Synthesis 178 NCHRP H TRANSPORTATION RESEARCH BOARD EXECUTIVE COMMITTEE 1992 Officers Chairman WILLIAM W. MILLAR, Executive Director, Port Authority of Allegheny County Vice Chairman A. RAY CHAMBERLAIN, Executive Director, Colorado Department of Transportation Executive Director THOMAS B. DEEN, Transportation Research Board, National Research Council Ex Officio Members MIKE ACOTI', President, National Asphalt Pavement Association GILBERT E. CARMICHAEL, Administrator, Federal Railroad Administration, U.S. Department of Transportation BRIAN W. CLYMER, Administrator, Federal Transit Administration, U.S. Department of Transportation JERRY R. CURRY, Administrator, National Highway Traffic Safety Administration, US. Department of Transportation TRAVIS P. DUNGAN, Administrator, Research and Special Programs Administration, U.S. Department of Transportation FRANCIS B. FRANCOIS, Executive Director, American Association of State Highway and Transportation Officials THOMAS H. HANNA, President and CEO, Motor Vehicle Manufacturers Association of the United States, Inc. BARRY L. HARRIS, Acting Administrator, Federal Aviation Administration, U.S. Department of Transportation LT. GEN. HENRY J. HATCH, Chief of Engineers and Commander, U.S. Army Corps of Engineers THOMAS D. LARSON, Administrator, Federal Highway Administration, U.S. Department of Transportation WARREN G. LEBACK, Administrator, Maritime Administration, U.S. Department of Transportation GEORGE H. WAY, JR., Vice President, Research and Test Department, Association of American Railroads Members JAMES M. BEGGS, Chairman, SPA CEHAB, Inc. (former Administrator, National Aeronautics and Space Administration) KIRK BROWN, Secretary, Illinois Department of Transportation DAVID BURWELL, President, Rails-to-Trails Conservancy L.G. (GARY) BYRD, Consultant, Alexandria, Virginia L. STANLEY CRANE, former Chairman & CEO of CONRAIL RICHARD K. DAVIDSON, Chairman and CEO, Union Pacific Railroad JAMES C. DuLONG, Director of Aviation, Philadelphia International Airport JERRY L. DEPOY, Vice President, Properties and Facilities, USAir THOMAS J. HARRELSON, Secretary, North Carolina Department of Transportation LESTER P. LAMM, President, Highway Users Federation LILLIAN C. LIBURDI, Director, Port Department, The Port Authority of New York and New Jersey ADOLF D. MAY, JR., Professor and Vice Chair, Institute of Transportation Studies, University of California WAYNE MURI, Chief Engineer, Missouri Highway and Transportation Department (Past Chairman, 1990) CHARLES P. O'LEARY, JR., Commissioner, New Hampshire Department of Transportation NEIL PETERSON, Executive Director, Los Angeles County Transportation Commission DELLA M. ROY, Professor of Materials Science, Pennsylvania State University JOSEPH M. SUSSMAN, JR East Professor of Engineering, Massachusettes Institute of Technology JOHN R. TABB, Director and CAO, Mississippi State Highway Department JAMES W. VAN LOBEN SELS, Director, California Department of Transportation) C. MICHAEL WALTON, Paul D. and Betty Robertson Meek Centennial Professor and Chairman, Civil Engineering Department, University of Texas at Austin (Past Chairman, 1991) FRANKLIN E. WHITE, Commissioner, New York,State Department of Transportation JULIAN WOLPERT, Henry G. Bryant Professor of Geography, Public Affairs and Urban Planning, Woodmw Wilson School of Public and International Affairs, Princeton University ROBERT A. YOUNG III, President, ABF Freight Systems, Inc. NATIONAL COOPERATIVE HIGHWAY RESEARCH PROGRAM Transportation Research Board Executive Committee Subcommittee for NCHRP WILLIAM W. MILLAR, Port Authority of Allegheny (Chairman) C. MICHAEL WALTON, University of Texas at Austin A. RAY CHAMBERLAIN, Colorado Department of Transportation THOMAS B. DEEN, Transportation Research Board FRANCIS B. FRANCOIS, American Association of State Highway and L. GARY BYRD, Consulting Engineer, Alexandria, Virginia Transportation Officials THOMAS D. LARSON, Federal Highway Administration Program Staff Field of Special Projects ROBERT J. REILLY, Director, Cooperative Research Programs Project Committee SP 20-5 LOUIS M. MACGREGOR, Program Officer VERDI ADAM, Gulf Engineers & Consultants DANIEL W. DEARASAUGH, JR., Senior Program Officer ROBERT N. BOTHMAN, The HELP Program IAN M. FRIEDLAND, Senior Program Officer JACK FREIDENRICH, The RBA Group CRAWFORD F. JENCKS, Senior Program Officer JOHN J. HENRY, Pennsylvania Transportation Institute KENNETH S. OPIELA, Senior Program Officer BRYANT MATHER, USAE Waterways Experiment Station DAN A. ROSEN, Senior Program Officer THOMAS H. MAY, Pennsylvania Dept. of Transportation EILEEN P. DELANEY, Editor EDWARD A. MUELLER, Morales and Shumer Engineers, Inc. EARL SHIRLEY, California Dept. of Transportation TRB Stafffor NCHRP Project 20-5 JON