Hi, i honestly dont know what that tag is supposed to mean. It's not something i modified at the time (It was a maproulette challenge about highway surface on italian main roads) and I don't recall any josm validator warning on that. 
I quickly looked at the way history (https://.osm.org/way/171687869/history, and I saw that it was added in 2019 (https://osm.org/changeset/73697206). By quickly studying that changeset, i believe the mapper meant to put maxspeed=120 on the bridge railway (https://osm.org/way/181456239), coherently with nearby railway tracks (osm.org/way/181456242 and osm.org/way/425965746), and mistakenly selected the underneath highway, also using a wrong tag, probably caused by some kind of autocomplete feature. 
That's the only reasonable explaination i can think of, because 120 kph is not a default highway speed limit in IT (we have 50, 90, 110 and 130).
If it's something that's fine with you, i can proceed to do this kind of edit to the area.

Best regards
Cybermanx