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NATIONAL COOPERATIVE HIGHWAY RESEARCH PROGRAM NCHRP SYNTHESIS 399 Systematic, well-designed research provides the most effective Project 20-5 (Topic 39-08) approach to the solution of many problems facing highway administra- ISSN 0547-5570 tors and engineers. Often, highway problems are of local interest and ISBN 978-0-309-09842-7 can best be studied by highway departments individually or in coop- Library of Congress Control No. 2009928789 eration with their state universities and others. However, the accelerat- ing growth of highway transportation develops increasingly complex problems of wide interest to highway authorities. These problems are best studied through a coordinated program of cooperative research. 2009 Transportation Research Board In recognition of these needs, the highway administrators of the American Association of State Highway and Transportation Officials initiated in 1962 an objective national highway research program COPYRIGHT PERMISSION employing modern scientific techniques. This program is supported Authors herein are responsible for the authenticity of their manuscripts on a continuing basis by funds from participating member states of and for obtaining written permissions from publishers or persons who the Association and it receives the full cooperation and support of own the copyright to any previously published or copyrighted material the Federal Highway Administration, United States Department of used herein. Transportation. Cooperative Research Programs (CRP) grants permission to repro- The Transportation Research Board of the National Research Coun- duce material in this publication for classroom and not-for-profit pur- cil was requested by the Association to administer the research pro- poses. Permission is given with the understanding that none of the mate- gram because of the Board's recognized objectivity and understanding rial will be used to imply TRB, AASHTO, FAA, FHWA, FMSCA, FTA, of modern research practices. The Board is uniquely suited for this or Transit development Corporation endorsement of a particular product, purpose as it maintains an extensive committee structure from which method, or practice. It is expected that those reproducing the material in authorities on any highway transportation subject may be drawn; it this document for educational and not-for-profit uses will give appropri- possesses avenues of communication and cooperation with federal, ate acknowledgment of the source of any development or reproduced state, and local governmental agencies, universities, and industry; its material. For other uses of the material, request permission from CRP. relationship to the National Research Council is an insurance of objec- tivity; it maintains a full-time research correlation staff of specialists NOTICE in highway transportation matters to bring the findings of research directly to those who are in a position to use them. The project that is the subject of this report was a part of the National The program is developed on the basis of research needs identified Cooperative Highway Research Program conducted by the Transpor- by chief administrators of the highway and transportation departments tation Research Board with the approval of the Governing Board of and by committees of AASHTO. Each year, specific areas of research the National Research Council. Such approval reflects the Governing needs to be included in the program are proposed to the National Board's judgment that the program concerned is of national impor- Research Council and the Board by the American Association of State tance and appropriate with respect to both the purposes and resources Highway and Transportation Officials. Research projects to fulfill of the National Research Council. these needs are defined by the Board, and qualified research agencies The members of the technical committee selected to monitor this are selected from those that have submitted proposals. Administration project and to review this report were chosen for recognized scholarly and surveillance of research contracts are the responsibilities of the competence and with due consideration for the balance of disciplines National Research Council and the Transportation Research Board. appropriate to the project. The opinions and conclusions expressed or The needs for highway research are many, and the National Coop- implied are those of the research agency that performed the research, erative Highway Research Program can make significant contributions and, while they have been accepted as appropriate by the technical com- to the solution of highway transportation problems of mutual concern mittee, they are not necessarily those of the Transportation Research to many responsible groups. The program, however, is intended to Board, the National Research Council, the American Association of complement rather than to substitute for or duplicate other highway State Highway and Transportation Officials, or the Federal Highway research programs. Administration of the U.S. Department of Transportation. Each report is reviewed and accepted for publication by the tech- nical committee according to procedures established and monitored by the Transportation Research Board Executive Committee and the Governing Board of the National Research Council. Published reports of the NATIONAL COOPERATIVE HIGHWAY RESEARCH PROGRAM are available from: Transportation Research Board Business Office NOTE: The Transportation Research Board of the National Acad- 500 Fifth Street, NW emies, the National Research Council, the Federal Highway Adminis- Washington, DC 20001 tration, the American Association of State Highway and Transporta- tion Officials, and the individual states participating in the National and can be ordered through the Internet at: Cooperative Highway Research Program do not endorse products or manufacturers. Trade or manufacturers' names appear herein solely because they are considered essential to the object of this report. Printed in the United States of America