There are 4 million miles of roads in the United States. One hundred years ago, roads were primarily unpaved and had half the number of miles of the present U.S. road system. As the system grew, roads became wider and more complex structurally to provide for more and heavier traffic. New construction technology and greater structural stability were needed to improve the road system.
All phases of road development—from construction and use by vehicles to maintenance—affect physical and chemical soil conditions, water flow, and air and water quality. Roads alter habitats, increase wildlife mortality, and disperse nonnative pest species of plants and animals. At larger scales, roads affect wildlife migration patterns. In some cases, roads can also enhance roadside habitats for native species.
The importance of integrating environmental considerations into all phases of transportation is emphasized in legislation. The Transportation Equity Act for the Twenty-First Century (TEA-21) of 1998 called for protection of the environment by initiating transportation projects that would improve environmental quality and support fuel efficiency, cleaner fuels, and alternative transportation. The act called for streamlining procedures to reduce red tape and paperwork in transportation project reviews without compromising environmental protection.
Consideration of environmental issues in road development has been an evolving process. The increasing awareness of environmental issues, regulatory changes, and new solutions have made road development more complex and controversial. Many believe that environmental protection can be compatible with streamlining the project approval process through effective planning and coordination. Suggestions on
how environmental and transportation goals can be better integrated have been developed by government agencies and nongovernmental organizations. Approaches include more integrated planning and interagency coordination, consideration of alternative designs earlier in the planning process, and consideration of mitigation strategies, such as installation of wildlife crossings and native vegetation management. As the road system expands and construction and management require additional resources, more is understood about the impact of roads on the environment, but much remains to be learned. To address these matters, better understanding of road ecology and improved methods of integrating that understanding into all aspects of road development are needed.
Over the past two decades, the Federal Highway Administration and state transportation agencies have increasingly recognized the importance of the effects of transportation facilities on the natural environment. The importance of this issue was reflected by congressional action in Section 5107(b)(4) of TEA-21, 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 to 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 authorized the secretary to arrange for a study of this relationship by the National Research Council (NRC). In response, at the request of the Federal Highway Administration, the NRC established the Committee on Ecological Impacts of Road Density (see Statement of Task in Box S-1). This committee’s report attempts to provide guidance on ways to reconcile the different goals of road development and environmental conservation.
The term “road density” is frequently used to mean the average total road length per unit area of landscape. However, roads also have widely varying widths; therefore, lane miles per square mile (or lane length per unit area) is a better measure of density because it takes into account the differences between, for example, multilane expressways and two-lane rural roads. The concept of road density was developed as a way of quantifying one aspect of a road network and is applicable at scales larger than a road segment. 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 can be used to describe road pattern and structure.
BOX S-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 functioning 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.
There are cases in which the meaning of the term “road density” is clear, but often it may be difficult to make useful comparisons between the ecological effects of different types of road networks. For example, several two-lane roads that have little traffic versus fewer, eight-lane roads that are heavily traveled. Therefore, the committee focused on variables that contribute to density, such as highway length and portion of land covered, rather than strictly on density, and used the broader concept of “scale” for evaluating environmental effects.
The committee focused on the ecological effects of federally funded paved highways in urban and rural locations. The committee did not focus on urban street networks, and no consideration was given to the ecological effects of unpaved roads, such as those found in federal forests, wilderness areas, wetlands, parks, and farms, or the ecological effects of state and local roads. The committee did not address global or regional climate effects, such as how potential climate changes might affect the interactions of organisms and the environment associated with roads and vehicles or how roads and traffic might influence climate. However, local climate interactions with road ecology are considered in this report.
Developing policy choices to balance mobility, economic growth, and environmental protection goals has been important and challenging for more than 50 years. Although the committee was not charged to evaluate such policy choices, it identified the ecological effects of roads that can be evaluated in the planning, design, construction, and maintenance of roads. The committee did not address human ecological factors; nonecological factors, such as safety; efficient movement of vehicles; or protection of farmlands, publicly owned recreation lands, and scenic, historic, and cultural areas. The committee also did not address such factors as urban sprawl or suburban growth; project costs; statewide, regional, and local planning goals; and the economic viability of the communities of users.
ECOLOGICAL EFFECTS OF ROADS
Perhaps the most noticeable ecological effect of roads is direct, vehicle-related mortality (animals killed by collisions with vehicles). Although it is not the most threatening effect of roads for most species, mortality can reduce wildlife-population densities and ultimately affect
the survival probability of local populations, including endangered or threatened species, such as the Florida panther and grizzly bear. In addition to vehicle-related mortality, roads—acting as barriers to wildlife movement—may affect wildlife-population structure by disrupting breeding patterns or impairing reproductive success because they can fragment and isolate populations. In extreme cases, the resulting limitation of gene flow could result in local extirpation of a species. Properly designed mitigation measures, such as wildlife-crossing structures, can facilitate wildlife movement across roads and reconnect isolated populations. Fish movement can also be blocked by road-crossing structures, such as culverts (usually a large pipe under a road where it crosses a stream) that are improperly designed or not present at all. Some fishes avoid moving through culverts, possibly because of the increased speed of the water flow, even if there are better habitat conditions on the opposite side. Reluctance to move, for example, downstream, could contribute to isolating upstream populations and, in some cases, localized extirpations.
