NOTICE: This publication has been reviewed according to procedures approved by a National Academy of Engineering report review process. Publication of signed work signifies that it is judged a competent and useful contribution worthy of public consideration, but it does not imply endorsement of conclusions or recommendations by the NAE. The interpretations and conclusions in such publications are those of the authors and do not purport to represent the views of the council, officers, or staff of the National Academy of Engineering.

Funding for the activity that led to this publication was provided by Cummins Engine Company, DaimlerChrysler Corporation Fund, Defense Advanced Research Projects Agency, Department of Defense Research and Engineering, Hewlett-Packard Company, Lockheed Martin Corporation, Lucent Technologies, Microsoft Corporation, National Aeronautics and Space Administration, Parsons Brinckerhoff, Inc., Pratt & Whitney, Sandia National Laboratories, Science Applications International Corporation, Xerox Foundation, Xerox Research and Technology, and the National Academy of Engineering Fund.

Printed in the United States of America

Copyright © 2001 by the National Academy of Sciences. All rights reserved.