My proposal is to make cell phone hail be as fast as -- and sometimes faster than -- street hail. But with reputation, security and the rest.

Yes, shared airport shuttles are not a great experience. With the airport it's made worse by having to unload bags, and arrange payment. In a system like this payment would be automatic from an accout, and people not doing airport trips would not have bags, often getting out at stop lights and with no diversion from the optimal route for the other passengers. Only true computerized coordination of trips can make the last one happen.

As I see it a jitney trip might involve taking a private cab to some suitable "transfer station" where one moves into the next cab with minimal wait. Likewise the last mile of the trip might also be in a private cab, to avoid taking the other passengers out of their way. This becomes more workable if the company can negotiate fees and there is not a mandatory flag drop (which makes 1 mile trips less economical.)