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Transportation Research Board 500 Fifth Street, NW Washington, DC 20001 ISBN 978-0-309-21381-3 90000 Subscriber Categories: Administration and Management · Law · Public Transportation 9 780309 213813 These digests are issued in order to increase awareness of research results emanating from projects in the Cooperative Research Programs (CRP). Persons wanting to pursue the project subject matter in greater depth should contact the CRP Staff, Transportation Research Board of the National Academies, 500 Fifth Street, NW, Washington, DC 20001.