1. We didn't see any desire even when all the right pieces were in place (origin, destination, timing match, long distance + high cost of parking, compatible and networked ridesharers -- they still couldn't be bothered).
2. In Europe, where ridesharing is catching on, it competes with bus and train travel which is expensive, not car travel. In the US, ridesharing is competing with going in your own car (or nothing) and it isn't very competitive. The costs of car travel are not significant enough (yes yes I know that it is 18% of household income and $8k per year -- remember, I spent lots of money and a couple of years on this business).
3. For short trips, it'll never work because the cost of any minutes of delay just aren't worth the reduced cost of travel to the driver.
4. As I'm sure you are thinking about dynamic real-time ridesharing -- everyone is -- answer this when you set about building your network: Robin driver posts her trip once, twice, four times (?), maybe keeps an open trip pending and gets no response. She stops posting and doesn't tell anyone about what a great service/app/method you are running. Ditto passengers. When you move to real time, you have reduced the likelihood of getting a match because now the window of opportunity is 5-10 minutes (if we couldn't find a match when we said any time you ever go to NYC email me, why would you find one if I said anyone wanna go in the next 5 minutes?)
5. Look at all the past efforts, and figure out honestly why your idea is different. Everyone so far has failed. Smart phone apps, social network connections, through employers, on narrow corridors, etc etc.
Driving is still too cheap in the US. People still aren't willing to make the effort in adequate numbers to make it work.
Sorry to be such a downer but I get asked this all the time. So many people starting and thinking about this space."
Note that she's using rideshare in the traditional sense, before Lyft/etc. took over the word within the tech community. (Lyft is awesome! Just different, for now.) So for short trips, using pseudo-hired drivers on shifts a la Lyft, it obviously works well, just like taxis work well.