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NOTICE: The project that is the subject of this report was approved by the Governing Board of the National Research Council, whose members are drawn from the councils of the National Academy of Sciences, the National Academy of Engineering, and the Institute of Medicine. The members of the committee responsible for the report were chosen for their special competences and with regard for appropriate balance.

This project was supported by Contract No. DTFH61-01-C-00036 between the National Academy of Sciences and the Department of Transportation. Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the organizations or agencies that provided support for this project.

Cover design by Liza R. Hamilton, National Research Council. Front photo by Anthony P. Clevenger, Montana State University. Back photos used with permission from Lance H. Gunderson, Emory University (double yellow line), Emory University Facilities Management (campus), and USGS (southeastern United States).

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