If the yellow cab drivers have phones on them as well, you could try to make "psuedo hails" using bluetooth or something where if you drive by a taxono.my user

If the yellow cab drivers have phones on them as well, you could try to make "psuedo hails" using bluetooth or something where if you drive by a taxono.my user who has hailed a cab both apps ping, and let the users know. Then you could just have the drivers app show a heatmap of hails throughout the city to get them in the right places.