The Detroit Department of Transportation, provider of transit bus service within Detroit, does have a couple routes that cross over into Downriver. These routes are the 19 (which lays over just barely into Lincoln Park) and the 41 (serving River Rouge). DDOT replaced the Department of Street Railways (DSR) in 1974.
Interestingly enough, an older version of a DDOT bus stop sign can be seen along Southfield Road in Allen Park, although no DDOT route goes that way, instead it is served by SMART Route 140. That pole, however, still contains the standard SMART schedule box.
Detailed info is on the DDOT entry on the Detroit Wiki.