Books by Gary H. Zehnpfenning:
Books in the extended shelves:
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Zehnpfenning, Gary H.: Impediments to bicycling and walking (Federal Highway Administration, 1993), also by National Bicycling and Walking Study (U.S.) and United States Federal Highway Administration (page images at HathiTrust)
Find more by Gary H. Zehnpfenning at your library, or elsewhere.