I fear that in these modern times -- with security cameras on street corners, recording our every move and providing evidence for the courts when necessary, and with monitoring devices in elevators, in trains, and behind every convenience store counter -- Clark Kent would have a very difficult time finding a place to change into Superman without being "seen."
I suppose he could use his heat vision or freeze breath to disable (temporarily or permanently) nearby recording devices that he knew about; and sure, thanks to his x-ray vision and super speed he could probably find all the nigh devices in a matter of moments, if he had the desire to do so. But if he was headed toward a rescue, where death was on the line, would taking the time to do that really be worthwhile? Not to mention he would be tampering with and/or destroying others' property. Should Superman destroy others' property for the common good?
"No," you might say. "Superman shouldn't destroy video cameras. He should just go about his job. So what if someone catches Clark Kent changing into Superman?"
I'll tell you what would happen. All of Clark Kent's loved ones will be in danger. Everyone at the Daily Planet will become a target. His elderly parents will be kidnapped, and Lois? Well, the woman puts herself in enough danger as it is, she doesn't need any more of it.
You see? You see the problem? This is a big problem.
Darn you, modern technology!