provide for larger volumes of traffic and heavier loads. Those complexities include larger, wider roads with changing techniques of construction and greater structural stability. Some of these changes were done, in part, to accommodate and mitigate environmental issues.
As the road system became more complex in the twentieth century, it became more expensive to build, repair, and maintain. Road develop-
ment in the early 1900s relied on local or private sources of funding. Funding is now derived from a variety of sources, and a large percentage originates from federally collected sources, particularly user-related fees on gasoline and transport equipment. Since the passage of the Transportation Equity Act for the Twenty-First Century (TEA-21), investment in highway infrastructure has increased; for example, investment increased by 14% between 1997 and 2000 (FHWA 2002a). Capital spending on highways totaled about $64 billion in 2000, but that amount is unlikely to meet future needs for maintenance (FHWA 2002a).
The process by which these monies are spent to administer, plan, select, and build projects and maintain roads is a complicated maze of bureaucracy. The process is centered in states—with some variation among states in planning and implementation—and involves five interacting groups: federal government, state government, metropolitan planning organizations, local government, and the public (Forman et al. 2003). These governments share funding of administration, construction, planning, and maintenance in various ratios. Governments construct and maintain roads to provide a number of social benefits, such as providing mobility, transporting goods and people, and sustaining economic growth. One of the considerations in the process of road development is the recognition and management of environmental concerns.
All phases of road development—from construction to vehicle use—change ecological conditions of an area. Roads change abiotic characteristics of the environment—from physical to chemical soil conditions to alterations in water flow and water quality. Vehicle use on roads contributes to air and water quality degradation (Forman et al. 2003). Other direct changes to the biotic components of ecosystems include alteration of habitats, increased wildlife mortality, and dispersal of nonnative pest species (plants and animals). At larger scales, roads can affect migration patterns (Forman et al. 2003).
Integration of environmental effects into all phases of transportation efforts has been part of an ongoing process for several decades. These efforts attempt to form a bridge between the transportation policy arena and the environmental science community. Federal legislation has long directed the Federal Highway Administration (FHWA) and recipients of federal funding to consider environmental factors when planning transportation systems and specific road projects. During the 1990s, the Intermodal Surface Transportation Equity Act and the TEA-21 (FHWA 2001a) contained provisions addressing environmental considerations for both transportation planning and project implementation. Combined
with programs to address cleaner fuels and cleaner vehicle emissions, many programs have aimed at addressing and reducing the environmental impacts associated with roads and the mobility roads provide. At the same time, there have been efforts to streamline environmental review processes to reduce red tape, paper work, and delay in project reviews without compromising environmental protections. Striking a balance between competing factors and policies has been and remains a daunting challenge.
The consideration of environmental issues in the road development process has been a source of debate. Some argue that dealing with environmental concerns adds unnecessary time and cost to road projects (GAO 2003). Others contend that it is just one of many factors that protract projects. Recent work by FHWA suggests that streamlining environmental considerations is possible through effective planning and coordination.
Research on road effects has been conducted for decades, but a larger emphasis has been placed on investigating road ecology in the past decade. Forman et al. (2003) provided a compilation of the ecological effects of roads, assessing the direct and indirect effects on vegetation, animals, and abiotic influences across a range of scales. They also argued for the development of a discipline of road ecology that combines an improved understanding of ecological effects with all dimensions (assessment, planning, coordination, construction, and management) of road development.
Suggestions on how environmental and transportation concerns can become more integrated have also been addressed by governmental and nongovernmental agencies. In a report published by the Transportation Research Board, Evink (2002) suggested that many issues associated with wildlife can be addressed through early planning or context-sensitive design, where structural solutions can mitigate some effects. White and Ernst (2003) recommended a number of ways to mesh conservation and transportation objectives, including integrated planning, conservation banking, agency coordination, wildlife crossings, and native vegetation management.
As more resources go toward road construction and management and as knowledge about the ecological effects of roads increases, the conflicts between the societal goals of developing transportation infrastructure and maintaining ecological goods and services (for example, soil production [a good] from decomposition of plant matter [a service]) become more apparent. Reconciling the conflicts is made more difficult
by the lack of knowledge of some ecological changes caused by roads—especially local, site-specific changes and processes at all appropriate ecological scales—and by the demand for rapid, efficient use of human resources for road development. The question is how can efficient use of resources in road planning, construction, and use be increased while attempting to conserve critical ecosystem structure, functioning, and services?
