Aren't most taxis hailed by phone in San Francisco anyway. I was down there a month or two ago and took at least two dozen taxis. I think one was a street hail and four were pick ups by cabs waiting outside my hotel. All the others were dispatched by phone.

Seattle is similar. I almost never hail a cab on the street. I use my cellphone, or have the restaurant make the call. Seattle has no restrictions on the number of taxis, but does have liability and other regulations. Every taxi we have seen is dispatched by one of the major taxi companies.

In effect, your call for a cab model is almost here. As for GPS, I'm not sure it's worth it. We often call from one location and walk to another location where we can wait comfortably for the cab. Besides, I'm not sure how well GPS works in the city anyway. It sure doesn't work when there are trees around, so buildings might interfere as well.

Jitneys might be a good idea for commuters, especially if you already have a transit system, but anyone who has taken one of those airport shuttles knows the problem with door to door shared drop off. Going the last few blocks from hotel to hotel often takes as long as the whole haul from the airport into town.