It is common for travelers who have sat down beside each other to have almost finished exchanging their life stories by the time they get to their destination.
It ~for ~to ~の構文はfor~の部分が主語になるので、主語を修飾する節の終わ りにスラッシュを入れ、他は前置詞の前に入れます。
It is common for travelers who have sat down beside each other /to have almost finished exchanging their life stories /by the time they get to their