COMMITTEE CHARGE AND RESPONSE
The increasing importance of the relationships of transportation facilities to the surrounding natural communities and their wildlife has been recognized by FHWA and state transportation agencies. The importance of transportation corridors and infrastructure was pointed out by the National Research Council (NRC 2000a) when it called them globally significant. The importance of this issue was reflected by congressional action in Section 5107(b)(4) of the TEA-21 (PL 105-178), which required the secretary of transportation to “study the relationship between highway density and ecosystem integrity, including the impacts of highway density on habitat integrity and overall ecosystem health, and develop a rapid assessment methodology for use by transportation and regulatory agencies in determining the relationship between highway density and ecosystem integrity.” Section 5107(d) of TEA-21 authorizes the secretary to fund a study of this relationship by the National Academies through a grant or cooperative agreement. In response, FHWA asked the National Academies to direct its investigative arm, the NRC, to review the scientific information on the ecological effects of road density. See Box 1-1 for details of the charge to the committee and Box 1-2 for the committee’s definition of paved roads.
In 2002, the NRC formed the Committee on Ecological Impacts of Road Density, a panel of 14 members that included a experts in environmental engineering, highway construction and engineering, land-use change, wildlife ecology, endangered species, habitat evaluation modeling, habitat impact assessment, economic development and planning, environmental law and policy, and biodiversity conservation (see Appendix A).
The committee held three public meetings—Washington, DC, Irvine, CA, and Seattle, WA—to collect and review available scientific information, hear briefings from scientists and transportation profession-
BOX 1-1 Statement of Task
A multidisciplinary committee will be established to review the scientific information on the ecological effects of road density, including the impacts of roads and highway density on ecosystem structure and function and on the provision of ecosystem goods and services. The committee will focus on hard-surfaced roads and will assess data and ecological indicators needed to measure those impacts. Cumulative effects will be considered. The proposed study will also provide a conceptual framework and approach for the development of a rapid assessment methodology that transportation and regulatory agencies can use to assess and measure ecological impacts of road density. To the degree that the committee can identify documentation of their effectiveness, it will consider the potential ameliorating effects of measures that might avoid, reduce, or compensate for the effects of highways and highway density on the structure and processes of ecosystems.
The committee will consider such questions as the following:
The study will focus on all classes of hard-surfaced roads. The committee will consider and describe as possible the various attributes of roads that have ecological significance, such as how the right-of-way is managed, surface composition, and the presence or absence of structures, such as overpasses and underpasses. It will consider the importance of the pattern of road layout on ecological systems. It will not address global or regional climate effects, since they are being studied under other initiatives. However, local climate effects are appropriate in the scale of individual project design, construction, and use and are directly related to ecosystem performance in both long- and short-term contexts.
als, and receive oral and written testimony from the public. The meetings included field site visits to transportation projects. The committee met two more times in executive session to complete its report.
BOX 1-2 Definitions of Paved Roads
A paved road is any road that is described by the following categories (FHWA 2002a):
LOW TYPE: an earth, gravel, or stone roadway that has a bituminous surface course less than 1 inch (in.) thick suitable for occasional heavy loads.
INTERMEDIATE TYPE: a mixed bituminous or bituminous penetration roadway on a flexible base having a combined surface and base thickness of less than 7 in.
HIGH-TYPE FLEXIBLE: a mixed bituminous or bituminous penetration roadway on a flexible base having a combined surface and base thickness of 7 in. or more; it also includes brick, block, or combination roadways.
HIGH-TYPE COMPOSITE: a mixed bituminous or bituminous penetration roadway of more than 1 in. compacted material on a rigid base with a combined surface and base thickness of 7 in. or more.
HIGH-TYPE RIGID: a hydraulic cement concrete roadway with or without a bituminous wearing surface of less than 1 in.
The focus of the committee’s work was on the ecological effects of federally funded, major paved roads within the United States road network—for example, highways in urban and rural locations. The committee limited its focus to highly urbanized street networks. The committee recognizes that paved roads run through a range of conditions, from undeveloped areas (such as, parklands or conservation areas) through rural and agricultural lands to high density, urban development. More is known of the ecological effects of roads in undeveloped and rural areas than in highly urbanized areas. Some effects, such as air pollution and noise, increase with development. Legal requirements, policy frameworks, planning and assessment that address ecological issues also vary by context. Hence, some topics in this report focus on undeveloped and rural areas, and others focus on urban settings.