UNDERWOOD, Texas Dept. of Transportation ROBERT E. SKINNER, JR., Director for Special Projects THOMAS WILLETF, Federal Highway Administration SALLY D. LIFF, Senior Program Officer RICHARD A. McCOMB, Federal Highway Administration (Liaison) SCOTT A. SABOL, Program Officer ROBERT E. SPICHER, Transportation Research Board (Liaison) LINDA S. MASON, Editor CHERYL KEITH, Secretary National Cooperative Highway Research Program Synthesis of Highway Practice 178 Truck Escape Ramps DAVID K. WITHEFORD Reston, VIrgInIa Topic Panel WILEY D. CUNAGIN, Texas Transportation Institute, College Station, 7X WILLIAM A. PROSSER, Federal Highway Administration, Washington, D. C. LARRY A. SCOFIELD, Arizona Transportation Research Center, Phoenix, AZ JUSTIN G. TRUE, Federal Highway Administration, McLean, VA EDWARD J. TYE, Eugene, OR JAMES C. WAMBOLD, Pennsylvania Transportation Institute, University Par/c, PA Research Sponsored by the American Association of State Highway and Transportation Officials in Cooperation with the Federal Highway Administration TRANSPORTATION RESEARCH BOARD National Research Council Washington, D. C. May 1992 NATIONAL COOPERATIVE HIGHWAY RESEARCH PROGRAM NCHRP SYNTHESIS 178 Systematic, well-designed research provides the most effective Project 20-5 FY 1989 (Topic 21-12) approach to the solution of many problems facing highway ad- ISSN 0547-5570 ministrators and engineers. Often, highway problems are of local ISBN 0-309-05305-6 interest and can best be studied by highway departments individ- Library of Congress Catalog Card No. 92-060469 ually or in cooperation with their state universities and others. However, the accelerating growth of highway transportation Price $9.00 develops increasingly complex problems of wide interest to high- way authorities. These problems are best studied through a coor- Subject Areas dinated program of cooperative research. Highway and Facility Design In recognition of these needs, the highway administrators of Safety and Human Performance the American Association of State Highway and Transportation Officials initiated in 1962 an objective national highway research Mode Highway Transportation program employing modern scientific techniques. This program is supported on a continuing basis by funds from participating member states of the Association and it receives the full coopera- tion and support of the Federal Highway Administration, United States Department of Transportation. The Transportation Research Board of the National Research Council was requested by the Association to administer the re- search program because of the Board's recognized objectivity NOTICE and understanding of modern research practices. The Board is The project that is the subject of this report was a part of the National Cooperative uniquely suited for this purpose as: it maintains an extensive Highway Research Program conducted by the Transportation Research Board committee structure from which authorities on any highway with the approval of the Governing Board of the National Research Council. Such transportation subject may be drawn; it possesses avenues of approval reflects the Governing Board's judgment that the program concerned is of national importance and appropriate with respect to both the purposes and communications and cooperation with federal, state, and local resources of the National Research Council. governmental agencies, universities, and industry; its relation- The members of the technical committee selected to monitor this project and to review this report were chosen for recognized scholarly competence and with due ship to the National Research Council is an insurance of objectiv- consideration for the balance of disciplines appropriate to the project. The opinions ity; it maintains a full-time research correlation staff of specialists and conclusions expressed or implied are those of the research agency that per- in highway transportation matters to bring the findings of re- formed the research, and, while they have been accepted as appropriate by the technical committee, they are not necessarily those of the Transportation Research search directly to those who are in a position to use them. Board, the National Research Council, the American Association of State Highway The program is developed on the basis of research needs identi- and Transportation Officials, or the Federal Highway Administration of the U.S. fied by chief administrators of the highway and transportation Department of Transportation. Each report is reviewed and accepted for publication by the technical committee departments and by committees of AASHTO. Each year, specific according to procedures established and monitored by the Transportation Research areas of research needs to be included in the program are pro- Board