I think a geo-twitter service like twinkle may be the right platform for ride sharing / car pooling, but here's the competition....
Startups That Are Reinventing Carpooling on the Web
As gas prices have soared, more commuters have made the switch to carpooling, and they’re turning to a host of free web-based services – from government-sponsored transit programs to iPhone apps and widgets — to meet their needs. But the plethora of options can be a mixed blessing: More options mean your perfect carpool match is more likely to be signed up with a different service. That’s not only a problem for you, the commuter, but also for the companies hoping to capture your business; to succeed, ride-match sites have to be able to guarantee that they can get riders where they want to go, when they want to go there.
So a growing number of ride-share startups are following in the footsteps of other Web 2.0 applications providers by targeting the enterprise market. Catering to businesses gives Carpool 2.0 startups both a captive pool of riders with a shared destination (and often similar schedules) and corporate execs that are increasingly willing to pay for customized services, like hosted web sites and mobile apps.