This site has public-transportation and public-issues. Yes, transportation is a subset of public issues. No, both tags are not necessary. Either remove public-issues in favor of more specific tags such as public transportation, or remove public transportation in favor of more general tags like public issues.
I actually see them as not overlapping at all.
public-issues comes across to me as those sorts of awkward things that of course have to happen in public. Like spilling your stuff everywhere in the middle of a crowded mall, or your stomach growling (loudly, at a quiet moment) in class.
public-transportation comes across as issues to do with, you know, public transportation (aptly named tag). Public issues can happen on public transportation, but public issues can also happen in a classroom, in the middle of the mall, or on an alien starship (does that qualify as public transportation?).