You know my idea is accept both. If you use second option could be interesting (but not mandatory) to add :id to the key traffic_sign= key to include the code you have mapped already.
So
traffic_sign:id=US:2009:R7-21
traffic_sign=regulatory
parking:fee=yes
parking:maxstay=1 hour
mutcd_compliant=no
Let’s discuss it in Large scale change of traffic_sign to traffic_sign:id - #100 by yopaseopor
Here it is the proposal to do it > https://wiki.openstreetmap.org/wiki/Proposal:Extended_traffic_signs_tagging#How_it_works._How_to_map
Interesting. I didn’t know this way of doing quests.