In evaluating the ecological effects of roads, it is important to consider the physical, socioeconomic, and legal context, as well as the ecological context. Each has spatial and temporal dimensions. The term “road-effect zone” means the distance from a stretch of road or road segment that ecological effects can be detected. The road-effect zone is usually asymmetric extending outward on either side of the road, with varying zone boundaries. The effect of distance varies, depending on the species, location, and disturbance type. For example, animals avoid roads by a distance that increases with increasing traffic volume, and that distance varies by species. Noise from high-traffic-volume roads reduces the breeding densities and distribution of many bird species within a 40-to 1,500-m zone. Increased traffic and road density negatively affect aquatic habitats and the species that depend on them. For example, wetland species diversity is negatively correlated with paved roads up to 2 km away. Other disturbances, such as heavy metals and chemical pollution, can degrade habitat quality in the road-effect zone up to 100 m and 200 m, respectively. Vehicle-generated pollutants (such as nitrogen oxides, petroleum, lead, copper, chromium, zinc, and nickel) are the primary pollutants associated with road use. Along with pollutants from spills, litter, and adjacent land uses, they accumulate on impervious roads and enter waterways via surface runoff or atmospheric deposition. Runoff contaminated with road salt can damage vegetation and potentially
cause a shift in plant community structure when salt-sensitive plant species are replaced by less-sensitive species, such as cattails and common reed grass. Salt-related vegetation changes can also affect wildlife by adversely altering habitat, inhibiting road crossing by amphibian species, and causing behavioral and toxicological impacts on birds and mammals.
Similarly, air pollution from vehicle exhaust (volatile organic compounds, nitrogen oxides, carbon monoxide, and particulate matter) can alter the composition of roadside vegetation, promoting a few dominant plant species at the expense of more sensitive species, such as ferns, mosses, and lichens. This effect can extend up to 200 m from multilane highways and up to 35 m from two-lane highways.
The underlying topography, aspect (the direction a site faces or its exposure), geology, soils, ecological conditions, and land cover all influence how a road affects the environment. For example, the environmental effects of a road that does not cross a river are different from the effects of one that crosses a river several times in a few kilometers. New patterns of water runoff can develop as the local topography is altered. Aspect can influence how quickly snow and ice melt off the road and adjacent surfaces. Original topography, geology, and soils often dictate the road path and provide construction constraints or opportunities. The environmental effects of a road also depend on the prevailing land cover and use, such as wildlands, wetlands, agricultural lands, or a river valley versus a ridge. In fire-prone landscapes, a road can serve as a firebreak if the road is wide enough or as a source of fire initiation if access to the surrounding environment is increased.
Ecological productivity is influenced by roads. The roadside between the paved road and prevailing land cover often has lower productivity and different composition than the surrounding landscape (especially for roads through forests). The native habitat conditions of a roadside are frequently altered, but when the surrounding landscape is greatly altered by development, roadsides can include some of the last remaining habitats, especially for certain native plant species and some insects, birds, and small mammals. Roadside areas can also facilitate the establishment of nonnative plants transported by vehicles, among other mechanisms, including the clearing of land during road construction. Biodiversity along roads typically is different from that in the surrounding landscape. Plants along roads must survive vehicular pollution, exposure to bright sunlight, dry soils, and regular mowing. Roadside plantings in the United States once consisted of grasses and herbs (often of European origin) known to thrive in stressful conditions. Now there is
an effort to plant vegetation along many highways, some of which is selected because it is native to the United States (but not always from the local area). The linear pathways of continuous terrestrial or aquatic habitat adjacent to roads can serve as corridors for animal movement. Some animals are attracted to roadside vegetation, road kill (an animal that has been killed on a road by a motor vehicle), or the light and heat often associated with roads, and other animals are deterred by disturbances in the road-effect zone.
The ecological effects of building a road typically exhibit several time lags. Some effects of road construction are not realized until several months or even decades after a road is completed as nearby trees and other plants slowly die, although the most severe (condensed and sudden) effects typically occur when construction begins. Vegetation reestablishment efforts may result in a quick pulse of plant growth after seeding and fertilization, but the new equilibrium of vegetation along roadsides usually takes some time to establish, particularly in locations with steep slopes, rocky or nonorganic substrate, or other conditions that encourage roadside erosion.
