While I can't imagine needing 45 min in RDU, this is exactly the kind of situation that can be avoided with online checkin, as everyone else has said.

I also am somewhat disturbed that it seems that UA can set whatever cutoff time they want on a particular day. While I understand that they tell you to get there 90 minutes ahead, that is not the rule, it is just a suggestion. Of course, everyone should try to get to the airport 90 minutes ahead, but occasionally that does not happen, in which case it would be nice if UA had some consistency as to what cutoff time really is.