These are great questions. I wanted to get a sense of what people are wondering before I dig into any research and this is a big help!
Ultimately we want to develop a web feature that will address these questions and deal with the ins-and-outs of how it works. But that might be a while, so let me quickly answer the few that I know already:
A definition of formally organized car sharing is kind of hard to pin down, as some of you noticed. I was originally thinking of membership-based services (like ZipCar and others). In these cases, the company owns, maintains and services all the cars and members rent them by the hour (as opposed to standard rental companies with daily fees.) This is cost-effective for occasional errands, but for full-day or weekend trips, you’re better off with a typical rental.
But, there are also companies that let you essentially rent out your own car to other people (which raise a different set of questions.) From your comments, it seems like people are a bit curious (but more cautious) about these services too. So I’ll look into that as well. These services are not as popular as the membership-style ones (at least not in the Boston-area).
The insurance and liability questions are much trickier so I’m going to not touch those until I’ve done a lot more research… but we will definitely be looking into that because it’s a big one.
Thanks again for your input and if you think of anything else, post it! I’ll check back periodically.
Yours in brainstorming,