Modesty

Its interesting. I heard a vort from my Rabbi that he heard from his Rabbi.

That a person when they go on a bus there may be distractions like woman etc... and keeping ones head down might work, but it wont work if ch"v they accidentally look up or see something they shouldn't what will they do then?

A person should simply focus on Torah thoughts, keep them in mind constantly. When a person is worried about a situation whether its modesty or some other shmutz, they should focus on learning/reviewing Torah.