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 study was supported by Contract # NASW-99037, Task Order #103 between the National Academy of Sciences and the National Aeronautics and Space Administration. Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the authors and do not necessarily reflect the views of the organizations or agencies that provided support for the project

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The cover design incorporates an image from a simulation of airplane traffic over the United States at 6:30 p.m. EDT on September 6, 2001. The simulation was produced using Future ATM Concepts Evaluation Tool (FACET) by the Aviation Systems Division at the NASA Ames Research Center, Moffett Field, CA.