Das ist dokumentiert:
“Currently, there are local communities using the key source:maxspeed=* for this purpose.”
Quelle: https://wiki.openstreetmap.org/wiki/Key:maxspeed#Values
Sowie:
“These tags can be used in the tag source:maxspeed=* as well, in addition to an explicit maxspeed=* value - which is currently preferred by some local communities.”
Quelle: https://wiki.openstreetmap.org/wiki/Key:maxspeed#Implicit_maxspeed_values
Und betreffend 30/20iger Zonen:
“This tagging is overlapping with the dedicated tag zone:maxspeed=* which is superfluous when specified in the maxspeed-tag itself.”
Quelle: https://wiki.openstreetmap.org/wiki/Key:maxspeed#Maxspeed_zones