もう少し早く出発していれば、渋滞に巻き込まれずに済んだ___、今となっては後の祭りだ。

「～ものを」 is used at the end of a clause to express dissatisfaction, regret, or complaint that a different, better outcome was possible but did not happen. It's similar to saying "...but instead..." or "if only...". In this sentence, the speaker is regretting not leaving earlier. 「ものか」 is a strong denial. 「ものだから」 gives a reason or excuse. 「ものがある」 expresses that one feels a certain way about something.