Although most of the current and foreseeable transportation projects in the United States are along established roads, the increase in traffic volume on these roads and the selection of sites for new roads bring to the forefront the potential for new ecological impacts—and associated, often delayed responses of the environment.
Understanding and Assessing Road Effects
As described above, a great deal is known about the ecological effects of roads, even though there is need for more and better information about cumulative, long-term, and large-scale effects. The available information, much of it reviewed, summarized, and synthesized in this report, should be used in all stages of road building and maintenance, including planning.
From planning through construction stages, ecological indicators are important in assessing road effects; however, determining the broader and cumulative effects of roads and their corridors also is important and often not captured by indicators. Ecological indicators are generally developed to quantify ecological responses to a variety of factors. Several indicators have been proposed to measure or monitor ecological effects, and some of them are applicable to the effects of roads.
Ecological effects of roads at local scales (within a few kilometers of the roads) have been widely studied, documented, and understood, while effects at large scales are less documented and understood. More is known about the effects of bridges, overpasses, and culverts on flows of materials and organisms than about the effects of roads on larger patterns and processes, such as watersheds or migratory pathways. The lack of information at large scales is related to many factors, such as (1) legal and policy directives that guide what components of ecosystems must be considered; (2) planning and assessment practices that restrict scales; (3) limitations of data, indicators, and methods at broad scales; and (4) limited financial and technical support for ecological investigations at large scales.
CONCLUSIONS AND RECOMMENDATIONS
CONCLUSION: Most road projects today involve modifications to existing roadways, and the planning, operation, and maintenance of such projects often are opportunities for improving ecological conditions. A growing body of information describes such practices for improving aquatic and terrestrial habitats.
Recommendation: The many opportunities that arise for mitigating or reducing adverse environmental impacts in modifications and repairs to existing roads should not be overlooked. Environmental considerations should be included when plans are made to repair or modify existing roads, as well as when plans are made to build new roads.
CONCLUSION: Planning boundaries for roads and assessing associated environmental effects are often based on socioeconomic considerations, resulting in a mismatch between planning scales and spatial scales at which ecological systems operate. In part, this mismatch results because there are few legal incentives or disincentives to consider environmental effects beyond political jurisdictions, and thus decision making remains primarily local. The ecological effects of roads are typically much larger than the road itself, and they often extend beyond regional planning domains.
Scientific literature on ecological effects of roads generally addresses local-to-intermediate scales, and many of those effects are well documented. However, there are few integrative or large-scale studies. Sometimes the appropriate spatial scale for ecological research is not
known in advance, and in that case, some ecological effects of roads may go undetected if an inappropriate scale is chosen. Few studies have addressed the complex nature of the ecological effects of roads, and the studies that have done so were often based on small sampling periods and insufficient sampling of the range of variability in ecological systems.
Recommendation: Research on the ecological effects of roads should be multiscale and designed with reference to ecological conditions and appropriate levels of organization (such as genetics, species and populations, communities, and ecological systems.)
Recommendation: Additional research is needed on the long-term and large-scale ecological effects of roads (such as watersheds, ecoregions, and species’ ranges). Research should focus on increasing the understanding of cross-scale interactions.
Recommendation: More opportunities should be created to integrate research on road ecology into long-term ecological studies by using long-term ecological research sites and considering the need for new ones.
Recommendation: Ecological assessments for transportation projects should be conducted at different time scales to address impacts on key ecological system processes and structures. A broader set of robust ecological indicators should be developed to evaluate long-term and broad-scale changes in ecological conditions.
CONCLUSION: The assessment of the cumulative impacts of road construction and use is seldom adequate. Although many laws, regulations, and policies require some consideration of ecological effects of transportation activities, such as road construction, the legal structure leaves substantial gaps in the requirements. Impacts on certain resources are typically authorized through permits. Permitting programs usually consider only direct impacts of road construction and use on a protected resource, even though indirect or cumulative effects can be substantial (for example, effects on food web components). The incremental effects of many impacts over time could be significant to such resources as wetlands or wildlife.
Recommendation: More attention should be devoted to predicting, planning, monitoring, and assessing the cumulative impacts of
roads. In some cases, the appropriate spatial scale for the assessment will cross state boundaries, and especially in those cases, collaboration and cooperation among state agencies would be helpful.
CONCLUSION: The methods and data used for environmental assessment are insufficient to meet the objectives of rapid assessment, and there are no national standards for data collection. However, tools for in situ monitoring, remotely sensed monitoring, data compilation, analysis, and modeling are continually being improved, and because of advances in computer technology, practitioners have quick access to the tools. The new and improved tools now allow for substantial improvements in environmental assessment.
