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NATIONAL COOPERATIVE HIGHWAY RESEARCH PROGRAM NCHRP SYNTHESIS 389 Systematic, well-designed research provides the most effective Project 20-5 (Topic 37-09) 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 978-0-309-09825-0 can best be studied by highway departments individually or in coop- Library of Congress Control No. 2008910982 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 2009 Transportation Research Board best studied through a coordinated program of cooperative research. In recognition of these needs, the highway administrators of the American Association of State Highway and Transportation Officials COPYRIGHT PERMISSION initiated in 1962 an objective national highway research program Authors herein are responsible for the authenticity of their manuscripts employing modern scientific techniques. This program is supported and for obtaining written permissions from publishers or persons who on a continuing basis by funds from participating member states of own the copyright to any previously published or copyrighted material the Association and it receives the full cooperation and support of the used herein. Federal Highway Administration, United States Department of Trans- Cooperative Research Programs (CRP) grants permission to repro- portation. duce material in this publication for classroom and not-for-profit pur- The Transportation Research Board of the National Research Coun- poses. Permission is given with the understanding that none of the mate- cil was requested by the Association to administer the research pro- rial will be used to imply TRB, AASHTO, FAA, FHWA, FMSCA, FTA, gram because of the Board's recognized objectivity and understanding or Transit development Corporation endorsement of a particular product, of modern research practices. The Board is uniquely suited for this method, or practice. It is expected that those reproducing the material in purpose as it maintains an extensive committee structure from which this document for educational and not-for-profit uses will give appropri- authorities on any highway transportation subject may be drawn; it ate acknowledgment of the source of any development or reproduced possesses avenues of communication and cooperation with federal, material. For other uses of the material, request permission from CRP. state, and local governmental agencies, universities, and industry; its relationship to the National Research Council is an insurance of objec- NOTICE tivity; it maintains a full-time research correlation staff of specialists in highway transportation matters to bring the findings of research The project that is the subject of this report was a part of the National directly to those who are in a position to use them. Cooperative Highway Research Program conducted by the Transpor- The program is developed on the basis of research needs identified tation Research Board with the approval of the Governing Board of by chief administrators of the highway and transportation departments the National Research Council. Such approval reflects the Governing and by committees of AASHTO. Each year, specific areas of research Board's judgment that the program concerned is of national impor- needs to be included in the program are proposed to the National tance and appropriate with respect to both the purposes and resources Research Council and the Board by the American Association of State of the National Research Council. Highway and Transportation Officials. Research projects to fulfill The members of the technical committee selected to monitor this these needs are defined by the Board, and qualified research agencies project and to review this report were chosen for recognized scholarly are selected from those that have submitted proposals. Administration competence and with due consideration for the balance of disciplines and surveillance of research contracts are the responsibilities of the appropriate to the project. The opinions and conclusions expressed or National Research Council and the Transportation Research Board. implied are those of the research agency that performed the research, The needs for highway research are many, and the National Coop- and, while they have been accepted as appropriate by the technical com- erative Highway Research Program can make significant contributions mittee, they are not necessarily those of the Transportation Research to the solution of highway transportation problems of mutual concern Board, the National Research Council, the American Association of to many responsible groups. The program, however, is intended to State Highway and Transportation Officials, or the Federal Highway complement rather than to substitute for or duplicate other highway Administration of the U.S. Department of Transportation. research programs. 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 http://www.national-academies.org/trb/bookstore manufacturers. Trade or manufacturers' names appear herein solely because they are considered essential to the object of this report. 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