I've been having similar fantasies about driving out into traffic. I feel OK with these thoughts because like you I would NEVER come close to acting on something that might injure others or even involve others with my own death that might cause them direct grief or guilt.

However I do worry that these impossible fantasies indicate that I might be closer to justifying a "safer" form of suicide for myself. If that might be the case with you I hope you are able to reach out for some help.