Theres an app for that
Changing booths in department stores
Mens rooms alleyways,back seats of cars..though he is super fast,life is not the same for todays Super-heros..he was sobpoena'ed the other day,sued for not putting out a fire fast enough..everyones a critic!
McDonalds or Walmart bathrooms - They are everywhere, and no one questions an oddly dressed person at either of this places.
he doesn't need to hide. no one looks up from their i-phones
Since he only exists in a pretend world, maybe the pretend world has pretend phone booths?
He does it in ladies' restrooms.
He must find where loise Lane is living to change. I believe , all Clark has to do get his business suit off, He always wearing blue leotard and tights underneath and ready to go into action.
There's an app for that...duh!
I believe phone booths have been replaced by a cut scene nowadays
And when he does, the Invisible man is using it...
He could change in plain sight since he has super speed.
Portapotties, like everyone else.