Não informado. (1968). Research Results Digest. National Cooperative Highway Research Program.
Chicago-referens (17:e uppl.)Não informado. Research Results Digest. Washington: National Cooperative Highway Research Program, 1968.
MLA-referens (8:e uppl.)Não informado. Research Results Digest. National Cooperative Highway Research Program, 1968.
Varning: dessa hänvisningar är inte alltid fullständigt riktiga.