It was outside on the committee’s charge to consider the ecological effects of large networks of roads that are not paved, such as those found in federal wetlands, forestlands, wilderness areas, farm roads, or state-
level categories. Global climate effects, either how potential climate changes might effect road ecology or how road ecology might influence climate were also not included in the committee’s charge. Local (smaller than meso-scale) climate interactions with road ecology are included in this report.
The committee was not formed to address the diversity of political, social, and economic factors that relate road development and traffic flow to urban sprawl or suburban growth, and these issues are not addressed in the report. The committee also did not consider the extremely important topic of human ecology in this report. Human ecology includes social, cultural, economic, and political dimensions, and it is important in rural and urban environments. The disruption of human communities—especially in urban areas—is well documented, but roads can profoundly affect humans even in remote, sparsely populated areas such as Alaska’s North Slope (e.g., NRC 2003). Roads obviously have enormous effects on commerce, many of which are by design, but many of their effects are unintended or indirect. The topic is broad and complex enough to warrant its own committee report. Such a committee would be constituted very differently from this one, and that is the main reason that the topic is not discussed in detail in this report. The committee was not constituted to address other important quality-of-life factors such as safety; efficient movement of vehicles; and protection of farmlands, publicly owned recreation lands or scenic, historic, and cultural areas. Nor does it address such important considerations as project and other related costs; statewide, regional, and local planning goals; and the economic viability of the communities of users.
This report strives to highlight the ecological effects of highways that should be evaluated throughout the decision-making process in the planning, design, construction, and maintenance of highways.
The term road density is used in the charge to the committee. Forman et al. (2003) defined the term as “the average total road length per unit area of landscape,” which is intuitively appealing because it is easy to measure. That definition is used fairly widely in the literature on the ecological effects of roads. However, roads also have width, which can vary widely, and therefore lane miles per square mile (or lane length per unit area) is a better measure of density. That measure takes into account the differences between major multilane expressways and two-lane rural roads, as well as any road in between. Therefore, when this committee discusses road density in general, lane length per unit area is meant. However, it often is more difficult to obtain information on lane length
than on road length, and many studies considered in this report use road length only. In specific cases where other studies are cited, the definition of road density used in those studies is specified.
The concept of road density was developed as a way of quantifying one aspect of a road network, and therefore the term is applicable only at scales larger than a road segment or for a system of roads. The term road density may be appropriate for measuring the structure of some existing road networks (especially those few urban or rural systems in a rectilinear grid), but it is not the only measurable term that could be used to describe road pattern and structure. The term may also be useful for assessing some ecological effects (such as a surrogate of impervious surface).
However, in attempting to evaluate road density, the committee came to recognize that the term does not refer to a simple or single variable. Density includes length, width, number of intersections, and other related variables. Although the meaning of the term was reasonably clear in some cases, such as in its relevance to the number of lanes in a highway, the committee often had difficulty identifying a good way to make comparisons. For example, the concept of road density alone does not allow a useful comparison of the effects of two road networks, one of which consists of many narrow roads that have little traffic and one of which has fewer roads that are wider and heavily traveled. In another example of using the concept of density alone, it is unclear how the effects of one 8-lane highway can be compared with those of two 4-lane highways. In the above comparisons and in many others, the roads must be observed on the ground and their specific effects studied.
Therefore, the committee has not devoted a great deal of attention in this report to density per se but has focused on variables that contribute to density, such as highway length, portion of land covered, nature of interchanges, and so on. It also has used the broader concept of scale for evaluating environmental effects. In a few cases, however, density is a tractable and useful way to think about the ecological effects of roads.
The committee was asked to consider the cumulative effects of roads. Cumulative impact is defined by the Council on Environmental Quality as “the impact on the environment which results from the incremental impact of the action when added to other past, present, and reasonably foreseeable future actions regardless of what agency (Federal or non-Federal) or person undertakes such other actions.” Cumulative effects can result from individually minor but collectively significant actions taking place over a period of time. The term ensures that environmental impacts of federal actions, such as transportation projects, are
considered in connection with other activities that can affect the same environmental resources. In this report, the committee considers the concept of cumulative effects more broadly than the regulatory definition to look at effects that arise from synergistic interactions, involving multiple factors and occurring over large spatial and long time scales. In assessing and managing cumulative effects, especially in response to legal and regulatory requirements, this committee agrees with an earlier NRC (2003) committee that it is helpful to focus “on whether effects under consideration interact or accumulate over time and space, either through repetition or combined with other effects, and under what circumstances and to what degree they might accumulate.” For an extensive discussion of cumulative effects assessment, see NRC (2003).
