Those unwanted spontaneous erections, just hate them! I suspect that your penis is responding to the movement of the particular vehicle you're in, bouncing up and down, rubbing inside your clothes..stuff like that. I'm not sure that there is much you can do about this except wear something you can easily cover up with as needed. Hopefully this will ease at some point. ALso, if you think about it happening during these times or get worried/concerned about it happening..chances are it'll happen. And by the way, these unwanted erections have nothing to do with sex, just part of being a guy.