Recommendation: Improvements are needed in assessment methods and data, including spatially explicit models. A checklist addressing potential impacts should be adapted that can be used for rapid assessment. Such a checklist would focus attention on places and issues of greatest concern. A national effort is needed to develop standards for data collection. A set of rapid screening and assessment methods for environmental impacts of transportation and a national ecological database based on the geographic information system (GIS) and supported by multiple agencies should be developed and maintained for ecological effects assessment and ecological system management across all local, state, and national transportation, regulatory, and resource agencies. Standard GIS data on road networks (for example, TIGER) could be interfaced with data models (for example, UNETRANS) to further advance the assessment of ecological impacts of roads.
Recommendation: The committee recommends a new conceptual framework for improving integration of ecological considerations into transportation planning. A key element of this framework is the integration of ecological goals and performance indicators with transportation goals and performance indicators.
Recommendation: Improved models and modeling approaches should be developed not only to predict how roads will affect environmental conditions but also to improve communication in the technical community, to resolve alternative hypotheses, to highlight and evaluate data and environmental monitoring, and to provide guidance for future environmental management.
CONCLUSION: With the exception of certain legally specified ecological resources, such as endangered or threatened species and protected wetlands, there is no social or scientific consensus on which ecological resources affected by roads should be given priority attention. In addition, current planning assessments that focus on transportation needs rarely integrate other land-management objectives in their assessments.
Recommendation: A process should be established to identify and evaluate ecological assets that warrant greater protection. This process would require consideration not only of the scientific questions but also of the socioeconomic issues. The Federal Highway Administration should consider amending its technical guidance, policies, and regulations based on the results of such studies.
CONCLUSION: The state transportation project system offers the opportunity to consider ecological concerns at early planning stages. However, planning at spatial and temporal scales larger than those currently considered, generally does not address ecological concerns until later in a project’s development.
Recommendation: Environmental concerns should be integrated into transportation planning early in the planning process, and larger spatial scales and longer time horizons should be considered. Adding these elements would help to streamline the planning process. Metropolitan planning organizations and state departments of transportation should conduct first-level screenings for potential environmental effects before the development of a transportation improvement plan. Transportation planners should consider resource-management plans and other agencies’ (such as the U.S. Corps of Engineers, U.S. Environmental Protection Agency, U.S. Fish and Wildlife Service, and National Park Service) environmental plans and policies as part of the planning process. Other agencies should incorporate transportation forecasting into resource planning.
CONCLUSION: Elements of the transportation system, including the types of vehicles and their fuels, will continue to evolve. Changes in traffic volume and road capacity, mostly through widening of roads rather than construction of new corridors, have smaller but nevertheless important ecological effects compared with the creation of new, paved roads.
Recommendation: Monitoring systems should be developed for the evaluation and assessment of environmental effects resulting from changes in the road system—for example, traffic volume, vehicle mix, structure modifications, and network adjustments. Data from monitoring could then be used to evaluate previous assessments and, over the long term, improve understanding of ecological impacts.
CONCLUSION: Much useful information from research on the ecological effects of roads is not widely available because it is not in the peer-reviewed literature. For example, studies documenting the effects of roads on stream sedimentation have been reported in documents of state departments of transportation, the U.S. Army Corps of Engineers, and the World Bank. Although much of this literature is available through bibliographic databases, it is not included in scientific abstracting services and may not be accessible to a broader research community. Also, the data needed to evaluate regulatory programs are not easily accessible or amenable to synthesis. The data are typically contained in project-specific environmental impact statements, environmental assessments, records of decision, or permits (for example, wetlands permits), which are not easily available to the scientific community.
Recommendation: Studies on ecological effects of roads should be made more accessible through scientific abstracting services or through publication in peer-reviewed venues. The Federal Highway Administration, in partnership with state and federal resource-management agencies, should develop environmental information and decision-support systems to make ecological information available in searchable databases.
CONCLUSION: Transportation agencies have been attempting to fill an institutional gap in ecological protection created by the multiple social and environmental issues that must be addressed at all phases of road development. The gaps often occur when problems arise that are not covered by agency mandates or when agencies need to interact with other organizations in new ways. Even when transportation agencies work toward environmental stewardship, they cannot always do the job alone.
Recommendation: Transportation agencies should continue to expand beyond their historical roles as planners and engineers, increasing their roles as environmental coordinators and stewards. Transporta-
tion planners and natural-resource planners should collaborate to promote integrated planning at comparable scope and scale so that the efforts can support mutual objectives. This collaboration should include federal, state, and county resource-management agencies; nongovernmental organizations; and organizations and firms involved in road construction. Incentives, such as funding and technical support, should be provided to help planning agencies, resource agencies, nongovernmental groups, and the public to understand ecological structure and functioning across jurisdictions and to interact cooperatively.