Ecosystem goods and services are terms used to describe those structures or processes that provide support for a variety of human endeavors (Daily 1997). This utilitarian construct, originally described by Daily (1997), defines components of ecological systems in terms of how they benefit and support human life. Ecosystem services include water purification, flood and drought mitigation, climate stabilization, carbon sequestration, waste treatment, biodiversity conservation, soil generation, disease regulation, pollination, maintenance of air quality, and the provision of aesthetic and cultural benefits. Ecosystem goods are produced by these services and include food, fiber, timber, genetic resources, and medicines. Most ecosystem goods are direct inputs into economic systems. The committee assumed that ecosystem goods and services are fairly well correlated with ecological structure and functioning. Some of the report addresses how these goods and services interact with road ecology. For the most part, however, the report focuses on ecosystem structure and functioning.
In evaluating the ecological effects of roads, the physical, socioeconomic, and legal contexts in which the roads are situated are important. Each context has spatial and temporal dimensions.
The spatial context is largely determined by the physical conditions of the environment. Topography, aspect (a position facing in a particular direction or exposure), underlying geology, stream network, and soils all influence how a road affects the environment. For example, the interaction between an environment and a road that does not cross a river is different from that between an environment and a road that crosses a river six times in 10 kilometers (km). Aspect can influence how quickly snow and ice melt off the road surface and thereby affect driving conditions, which influence both traffic speed and volume. Rapid snowmelt could
also affect stream hydrographs. Original topography, geology, and soils dictate the road path and provide construction constraints or opportunities.
Roads change the local physical environment by interacting with underlying topography, aspect, geology, soils, ecological conditions, and land cover. For example, new patterns of water runoff can develop as the local topography is altered, and those changed patterns can result in altered storm hydrographs, changed groundwater recharge, and increased delivery of sediments and contaminants.
The effects of a road also depend on the prevailing type and intensity of land cover and land use. Environmental effects are largely influenced by whether a road runs through wildlands (removing natural habitats and opening up new areas to human access), wetlands, agricultural lands, or a river valley or whether it lies on a ridge. In fire-prone landscapes, roads can serve as a firebreak if the road width is enough to deter spread of ground fires.
Ecological productivity is also influenced by the presence of roads. The roadside between the paved road and original land cover often has lower productivity than the surrounding landscape (especially for roads through forests). Roadside maintenance is designed for driver safety and often involves the use of chemical deterrents or mechanical removal of vegetation. The linear pathway along a road can serve as a corridor for movement of native and nonnative plants and animals. For example, some highways in Georgia are bordered by the kudzu vine, which was introduced and grows into the native pine canopy and eventually kills the trees.
Biodiversity along roads typically is quite different from the surrounding landscape. Plants along roads tolerate vehicular pollution, exposure to bright sunlight, dry soils, and regular mowing. Roadside plantings in the United States once consisted of forbs and herbs (often of European origin) known to thrive in extreme conditions. Now there is an effort to plant vegetation along many highways, some of which are selected because they are native to the United States (but not always from the local area). For example, California poppies are now abundantly grown outside their original range. Sometimes roadsides function to protect native plant communities and may be the only way some plants are protected from land alteration. Some animals use habitats found near roads, such as deer feeding on vegetation, snakes basking, or many animals feeding on road kill (an animal that has been killed on a road by a motor vehicle). Some animals appear indifferent to a road, and other
animals shy away from the noise and chemical pollution from surfaces and vehicles.
To capture the diverse effects of roads on their environment, Forman et al. (2003) refer to a “road-effect zone” over which significant ecological effects occur. Those effects can include changes in the abundance (or even presence) of plants and animals; barriers to movement of both terrestrial and aquatic animals; changes in water levels, flows, and water quality; and other habitat changes that affect populations and biological communities. Because these factors vary over space, the road-effect zone is usually asymmetric extending outward on either side of the road, with varying zone boundaries.
Temporal changes on the land are influenced by roads. The ecological impact of building a road occurs with several time lags in response. Some effects of road construction are not realized for several decades after a road is completed, when trees and other plants die and wildlife mortality affects population persistence. Reestablishment efforts may result in a quick pulse of plant growth after seeding and fertilization, but the new equilibrium of vegetation along roadsides may take some time to establish.
