I have subscribed to two subreddits that sort of cover this. The first is /r/SideProject. It is a place that people post their side projects and occasionally blogs/articles about their side project. The pros of it is that people don't frown on self promotion but the con is that there doesn't seem to be many comments going on to help improve your offering. The second is /r/lookatmyprogram although that sub seems to be dead. The bigger (if not the biggest) software subreddit is /r/programming but they state that "App demos should include code and/or architecture discussion." so you wouldn't be able to flat out promote.

I'd suggest to find a/the subreddit that corresponds the most to your side project. In the case of the OP: https://www.reddit.com/r/RedditCarpool