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The attempt to express, much less understand, the nature of the relationships inherent in transportation and its myriad components is a challenge for transportation educators. Fortunately, the book you hold in your hands explains many of these interactions and complexities, as well as the design problems that go with them. Another challenge that transportation engineering poses, as a comparatively young discipline, is that professionals have to constantly deal with a plethora of new problems that seem to crop up faster than they can be tackled. In short, transportation is an exciting field. The purpose of this edition, like previous editions, is to cover the major areas of traffic engineering and transportation planning at an introductory level, and provide the student with a basic rationale and a set of design concepts. The primary audience for this textbook is upper-division undergraduates and master-level students studying for degrees in civil engineering, as well as those in urban planning, economics, management, and other related disciplines. The orientation of the book is pragmatic and multidisciplinary, providing both students and practitioners with a variety of worked examples illustrating the basic concepts, covering an unusually wide spectrum of topics in transportation planning and traffic engineering. While the general structure of this third edition remains generally the same as in previous editions a number of chapters have been thoroughly revised in light of the Highway Capacity Manual, published in 2000 and issued by the Transportation Research Board, as well as the latest material on geometric design from the AASHTO Publications. In addition, three new appendices have been added to this edition. Appendix B covers the elements of statistics and probability that are frequently used in transportation engineering. Appendices C and D consist of material drawn from the Bureau of Transportation Statistics, U. S. Department of Transportation, Washington, DC. Minor additions have been made in many of the chapters to reflect changes in design practice and policies that have occurred since the last edition. Many students, professionals, and colleagues have been curious to know the significance of the Mobius band that you see on the cover of the book. We are flattered, and a word regarding it is perhaps in order. The Mobius band throws some light on the interdisciplinary character and complexity embedded in transportation, which is one of its unique features. It is a paradox with geometrical and topological characteristics that are fascinating and special. In reality, the Mobius band has only one edge and one side. If you imagine yourself standing on the band, you see two edges and two sides. But when you begin walking along the band, you eventually return to your starting place and find yourself on the other side of the band. Indeed, the Mobius band resembles the interdisciplinary nature of transportation engineering, embracing several areas of knowledge blending together--it is the one and yet it is the many. It drives home the point that it is just an accident of history that transportation engineering finds its home in civil engineering and that the recent advances in this discipline easily warrant a department of its own. The authors are grateful to many colleagues, professionals, and students who have suggested improvements, and many of these have been incorporated in this edition. Several of our students, past and present, have assisted us in revising this edition; namely, Dr. PS. Sriraj, Dr. Cemal Ayvalick, Turan Arslan, Porson Chantra, Sagar Sonar, Sameer Patil, and Edward Anderson. We would also like to thank Peter T. Martin, Utah Traffic Laboratory, and Shashi Sathisan Nambisan, University of Nevada-Howard K. Hughes College of Engineering, for reviewing the manuscript. Jenny Kincaid and Margaret Lall lent invaluable assistance in proofreading sections of the book. We also acknowledge admirable support provided by Kevin Bradley, Sunflower Publishing Services, and Laura Fischer of Prentice Hall. Finally, we would appreciate any suggestions, criticisms, and corrections from our readers. C. JOTIN KHISTY B. KENT LALL Excerpted from Transportation Engineering: An Introduction by C. Jotin Khisty, B. Kent Lall All rights reserved by the original copyright owners. Excerpts are provided for display purposes only and may not be reproduced, reprinted or distributed without the written permission of the publisher.