Nextbus.com already does this with several public transportation systems, including Boston and San Francisco. I'm actually in the process of putting together a

Nextbus.com already does this with several public transportation systems, including Boston and San Francisco. I'm actually in the process of putting together a service to alert riders when the bus is about to arrive, based on that data. It's still in beta, it only supports Boston, it's ugly as all hell and probably broken in places, but if anyone wants to look around, it's live here: http://www.buscalling.com http://www.buscalling.com PS: if anyone wants to beta-test, let me know!