Great idea.

If you plan to create a social network that would be used for roadside assistance, then your biggest obstacle will definitely be competition from existing social network giants. People can currently use Twitter or Facebook to reach out to friends for help.

If, instead, your service will use existing social networks, I think your big obstacle will be ensuring the safety of users. A user will need to share his or her location to get assistance. You have to figure out a way to prevent criminals from taking advantage of that information to find easy victims for theft or worse. Also, criminals could pose as distressed travelers to lure unsuspecting good samaritans into a position where they could become victims. Once you solve that, your'll have to market your service as both reliable and safe.

Good luck to you!