back off ：滚开，后退。比较口语化，用于辱骂等命令语气。back up:交通堵塞。如traffic backs up around the corner...写作时可用这句，不要用traffic is busy...back up还有倒退之意The economy has been always backing up(off)....经济持续倒退。这里也可用back off.