The existing road system and the addition of new roads to that system often have different impacts and management options. Although most current and foreseeable road projects in the United States are along established roads, the selection of sites for new roads carries the potential for new ecological effects. New roads can affect the movement of plants and animals and change the physical environment, as can increases in traffic volume on existing roads.
Ecological indicators are important in assessing the effects of roads during planning through construction stages of road projects, and they are important in determining the broader and cumulative effects of the road and its corridor. Ecological indicators are generally developed to quantify the ecological response to a variety of factors (Hunsaker and Carpenter 1990, Suter 1993) and are further discussed in Chapter 7.
WHAT IS DIFFERENT AND NEW IN THIS REPORT?
This report attempts to provide guidance on reconciling the different goals of road development and of environmental conservation in two ways. The use of scale is an important factor in understanding the ecological context of roads, and the integration of social and ecological di-
mensions is important in assessing, constructing, and managing road ecology. These themes are discussed in the following sections.
Scales of Observation
In traditional use, the word scale has at least two meanings. One defines a unit of measurement. A meter and a foot are different scales, and measures of objects are made using multiples or fractions of these units. For example, the paved surface of a typical two-lane highway is about 26 feet (ft) or 8 meters (m) wide. The other definition of scale has to do with relationships among units and is derived from the Latin word scalaris for ladder, which is used for such items as the scale of a map. For example, the scale of a road map is 1:50,000, where 1 inch (in.) on the map equals 50,000 in. on the ground.
In this report, two terms, grain and extent (O’Neill et al. 1986), are associated with scale. A grain is defined as the unit of the smallest resolution within a data set and is similar in meaning to the first definition of scale in the preceding paragraph. Grain and extent can be applied along either temporal or spatial dimensions and thus are useful descriptors in assessment and planning. In one-dimensional spatial data, such as a transect, the grain is the unit of measure or step length. In two-dimensional spatial data, such as a map, a pixel or grid cell size is the grain of the data set.
The extent of a data set defines the bounds in space or time. The extent could be the length of a transect for one-dimensional spatial data. For two-dimensional spatial data, the extent is defined as the window of the map. In temporal data, the grain is usually defined as the minimal time unit, such as minute, day, or year, and the extent is the period of record used in analysis. Therefore, scale is defined here by two components: the grain and extent.
As a demonstration of these meanings of scale and its relevance to road ecology, consider the set of Figures 1-1 through 1-8. The concept of viewing the world at different scales was originated by Boeke (1957) and can be found in a book and video called Powers of Ten by Morrison and Morrison (1982). Both works generated a set of images that were differentiated in size by an exponent of 10 (101, 102, 103, and so forth). In this sequence, there is a photograph of the center of a road on the campus of Emory University in Atlanta, Georgia (Figure 1-1). The extent of the photograph is 1 m, a grain being about 2 millimeters (mm).
Each subsequent image increases the extent by an order of magnitude or power of 10 (for example, 10 m, 100 m, 1 km, and so forth) while retaining roughly the same central point. The sequence ends with an image of North America—an extent of about 10,000 kilometers (km). The set of images provides a glimpse into the complexity of the ecosystems at each of the scales.
Figures 1-1 through 1-8 depict distinct changes in structures as the scale of observation changes over 8 orders of magnitude. At the smallest or finest scales, the road surface is visible (Figure 1-1). Road structures—markings, sidewalks, curbs—are apparent in the next scale (Figure 1-2). As the scales become larger, buildings, parking garages, athletic fields appear (Figure 1-3); then road networks appear through campus and suburban neighborhoods and metropolitan features appear—areas of intensive development that are linked by roads (Figure 1-4); road patterns are evident, yet individual roads are scarcely visible (Figure 1-5); land uses of urban areas, suburban housing, agricultural fields, and forests appear (Figure 1-6); geological and hydrological features, such as mountains ranges and coastlines, with large urban areas are still visible (Figure 1-7); and finally, geomorphological structures and land, ocean, and atmosphere interactions that mediate climate change and sea-level change appear (Figure 1-8).
