Superman used to change out of his Clark Kent identity into his Superman costume in a phone booth. There haven't been any full size phone booths for several decades.
Where is Superman changing his clothes these days?
I want to suggest some answers.
* Starbucks johns...the only public toilets in most American downtowns.
* Metropolis has saved the old phone booths but with no phones in them.
* He has a big yoga mat he unrolls and hides behind.
* The Clark Kent costume burns up when Superman flies. The suits came from Goodwill anyway.