http://www.loc.gov/mods/v3 http://www.loc.gov/standards/mods/v3/mods-3-2.xsd
the complete study of passenger/pedestrian oriented symbols developed by the American Institute of Graphic Arts for the U.S. Department of Transportation
"[the 1974 and 1978] studies have been combined and reprinted here [with additional material from 1985]."-- page 8.