Examination from the perspective of a square meter in the middle of a road to that of a continent reveals three observations. First, as the extent and grain of scale increases, distinct objects appear and persist over distinct scale ranges and are replaced by others that are either aggregates of objects at smaller scales or new objects. For example, the double strip marking in the center of the road is no longer visible at extents of a kilometer, and road segments visible at smaller extents aggregate to become road networks in larger extents. At each such range of scales, the identifiable objects have geometric properties. For example, the metric of road density is only applicable at scales with extents greater than about 1 km. The second observation is that the patterns and structures change across scales; that is, these complex systems are not self-similar and amenable to simple scaling relationships. Ecological structures persist over given scale ranges, then change as the sets of processes that organize those structures change.
For example, the effects of runoff from a road on accumulation of toxic material and subsequently on individual plants may include several scale ranges.
The chemistry of interactions among compounds occurs at small scales (orders of magnitude smaller than the first image (Figure 1-1). Effects of toxins on individual biota are evaluated at scales depicted in Figure 1-1 or 1-2. At scales of the region (Figure 1-7), local accumulations of toxins may be undetectable. The third observation is that the effects change with scale and medium (water, air, or land). Human effects on the atmosphere cross a wide range of scales—exhausts from individual vehicles can accumulate and aggregate to have planetary effects on atmospheric carbon dioxide and other greenhouse gases. Human influence on land, however, appears more local. For example, roads and even road networks affect hydrological and geological processes but only at scales up to a watershed and not at continental scales.
The sequence of photographs suggests that road structures and attributes vary with scale. The attributes of paved roads (materials of asphalt, gravel, and concrete) and thicknesses are described in spatial extents of observation from millimeters to centimeters. At scales of centi-
meters to meters, road markings, curbs, drains, and culverts are objects of design and construction. At scales of a few meters, the structures of pavement width, shoulders, travel lanes, and paved envelope are germane. Many of the ecological effects are contained within a kilometer of the paved roads, an area often referred to as the road-effect zone. At scales of multiple kilometers, road densities, patterns, traffic use, and alignments are elements of the road system.
These simple models of scales—especially scales at which roads affect ecological systems—and the integration of different disciplinary approaches to road ecology lay the foundation for the discussions in this report.
Integration of Ecological and Social Systems
Perhaps one of the most difficult challenges facing society is the integration of human-development activities and ecological-resource con-
servation. This challenge is particularly true of all phases of roads—from construction to use to removal—as all of these actions interact with and alter ecological systems (Forman et al. 2003).
Ecological systems are defined as systems comprising biotic (organisms) and abiotic (physical) components (Odum 1983). The biotic components interact with abiotic components (such as solar energy, water, air, and nutrients) in ways that generate complex and diverse structures. The interaction between structures and processes has been described as self-organization (Odum 1983; Levin 1999), where structure and process mutually reinforce one another. In the context of this report, the committee uses the term ecological systems to describe organizational units that cover a wide range of spatial scales, from centimeters (such as a Petri dish) to thousands of kilometers (the Pacific Ocean). Even though ecological systems are not restricted to any particular temporal or spatial scale, the spatial scale and time are two key dimensions (measurable extent) for understanding ecological systems. The components of a generic forest ecosystem also cover wide ranges of scale, as
shown in Figure 1-9. Some of these structures cover only a distinct range of scales due to processes that limit size. Cells, leaves, and trees are limited in size. Other structures and processes have wide ranges of scale. Emissions from the combustion of fossil fuels can aggregate to produce global scale effects.
A human social system is defined as a group of people who share understanding, norms, and routines to accomplish activities or fulfill key functions (Westley 2002). They may be organized to achieve goals or objectives or fulfill other needs. Ecological systems are organized around space and time dimensions, and human systems are organized around the number of people. They may be as small as a family of two or as large as a nation.
There are several ways to conceptualize the relationship between ecological and human social systems.
The most appropriate approach for this report is a “linked” system perspective and suggests that ecological and human components have a
set of rules and structures and that it is most important to focus on linkages and feedbacks between these components. Berkes and Folke (1998), NRC (1999), and Gunderson and Holling (2002) all underscore the need for new science and approaches to understand the dynamics and complexities of these linked systems of people and nature. For example, Sutter (2002) traces the roots of the wilderness movement as a response to road construction in the early twentieth century. The committee adopts a linked perspective in this report and develops it in subsequent chapters.
ORGANIZATION OF REPORT
This report addresses and is limited to the statement of task as agreed on by the NRC and FHWA. To address how ecological consid-
erations could better be integrated into all phases of road development—assessment, planning, construction, and management—the remainder of this report is structured in seven chapters.
