some links:
http://wiki.openstreetmap.org/wiki/United_States_roads_tagging
http://wiki.openstreetmap.org/wiki/United_States
http://wiki.openstreetmap.org/wiki/Map_Features#Highway
http://wiki.openstreetmap.org/wiki/TIGER

pittsburgh road classification examples(as from TIGER):
state route 28: highway=primary why not motorway?
highland park bridge: highway=secondary
washington blvd.: secondary
negley/stanton ave: tertiary
I-579/I-279: motorway