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.

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