Buses can be high pressure but only really at the start before you start chatting. If the conversation is flowing nicely it's easy enough to close.

Don't do it after a silence. Do it while you're both talking like when you or her have just finished or are nearly finished a sentence or whatever is being said, while it's high energy eye contact & smiles. Just pop in "we should exchange numbers so we can chat another time too"

Well, that's what I do.