Chapter 2 provides a brief history and baseline description of the current state of the road system in the United States. The description includes function definitions of types of road systems (interstate, arterials, and collectors), the layout or patterns of roads, and ownership and maintenance responsibilities. Chapter 2 also describes the current status of pavement, bridge conditions, and future projections of spending and other required capital investments.
Chapter 3 addresses the effects of roads on ecological conditions by using spatial scale to sort ecological effects. It also examines how ecosystem goods and services are altered by road activities. The focus of the chapter is a literature review of the documented changes in ecological structure and functions (and goods and services) by scale of impact. A
large part of the scientific knowledge of ecological effects of roads has been based on short-term studies focused on narrowly defined objectives and have generally been related to specific construction or planning needs. As a result, little is known about ecological effects that occur over large spatial areas or long (decades) time frames. Ecological conditions are affected not only by the construction of the road and road appurtenances (bridges) but also by the traffic on the road and, at larger scales, by increases in road density. The committee’s review has shown the ecological effects of roads to be much larger than the road itself and can extend far beyond regional planning domains. Many studies have failed to address the complex nature of the ecological effects of roads. Studies assessing ecological effects are often based on small sampling periods and therefore do not adequately sample the range of variability in ecological systems. Little is known about how roads affect the different components of biodiversity (genetic, species and population, community, and ecosystem). Information on the resiliency of biodiversity compo-
nents to road-related disturbances is needed to better understand the effects of roads on ecological systems.
Chapter 4 outlines existing and potential opportunities at different scales for mitigating the ecological effects associated with three major phases of road projects: planning, construction, and maintenance.
Chapter 5 reviews the existing laws, regulations, and policies and their influence on the interaction between ecology and roads. Although a wide range of laws, regulations, and policies require some degree of consideration of ecological effects of road construction, the existing legal structure leaves significant gaps. Road projects need only permits to affect certain types of ecological features—wetlands, endangered species, and migratory birds—and generally at a small scale. Moreover, the permit programs generally only consider direct effects of road construction or use on the protected resource. With few exceptions (for example, wetlands and endangered species), existing law authorizes ecological con-
cerns to be balanced with goals of transportation mobility, capacity, and other social needs in determining whether and how to undertake transportation projects. This can create controversy between supporters of a project and opposers who raise issues of ecological protection. These are primarily federal level acts that either directly or indirectly influence activities at smaller spatial scales.
Chapter 6 addresses the current practices of planning and assessing road projects. Large-scale planning processes (such as long-range transportation plan) are required to address only air quality issues (such as attainment of standards) and generally do not address other environmental issues. Integration of environmental concerns into transportation planning should be done earlier in the planning processes. Such organizations as the metropolitan planning organizations should conduct first-level screenings for environmental considerations in transportation improvements before the development of a transportation improvement
program (TIP). TIP is a fiscally balanced, itemized list of all federal and regionally important state-funded transportation projects planned for the metropolitan area usually covering 2 years. Federal transportation agencies, such as FHWA and Federal Transit Administration, require that all projects using federal funds come from an adopted transportation improvement program. Transportation and conservation planning at the state level should be integrated. Further development of a first-level screening assessment (rapid assessment) should be conducted for use early in the planning process. To streamline environmental assessments, two steps are needed: (1) more spatial and temporal environmental data should be gathered, and (2) a set of models must be developed for using those data in assessments to address concerns dealing with scale, feedbacks, and mixed criteria for environmental protection. Transportation agencies have an opportunity to be information brokers and to foster planning forums that integrate environmental planning and assessment across governmental agencies, nongovernmental organizations, stakeholders, and the public. This chapter describes the new and emerging technologies that could be used to improve the practices. It also describes some conceptual approaches to achieve better integration of social and ecological objectives and to assess environmental concerns of road projects.
Ecological systems cover wide ranges of spatial and temporal scales. The complexity and cross-scale interactions in ecological systems generates problems for assessment and planning. Multiple assessments must be developed, each at different spatial and temporal scales to address key ecosystem processes and structures. Ecological concerns should be included early in the assessment and planning processes. Although great progress has been made in understanding and mitigating effects of roads, much more is needed. The development of a broader set of robust ecological indicators and learning-based institutions will help facilitate assessment and planning. Better integration of the social institutions will likely require the development of new relationships among existing institutions. Transportation agencies have an opportunity to play a key role in interconnecting and integrating planning and management of environmental issues. New types of institutions are needed to address the mix of socioeconomic and ecological concerns.