Common use of the term viaduct around Chicago, usually in reference to a flooded underpass, is part of what prompted my question. I’m begining to think the entire elevated ROW, mandated by law in Chicago, is the actual viaduct and people began refering to any part of it using that name. This would be consistent with the elevated heavy rail ROW’s mentioned in in New York and some other cities. Unlike the L, these structures are a combination of concrete retained fills, plate girder bridges, and some other types of bridges.
A show on the Discovery Channel about building a very long, very high Autobahn crossing of a very large valley used both terms in reference to the same structure. A series of bridges wouldn’t seem to qualify as then the entire Bay Bridge complex in Frisco would be a viaduct, but I’ve never heard it called that.