It's much easier to coordinate ride sharing when there's an interested third party in place. ad-hoc ride-sharing for a reduces cost could result in the average car traveling at much closer to full capacity much of the time. If you set it up ahead of time, it could even optimize a route that picks up and drops off along an optimal route.
Don't think of it as replacing what historically was walking outside and hailing a cab, think of it as setting up a carpool with office mates except you don't have to know them, and you don't have to work out all the details, it will be handled for you.
Don't think of it as replacing what historically was walking outside and hailing a cab, think of it as setting up a carpool with office mates except you don't have to know them, and you don't have to work out all the details, it will be handled for you.